Lazy Hacker Checks Fuel System For Leaks, The Easy Way

Old cars are great. They represent a different time, reflecting the state of society at the point of their design and manufacture, and can charm and delight while also providing useful transport. Except, well… old cars are great, except when they’re not.

With my Volvo 740 hitting its thirtieth birthday and cresting over 200,000 miles, to say its a little worse for wear is an understatement. The turbo dadwagon has suffered transmission issues, and cold starting woes… but most frustrating is the sudden spike in fuel use. After some work, my humble daily driver had slid from using an acceptable 21 miles per gallon, to getting just 15. Add on the fact that the turbocharged engine demands premium fuel, and you can understand my consternation.

Now that I was haemorrhaging cash on a gargantuan weekly fuel bill, I had plenty of motivation to track down the problem. Busy, and eager for a quick solution, I deferred to a mechanic recommended as the local expert in all things Volvo. Sadly, the results were inconclusive — initial appearances were that all the engine’s electronic controls were functioning to specifications, and I was told that it was “probably a bad batch of fuel”.

Unfortunately, several expensive tanks later, sourced from all over town, revealed that the problem was in fact real. With a supposedly reliable report that the fuel mixture was correct, thus ruling out culprits like the oxygen sensor, I began to wonder, was I simply pouring fuel out the tank?

A Safety Practice Began My Problems

I’d come to this worrying suspicion due to an amateur mistake I’d made when changing the transmission filter weeks prior. After the job was done, I began to jack up the car in order to remove the axle stands holding it up. Upon letting the car down, there was a resounding crunch.

I generally leave tyres, bricks, or in this case, ramps under the car as a backup in case a stand fails. I’d stupidly forgotten to remove them this time, and the car’s full weight was now on the ramp I’d left sitting directly under the fuel tank. I hastily jacked the car back up and removed the ramp, inspecting for damage. There was nothing obvious, but in the following weeks, my fuel economy had become terrible.

This led me to believe I’d perhaps damaged a fuel line or other tank fitting, and that I was pouring petrol out of the tank when going around corners. This needed to be checked, but how? Dropping the tank for inspection promised to be a big job, and ideally would be done when the tank was empty to avoid being crushed under the weight of 50 liters of fuel. There had to be a better solution.

An accomplished colleague of mine asked if I’d considered pressure testing the system. Of course! It was so simple. All I had to do was find a way to pressurize the fuel system with air, and see if it held pressure, or if I could hear leaks. Easy!

Bike Pumps: A Staple of the Hacker Toolkit

A bike pump, which can be used to pressurize the fuel system. I scored mine for $20.

I scrounged around the house and put a plan together. First, I popped the hood of the car. Taking a look at the fuel rail, I determined it would be easiest to remove the return line from the fuel pressure regulator, which was held on with a simple hose clamp. This will vary a lot from car to car; I was lucky that this was fairly accessible. I then grabbed my trusty bike pump, and removed the valve adapter from the end, leaving just the air hose. As luck would have it, this fit snugly within the fuel return line. With the fuel delivery line still in place and any flow blocked by the regulator, it was now possible to pump air into the tank.

The aim here isn’t to put the fuel system under a great deal of pressure; a couple of psi above ambient is fine. Unfortunately, the gauge on my K-mart pump isn’t accurate below 20psi, but I was able to start pressurizing the tank and lines. After a few pumps, I heard a sound that I can only describe as a disgruntled moan. This was the fuel cap releasing the excess pressure. After a few more pumps (and awkward sounds), I looked under the car to check for any fuel leaking, and thankfully, none could be found. Just out of interest, I undid the fuel cap, which released the last of the pressure with a hiss. If this isn’t done, the return hose will tend to spew fuel all over the engine bay when the air hose is removed which can be a bit of a hassle.

The pump was hooked up by inserting its output air line into the tank return line. Various cars use different setups which may make this test easier or harder depending on how the fuel lines are set up.

In the end, it appeared that there was no leak at all. My hunt for a fuel economy problem goes on. But the great thing about this technique is that it’s readily achievable and requires no special tools – just the ingenuity required to mate up a few hoses. It can readily help you determine if your fuel system has a serious leak or not. If you’d like to see it done on video, we’ve got you covered — albeit, I confused the return and delivery lines. If, however, you’ve got a slower leak, like a dead fuel injector, or you’re trying to diagnose a failed check valve in a fuel pump, you may struggle to find it with this technique. In that situation, you’ll want to lay your hands on “the right tool” which is a fuel pressure test kit, and undertake a deeper investigation. [EricTheCarGuy] has a great guide on how to do just that, though it does require some more specialized equipment.

Overall, it’s a simple check that’s accessible by the shadetree mechanic that might help you spot a problem before it gets out of hand. We’d love to cover more garage hacking. How do you approach diagnosing fuel economy problems on your fuel injected rides? What are the other automotive hacks you’ve taken on over the years?

107 thoughts on “Lazy Hacker Checks Fuel System For Leaks, The Easy Way

    1. Seconded. What kind of a shaggy dog story is this?

      A few ideas, based on Volvo 240s but generally applicable to the 740:

      There are two fuel pumps in series, a main pump under the body and an in-tank pump. It’s possible the in-tank pump, the fuel pickup, or the fuel lines are crushed or otherwise damaged in a way that restricts fuel. The in-tank pump is accessible through a panel in the floor. Check for low fuel pressure. Check the spark plugs for lean operation, and check the duty cycle of the injectors. This might indicate the ECU desperately trying to compensate for fuel starvation.

      Jack up the rear end and check that the wheels rotate freely. Maybe the brake lines or parking brake cable are crushed, and your rear brakes are always engaged!

      Check that the overdrive is engaging at highway speeds. The overdrive is a separate gearbox bolted to the back of the transmission, and engaged via solenoid. There’s a lot to go wrong with this system: flaky solenoid, connectors, wiring, relay, or even the button on the shifter. There’s a light on the dash which indicates if OD is disabled, but sometimes it doesn’t work. Probably not related to the jack incident.

      Last but oh boy, not least for a turbobrick with 200K miles, check all the air intake hoses for any degradation or loose fit. The turbo will happily blow hot oil into the air intake system, causing the rubber hoses to swell and disintegrate. Then boost escapes under acceleration, and air is sucked in at idle, all behind the air mass meter which is supposed to tell the ECU how much air is coming in. This is generally coupled with very poor performance, but a small leak could hurt mileage without being noticeable. Again, not related to the jack incident.

      1. Not sure what a shaggy dog story is.

        The fact that the system held pressure (until the fuel cap released it) indicated there was no leak. I could have explained that better.

        Thanks for the tips. A fuel pressure test kit is on my shopping list.

          1. Ah yes. That was a conscious decision, but I’ll admit probably somewhat grating for someone reading this who just wants the technical meat of the matter.

  1. I found the Horror Fright Mechanic’s Stethoscope useful in finding dead fuel injectors on my Suzuki Sidekick (R.I.P.)
    When the engine was idling, I would place the stethoscope probe on each fuel injector. A good injector made a “tika-tika-tika” sound, and a bad injector made a “nothing-nothing-nothing” sound.

    But, when a fuel injector died on the Sidekick, MPG dropped to less than half. The O2 sensor would trigger the ECU to engage “limp home” mode. When that happened, a butane lighter placed into the exhaust stream, about a foot away from the end of the tailpipe would result in a large flame about a half foot in diameter a couple feet long (and singed finger hair).

        1. When gas stations switched to 10% ethanol, I experienced a 10% drop in fuel mileage (I have the Excel spreadsheet to show it).
          So… if I had put 85% ethanol (E-85) into it, I reckon mpg would have dropped 85% B^)

          1. I’m pretty sure that when I moved cross country, switching from pure gasoline in Virginia to 10% ethanol in Oregon gave me a hit of MORE than 10% to the mileage on my motorcycle (carbureted). I *very much* doubt that putting ethanol in the gas is actually reducing emissions; in some cases it seems to result in MORE gasoline burned per mile travelled–not more gasohol, more *gasoline*.

            Also, pulling all that corn out of the food supply raises food prices, hurting the most vulnerable members of our population. I am *100% for* protecting the environment, but gasohol is *not doing that* and should be dropped.

          2. If you don’t like it, get working on recombinant DNA tech like they use to make insulin through use of bacteria, then design a system which uses food waste to feed the bacteria for ‘free’ petrol, then vary the process for diesel and all our other favourite complex chemicals

          3. Yes, ethanol has less energy content than gasoline. However, that’s not the only factor here. It also needs a much richer air to fuel ratio. That further reduces your fuel mileage even more.

          4. Likewise the only way you are going to get the full power/ mpg ( due to right gearing for the power ) performance from new engines is to use 98RON they are calibrated for …. & not the 95 stuff mostly available in europe.

        2. Ethanol mix would have eventually eaten the seals anyway :(
          It absolutely wreaks havoc on older boat motors as well. I had to completely overhaul two last year.

    1. But, injectors can still click and not be functioning as designed. The best way to isolate a bad injector (after listening to them as you described) is to pull the plug wires one at a time while engine is at idle. If the engine is missing from a faulty/dirty injector, the plug wire you pull that makes no difference in the roughness of the engine is the bad injector. I just went through this on my old Mercury and, since you have to tear down the engine to reach this particular injector, I was able to “repair” it using 3 cans of sea foam mixed as per instructions. That was 5,000 miles ago and the engine is still running great.

      1. Another way of checking if a cylinder isn’t firing is to start (cold of course) and feel the exhaust manifold ports just as they exit the head. The hi-tech way was to use a FLIR (I used this to confirm my “by hand” tests before I pulled the head to replace a burnt valve).

        You then have to decide if it was the spark, the injector, poor rings, bad valve, blown head gasket, a hole burnt in the piston…

        1. As long as your wires are in decent condition and you are not directed grounded, you will not get zapped. I have a single coil and I pulled the wires from the distributor end as it was easier to reach all of them and I have never been shocked doing this. I have been shocked by grabbing the wrong place while checking for spark with a plug pulled from the cylinder and grounded…yes, it does hurt. It would pay to be careful when using my method but, from my experience it is not that dangerous.

          1. I’ve gotten 12 volt shocks through my jeans, (it feels like a pin prick) just leaning against the (metal) fender and touching the 12 volt positive battery post. So, “directly grounded”, is a relative term. B^)

            “I’d piss on a spark plug, if I thought it would help!” the General in “War Games”

          2. Can you even feel a 12 volt shock? I mean, I can spit on my fingers and touch both the + and – posts of my van battery and feel nothing. I can, however, feel the very unpleasant bite of one of my 400 volt Joule Thief circuits.

          3. [PirateLabs],
            as I said/wrote…
            (it feels like a pin prick)

            I’m working under the hood, (probably tightening or loosening the Positive battery terminal) and then my train of thought gets interrupted by this uncomfortable feeling on the front of my legs. I pull away checking to see if I’m being bitten by an ant or something, and there is nothing there.
            I lean back under the hood, and a moment later, the prickly feeling returns. Repeat a couple more times, and then it dawns on me. I also might have seen a tiny spark flash between my clothing and the fender.

  2. Too bad pumping up the fuel tank didn’t remove the dent caused by the ramp!
    I was wondering if the ramp impact didn’t break crud loose from the bottom interior of the tank and cause problems.
    Is the dent causing problems with the fuel level sensor?
    (oh, I haven’t watched the video)

    1. Fuel tank wasn’t dented, it’s a poly tank and the impact wasn’t so bad.

      Fuel level sensor is functioning, and I’m calculating fuel economy based on litres input at fill time anyway.

      1. 98 Euro octane (Research Octane) is about 93 or 94 on the crazy octane scale the US government created because they couldn’t decide between Research Octane or Motor Octane rating methods and averaged them.
        Depending on where in the US you live, premium gas will be anywhere between 89 (I’ve only seen this at high altitudes) to 95 or so. The racing gas may not usable in modern street legal cars

        1. The Premium Non-ethanol gas available at some gas stations around here is about $1 higher per gallon than 10% ethanol Regular. (And a label on the pump says it is only to be used in non-automotive small engines/motorcycles.)
          Octane is raised by the addition of ethanol, so Regular Plus, is about 2 Octane higher through the addition of even more ethanol.

    1. EU and North American octanes are calculated slightly differently. EU uses RON, North America and some other countries use (R+M)/2. 98 RON is 93 to 94 (R+M)/2 which is pump gas in Canada and the US.

      1. German here, regular pump is 95RON, premium is 98RON, superpremium Shell V-Power is 98-100RON (they don’t guarantee 100RON anymore) and the stuff i have to put in my JDM Subaru is Aral Ultimate 102 with 102RON.

        The funniest thing is the Aral stuff being better, both regarding RON and additives, and cheaper than the Shell cocktail.

    2. And just a gentle reminder, the higher the Octane rating, the “less explosive” the gasoline (petrol -Jenny).
      Higher Octane ratings reduce pre-ignition (A.K.A knocking/pinging).
      Often modern engines have a “knock sensor” on the engine. If knocking is detected, ignition timing is retarded, although I’m not sure how delaying the spark will reduce pre-ignition.

      1. Ignition timing is a game of firing the spark at such a time that the combustion process will start to expand the gasses just as the piston crests TDC. Higher octane fuel has a slower combustion, and therefore can be ignited when the piston is before top dead centre (BTDC), aka “advanced”. If the mixture combusts quicker (because it is lower octane), that will result in it producing pressure before the piston has crested TDC, which exerts forces against the piston in the opposite direction to the crank (not good). Retarding the ignition – causing it to fire later – results in the expansion happening after the piston has crossed TDC. However, too retarded, and you’re firing the cylinder where it’s already on the downstroke largely because of the action of other cylinders (all connected to the same crankshaft), and you consequently don’t get as much power.

        Remember, ignition timing isn’t static – there can be mechanical (centrifugal weights), vacuum, or electronic timing adjustments – generally, as the engine is running at higher RPMs, it is further advanced, which can bring on detonation. Traditionally, one sets and checks timing at a modified idle speed, but I have some performance engines that you set the timing at 3K RPM, because that’s more of the band where the timing is going to be more critical – it’ll be a bit more advanced above there and less below, but it’s a good midpoint for the timing adjustment.

        Additionally, it is more difficult for the spark to jump a specific gap under increased pressure. As the A:F is compressed – as you near TDC – you’re slightly more likely to have a misfire, another reason you would normally want to run your engine as far advanced as it can properly run at.

        Too much advance and rapid burn (low octane) will lead to high cylinder temperatures, which can lead to hot spots, which can cause pre-ignition. Oil contamination can contribute to igntion troubles as well. Advance can also lead to high NOx.

    1. You might be on to something here. If there’s no telltale gasoline smell, mechanical drag could be the issue, though it could also be issues with the injectors (though I got the impression those were checked already, but I could be wrong)

  3. You mentioned replacing the transmission filter, so I assume it’s an automatic. Have you verified that it is still shifting through all the gears? Damage to wiring, or internal issues in the transmission, that keep it from going into the highest gear would increase fuel use quite a bit.

      1. But, it would be a good idea to see if there is an O/D (overdrive) defeat switch (hidden somewhere on the dashboard, sometimes called a Power/Economy switch) that may be keeping a transmission from engaging overdrive.

        1. The 740 my family had when I was a kid had an O/D button on the shifter. At one point the O/D got stuck on regardless of the button position or dash light state. It ended up being a wire fault (can’t remember if it was a short or open) on the harness going to a solenoid on the transmission, and was apparently a common problem.

    1. Good tip. Fairly certain the auto is functioning at least decently. It no longer gets stuck too long in low gears since I changed the filter.

      Also can confirm OD solenoid functions properly.

      I’m now wondering if maybe I just overfilled the transmission fluid and that’s causing my lost mpg…

  4. Oh, for the uninitiated, don’t check the O2 sensor with an ohmmeter!
    As I understand it, the current pushed through it from the ohmmeter will ruin the sensor.
    A better way (if a lot harder) is to remove the O2 sensor, place it in a vise, attach a volt meter (looking for less than 1 volt DC) to the sensor leads. And apply heat (heat gun/propane torch) to the sensor and look for a voltage change as it heats up, (about a half volt change).

    1. Some O2 sensors have a heating element in them, so if your sensor has more than 2 leads, find out which 2 have the sensor output, and you could possibly check the heating element with an ohmeter (though I don’t if that part is true).

      1. Funny, is I swapped my O2 sensors out since that’s what the code was noting on the OBD readout along with a misfiring on one of the cylinders not to long afterward swapping the sensors… never went through a full engine light reset time period. Then after thinking why isn’t the sensor going off and seeing the new code I wet’ed (licked) my finger like I was going to test the wind direction and moved my finger tip around the manifold that was causing the new error code regarding a misfire. Found a missing gasket and looks like the bolts might be broke… though for sure the gasket was gone. At first I thought the manifold was warped. Strange… sucks… I’m already anticipating a failed screw extraction and drilling and tapping a new hole for the bolt(s) since looks like rust haven. I’ll see when I get around to. High temp silicone seems to be working somewhat with a jb weld outer. I was almost going to try sodium silicate with some titanium or aluminum oxide filler for less shrinkage along with maybe some calcium silicate and/or hydrated lime. Does cause the engine light to turn off for a few runs and performance improves considerably.

      1. I don’t believe you can do that on mine, as it’s LH2.2, but I’d love to be corrected.

        I think you can use an LED to set the mixture though by adjusting the AFM.

  5. The most compelling reason in Europe to use the premium fuels is actually that they don’t contain (mandatory) ethanol, which will wreak havoc on the cars rubber seals and workings. Even cars as “recent” as my 2002 Volvo V40 just don’t like ethanol containing fuels.

  6. All Eu cars have two horns, but correct me if I’m wrong.
    Concerning the fuel usage. I know from experiance that you will smell a leak in the fuel system. : )

    I would seek in the direction of a bad AFR. So check your lambda sensor, temp senor, and injectors.

    Good luck!

    1. As a german mechanic I have to say: No; not every EU-car has two horns. A lot of them do but definitely not all of them.
      But there is a possibility that every EU car imported to the US has two…

    1. Generally, the Air/Fuel Ratio (AFR) inside the fuel tank is outside the ignition point. Too much fuel/fumes, not enough O2. But firefighters/emergency personnel are more concerned about an empty fuel tank than a full one.

    2. I have to agree. While it is remote maybe, you are pressurizing the combustible vapor with air so combustion limits may be reached due to partial pressure … Not an exact anology of course but look up apollo 1. Use inert gas or maybe someone knows if theymake red dye for gas like for kero, heating oil and off rd diesel.
      PS my van also got 11% less mileage w 10% ethanol AND it was more expensive than straight gas! Double whammy. (Obviously still aggravated… Let it go… let it go…)

      1. I highly doubt anyone will manage to get to enough pressure and air volume to make gasoline self-ignite in a tank.
        As an example, a Subaru EJ205 Turbo has a compression ratio of 8-9:1 depending on the exact model and at operating temperatures it still doesn’t self-ignite but still requires a spark plug to fire. A non-turbo EJ20 is at 10:1.

    3. It was a thought that crossed my mind… I’ve done vacuum tests on coolant systems before, same concept, but a negative pressure differential instead of positive, pull the air out, see if the system can hold a vacuum, probably doesn’t work as well for gasoline with it’s lower vapor pressure though.

      and aren’t gas tanks supposed to have a carbon canister that allows them to vent excess pressure on modern cars?

      Personally I say replace the fuel filter… at 200k miles it’s time. and a blow to the gas tank probably stirred up a bunch of crud into suspension that was just sitting at the bottom of the tank previously.

  7. If those MPG numbers are in Imperial units it’s really bad.

    21 MPG Imp = 17.5 MPG US.
    15 Imp = 12.5 US

    My 3 short ton Chevrolet Avalanche with a 5.3 litre engine gets the same fuel economy, top number highway and bottom number city, on E10 petrol. Knock off 30% for E85.

  8. Could be a problem with the airflow meter. Some of those old Volvos use a mechanical one, called a VAF or Vane Airflow Meter. They’re literally just a lightly spring loaded flap connected to a rheostat or potentiometer. If something blocks it from its full range of travel or the ‘stat is worn, the fuel flow won’t match what’s needed for the engine RPM.

    1. 240 turbos used Bosch K-Jetronic mechanical fuel injection, with the vane air meter. The vane is connected to a mechanical valve which meters fuel to the injection system. I’m not aware of K-Jet ever being factory-fitted to a 740; those used the electronic L-Jetronic system, with a hot-wire air mass meter. The AMM can degrade or die, sending the car into “limp home mode”. Fuel economy would definitely suffer, but the car would also be nearly undriveable.

      1. My LH2.4 240 had a bad AMM (it got fried by the warm air flap in the air filter box getting stuck open.) All it resulted in for me was poor cold running (a lot of hesitation,) and really awful gas mileage. The power levels greatly improved when I replaced it, though…

  9. Lewin what are you doing!

    Getting only 14L/100 is bad to start with !

    Unless your tramping it around town you you should be down around 9-10L/100 .

    I’m presuming you’ve got the 2.3l turbo.

  10. I killed the Air mass meter in my Nissan 280ZX when it ingested water (long story- really…) The motor was Ok, but the bloody thing never ran reliably one corrosion set in within some circuitry. All up I had to replace the thing. It used to get better than 28 MPG (I’m old School ) on highway cycle. Bottom line, and with my respects to your mechanic, get a good fuel injection specialist to run over your system . I have done this more than just a few times with both the Nissan and a couple of BMWs and I got results.

  11. I don’t know anything about Volvo’s but common rail fuel injection doesn’t vary much.

    This could be just coincidental failure and not related to the mishap.

    There will be a removable panel to get access to the fuel pump from inside the car floor. That will make some inspection of the fuel lines easy. I use a digital macro camera with flash to view things in places I can’t get my head.

    The next step is to check the fuel pressure in the common rail. If it’s within spec then you most likely have a coincidental failure not related to the mishap.

    Lastly, lets hope it’s not the in-tank pump. If you sell the car and find some extra money you will probably be able to afford a replacement in-tank pump.

  12. Next time you get your tyres changed and they pull out the old valves from them ask to keep them and perhaps grab a few more too for good measure.
    Once you remove the rubber from them, they are a great little end to terminate hoses with a valve port, be it for air, liquid, or xyz.
    To remove the valve from the body dont buy the tool, grab a busted screwdriver and an angle grinder to make one.

    Coupled with Leff beer bottle corks they make for extremely good inserts for 1.5ltr plastic bottle rockets.

  13. This is about 0.02 gallons of gasoline per mile. If there was a leak you would be gagging from the smell.

    It’s also about 4.4 cubic inches.

    According to Wikipedia, gasoline energy is roughly 45 HP – hours/gallon, so you are looking at a loss of .85 horsepower-hour per mile. At 60 miles per hour, or one mile per minute that is over 50 hp. If that was a low tire or bad brake caliper something would be on fire after a short shot on the highway.

    Perhaps someone is siphoning gas from the car or the oil pan is filled with gasoline.

    1. Yea but once you put the petrol through the engine and burn it and it turns the wheels through the gearbox you’re probably getting something like 20% efficiency. so 10HP or 7.5kW. A dragging brake would be hottest when you’re going fastest which also gives the most air flow through the brake. Sure it would be hot but not on fire, probably not even glowing.

      Perhaps its worth checking the brake pads for uneven wear in any case?

  14. On the topic of MPG… For a 2nd gen CRV, will pretty much any cheap ebay bluetooth obd2 module provide good realtime MPG stats with android?

    I previously did this on an old vehicle using mpguino to measure fuel injector pulse duration directly. That was very accurate and hackable.

  15. Wouldn’t it be much easier just to check if the fuel pressure was in spec. A leak that large should affect fuel pressure. I agree with the statements about looking at the transmission control. Also, check the OBD for any codes. Make sure you have no underinflated tires. I find it hard to believe that the engine could become that much less efficient without throwing a code.

  16. Aren’t modern cars wonderful? (Not) Just about all of the above suggestions could be a factor in why the mileage had dropped. I didn’t count them all but that sure is a lot of possibilities and I am sure we left out a few too. Give me a car/motorcycle with points and a carburetor and I can fix it in a few minutes after a quick diagnostics session requiring just a few simple tools. Ahh…for the old days.

  17. Geez, call in a “pro” and save yourself the embarassment of having played “shade tree” mechanic. For the arfduino-everything crowd, its surprising that VOL-FCR software was never mentioned. As an “ASE certified master tech”, the first thing I would’ve done is RTFC (Read-The-F’in-Codes!), typically (but not always), it will point you to the relevant sub-system that needs corrective work.

    Not that familiar with Volvo (having worked on GM all my career), but if you’re MPG suddenly took a drastic hit. There are potentially numerous sub-systems that could cause it. Without accurate diag codes, these are WAG’s.
    1. Fuel injectors (check on time of each injector)
    2. Fuel injectors (dumping too much fuel into cylinders)
    3. Fuel pressure regulator – see 2. above (check with gauge on schrader valve)
    4. Engine issue: do a cylinder compression check
    5. Lean condition being reported to ECM, causing injectors to dump more fuel.

    Your basic ASSumption is a serious WAG (a fuel line leak?? come on.. sheesh! –
    and using a tire pump…smh….IF you had a physical leak, you would see liquid in
    obvious areas… eyes = great diag tool). In absence of physical evidence of a leak,
    there is obviously something else going on inside the closed system.

    Career advice: don’t give up your day job to become an automotive tech.

    1. LOL.

      In the Authors country ODB and indeed ODB-II compliance was not required or enforced until 1997. This car is a 1980’s vintage car. Put simply there are NO codes to read!

      1. Of course there are. Vintage Volvos (740s for sure!!) have a self contained diagnosis box in the engine compartment with a LED, a pin, a momentary switch and several holes to put the pin in. The holes have numbers on it which correspond to different units in the car. “1” is the ECU for the injection system, “2” is for the ignition module and so on. Put the pin in the desired hole, push the switch and the LED blinks the fault codes.


      2. You missed “in the authors country”.

        Ok you stick a wire in, press a switch and the LED flashes X times. What then? The manufacturer didn’t release the codes or what faults they related to IN THE AUTHORS COUNTRY UNTIL 1997.

        Manufacturers deliberately made different boxes and protocols for the AUTHORS COUNTY to lock owners into dealership servicing.

        I just checked on this and it was actually much later for most cars (2005/2006) –

        And for Specifiically Volvo’s –
        Volvo C3 2007 +
        Volvo C7 2002 +
        Volvo S4 2005 +
        Volvo S6 2002 +
        Volvo S8 2002 +
        Volvo V4 2003 +
        Volvo V5 2005 +
        Volvo V6 2008 +
        Volvo V7 2002 +
        Volvo XC6 2008 +
        Volvo XC7 2002 +
        Volvo XC9 2003 +

    2. I wanted to see if a physical impact caused a fuel leak. I devised a way to do it with tools at home. Not sure what’s embarassing about that.

      Not giving up my day job to become an automotive tech. I am however investigating what’s wrong with my project car because nobody else is going to do it.

      You’re welcome to try and “read the f’ing codes” on a 1988 740 Turbo with LH2.2, no diagnostic LED proivision and no OBD II, but for those of us without access to the obscure bespoke Volvo dealership diagnosis unit, it’s a little harder. For the sake of it, I had that done anyway with no conclusive results!

  18. My fuel efficiency dropped from 21mpg to around 15mpg as well. The root cause was not a leak in the tank, but external – some freeway road work had been completed, now allowing me to travel that section of road at 260km/h (160mph) instead of 80km/h (50mph). Unfortunately, I have not yet found an easy fix for that…

  19. Thanks for the tip. As others have noted the article kind of leaves us hanging. Maybe a final sentence just to wrap things up “I never did find the cause of my fuel economy woes, but at least I learned a neat way to check for fuel system leaks!”

  20. If the weather is getting colder outside:
    – Maybe a piece of card board or something with a higher temp rating to cover the grill or radiator.
    – Maybe instead of cold air intake… pre-heat air intake.
    – Fuel Air ratios and related sensors testing
    – Air, oil and fuel filter. Inspect so you know and upgrading isn’t a bad thing either. Easier to access and better quality can be hacked in without an issue.
    – With age… time to upgrade to synthetic blend or synthetic
    – I’ve heard in higher elevations people add graphite… though haven’t done that one.
    – Bearings… always do for the best and keep up on maintenance.
    – Brakes have already been noted… though tuning them especially with drum brakes.
    – Oil, oil and always oil… U-joints also if you can grease (I know sealed bearings some days seem like a curse). Oil and grease like you’re addicted.
    – I guess the fan clutch maybe and if you can hack in an electric fan… more power to ya… pun intended.
    – I guess if you can hack in an electric power steering pump… note as above. I think even an electric oil pump can be hacked in… though I’ve not done or even seen that one before.
    – Lower drag by taping seams (I use gorilla glue clear tape) or I’ve seen people use expanded foam then shape to contour to seal areas. I’ve used hot glue also. Covering the bottom with a skirt will lower drag also.
    – The funniest I’ve seen and almost am about to have the capabilities… though I am not way going to remove the alternator… though am wondering about how can I improvise with an electromagnetic clutch like on the A/C units the alternator pulley since I want to be able to turn on when needed and by default be off… remove the alternator and add a deep cycle battery.

  21. Hey there, one thing to check is the fuel pressure regulator. I’ve worked on a few of these, and the vacuum diaphragm has a leak between the fuel side and the vacuum/pressure side.

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