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Mazda Ed
Mazda Ed, Auto Service Technician
Category: Car
Satisfied Customers: 2223
Experience:  ASE Master Technician, Mazda Master Technician, GM and Ford factory trained. 15 years experience!
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I have a MazdaSpeed3 (actually a Mazda 3 MPS, the European

Customer Question

I have a MazdaSpeed3 (actually a Mazda 3 MPS, the European version), 2007, 60k miles. Since a month or so ago, the average gas consumption went up by ~30% all of a sudden, without any apparent cause. There is not light in the dashboard, no error code in the computer, the dealership said they found nothing wrong with it. But the steep increase in consumption is clear and there, and noticeable mostly at cruising speed. Please note most of the miles are done commuting over the same 50 mile constant driving route. I have done it hundreds of times and I can tell the difference in consumption is not coming from the changed characteristics of the road, driving style, etc. The initial increase happened at the same time with a significant decrease in air temperature (winter), but now temperature is back up and the consumption is still there. Cold air should help anyway...
Submitted: 4 years ago.
Category: Car
Expert:  Kalamykid replied 4 years ago.
Colder air requires more fuel to keep the air to fuel ratio correct. The colder the air the smaller the air molecules are, ie the colder the air is you get more air into the engine the more fuel has to be injected into the cylinders. As for the increase in consumption, Have you had the spark plugs replaced? Is there any oil or moisture on the ignition wires / boots? Have you had a good tune up done? Any of the things listed can steal the ignition spark and cause a loss in fuel mileage.
Customer: replied 4 years ago.
Hi Jamie,

Thanks for your unswer, but I am not yet satisfied with the cold air explanation for the following reasons:
1. This was actually the 3rd winter the car has been through, but only the first time I notice this behavior.
2. The engine can measure and control how much oxygen it is intaking. If it needs less volume of air for the same quantity of oxygen, it will let less volume in. It is only in full acceleration that in the same maximum volume flow limited by the intake (which is still stock), it might get more oxygen in colder air, thus using more fuel for significantly more power. This is not what I noticed, if anything power has gone down with the increase in consumption. Other way of looking at it is that people are paying money for "cold air intakes" which have exactly the effect of sucking air from where it's colder (not right in the engine bay), and allowing more to go in, for increased power. Cold air should have had a similar effect, but what I notice is the opposite.
3. As stated in my question, the tempreature of the air is now back up but the consumption is still there (possibly even getting worse).
4. As I stated, the most anoying and easily noticeable part of the consumption increase is at cruising speeds, where the engine is not running full load and full throttle, but optimizing (including the air flow) for maximum efficiency. All of a sudden its not doing that anymore.

Regarding the spark plugs, I did not have them checked yet but I had the car in the dealership for a full test and they found nothing wrong with it on a tester. Would problems with the spark plugs have not other symptoms than increased consumption? No error codes? No shaky engine?

Expert:  Kalamykid replied 4 years ago.

1. As the spark plugs wear over time they loose there integrity and the ability to burn all the fuel in the cylinders. This is why the problem is not showing up till now.

2. You are correct in your thinking almost. The engine can not control how much oxygen it takes in. Please do not think I am saying you do not know what you are saying. Basic physics tells us the colder the object the denser it is. The colder the air the smaller the oxygen is, thus the more oxygen you get for the same amount of volume of hot air "same for the fuel". If you drive the car the exact same way hot or cold out side, you will use more fuel in the colder days then the hotter days.

3. On colder days the weaker spark will be able to burn more of the air and fuel due to the added amount of air and fuel the engine has taken in. On hotter days there is less air and fuel taken in so the spark has to burn stronger to be able to create a better combustion in the cylinders.

4. If the spark plugs are burning the fuel in all cylinders the same then no the engine will not miss, shake, or set a trouble code in the pcm. The pcm only detects if a cylinder is off balance from the rest of the cylinders.


I hope you understand I am not a physicists, I am a auto technician. I hope I have made it easy for you to understand and I am sorry if I am not succeeding. Have the spark plugs replaced and see what happens with your next two tanks of fuel. Make sure to use only factor recommended spark plugs. Most tune up places do not use factory replacement plugs," they use the cheapest thing that will make your engine run". At least that is what most do here in the states. Second of all, using the cruse control will use more fuel then if you where to control your speed. Cruise control does not hold the speed at a set speed, It will accelerate to get you up to speed then will back off till you drop to a certain point then accelerate again to bring you back up to speed. If you control the speed your foot will hold the throttle at a more constant speed and use less fuel.

Customer: replied 4 years ago.
Hi Jamie,

I am still not satisfied with the cold air explanation (and while I would be able to go into a more detail scietific disertation as to why, since I happen to have a Bachelor precisely in Phsyics and Engineering from a quite reputable US University, I don't think it is in the scope of this dialogue :)).
However, I believe that the there is a non-negligible chance that the spark plugs are the right thing to chase. If that proves to be the case, you are more than entitled to the payment (and I should charge it to the dealership incapabable of figuring this out themselves :)).
I have not used justanswer before... Is it ok if I wait with the acceptance until I actually try replacing the sparkplugs (scheduled for Friday 12.03) and then accept it based on the result? I think this is fair, since I am risking a significant gratuituous expense (parts + work) in case your answer proves wrong...


Expert:  Kalamykid replied 4 years ago.
Yes that will be fine.
Expert:  Kalamykid replied 4 years ago.
Not to make you mad but, If what i was telling you was wrong about the air fuel mixture "cold and hot" why do race car drivers try to keep the engine, fuel and air cold? The reason is that colder air and fuel make more horse power do to the fact they can get more air and fuel into the engine when it is colder.
Customer: replied 4 years ago.
You are not making me mad at all, and I don't even disagree with your last statement (about cold air). I even agree with it, and I have even stated it in one of my previous answers: cold air is better, for power and efficiency. And yes, the same volume of cold air will contain more oxygen (not because the molecules are "smaller", but because they move at a slower speed in the gas, they bounce against each other less often and less hard, and on average they allow themselves to be closer together in the air). And yes, more oxygen can and will burn more fuel, generating more power. This is also better for efficiency, because we both know maximum power is not achieved at maximum efficiency of the engine. Thus, if due to colder air, the same power is achieved - say - at lower rpm, it is also closer to the efficiency otpimum of the engine. For example, if in warm air my engine makes a peak 250 HP at 5500 rpm, in cold air it might make 255 hp at 5500 (using a tiny bit more fuel than before at 5500). But more importantly, it makes the 250 HP from before at only 5400 rpm, closer to the efficient range of the engine, thus using a tiny bit less fuel than at 5500 before (or at least not more fuel). The numbers are just examples they do not necessarily make sense, I did not do the calculation...
What I am arguing is that colder air does mean more consumption for more power, but at worst an increase in consumption proportional with the increase in power (if not less). Based on that reasoning alone, a 30% increase in consumption would give at least a 30% increase in power... Well I haven't noticed any increase in power at all, on the contrary, the power decreased, if anything.
I guess ultimately you are suggesting a concurrent effect of the two factors (cold and spark-plugs). This might be. As a matter a fact I very much hope this is the problem and will be fixed by replacing the spark-plugs. Given my current frustration with the car, I am looking forward to making the payment for this answer some of the best dollars I ever spent... :)

Expert:  Kalamykid replied 4 years ago.
Thank you. Let me know what happens after the plugs are changed.
Customer: replied 4 years ago.

I changed the spark plugs last Friday (they were actually part of the 90,000 km maintenance anyway, so no waste of money there, at least) and drove some 500 miles since... Unfortunately the consumption is still high, and the performance of the engine still relatively low. Practically I don't feel any change. I have a new appointment this Friday to have the engine checked in detail. Also the air temperature is back up in the 60's F, so cold air is also not the cause...
Any other suggestions? I will keep you posted with the results of the further ivestigations, but so far unfortunately I cannot accept the answer as being the right one...

Expert:  Kalamykid replied 4 years ago.
At this point the only other thing I can think of would be the clutch in the torque converter is not fully engaging. When you take it in have them check the converter slip rate when the car is in over drive.
Customer: replied 4 years ago.
Thanks for the suggestion, I will tell them. Please note though, that in the mean time since the high consumption started, the hydraulic cylinder from the clutch failed (it is a manual), and it was replaced under warranty... Nothing changed with the consumption. I don't know how this affects the chances of the clutch slipping, but I am guessing it decreases them... Also constant slippage should make it really hot, right? Which I did not notice...
Expert:  Kalamykid replied 4 years ago.
I am sorry I thought you had an automatic trans. It has been a few days and i forgot you said it was a manual trans. Please disregard the last post. Please let me know what they find and what the cure was. Thank you for letting me try to help you with your problem.
Expert:  Mazda Ed replied 4 years ago.



It appears that Jamie is finished working with you based on his last post.


I don't get the pleasure of working on too many of the speed vehicles and let me say that I envy you being a speed owner. Now, that out of the way... down to the nitty gritty :)


One of the big problem areas with the 3 is the purge valve. I lost count how many of these thing I had to change. Many times they will not set a code and you may not notice a running condition especially in the cold. When they fail they suck fuel vapors into the engine all the time. I have also seen them suck raw fuel in! The engine is pretty good at burning the fuel/vapors that are sucked in but in never seems to give increased power. The symptom more experienced is a lag on initial acceleration. I figure this happens because the engine is temporarily flooded. Based on your details you would be less likely to notice this condition since you do a highway drive everyday.


You can do a superficial test of the purge valve by removing it from the 3 and trying to blow through it. If you can blow through it then it is stuck open and needs to be replaced. The problem with this test is that they don't usually stick open and stay open so when you test it, it may be working properly. If you haven't ever replaced the purge valve/solenoid then it is highly likely it has failed and is creating an overly rich condition.

Customer: replied 4 years ago.
Dear Mazda Ed,

Thanks for offering your suggestion to my question. Within my limited technical understanding, it does sound like a possible cause and I am (again) hopeful this is it, and it can then be solved.
I now realize some additional piece of information might be relevant:
I have a ScanGauge on the car, and the consumption figures are taken from it, adjusted already for real consumption. I noticed since the beginning (when the car was running "fine") that the Board Computer "lies" by about 15% when reporting (average) consumption (the correction factor I put in the ScanGauge after measunring actual liters and kms). Since the increased consumption period, I had to increase this correction factor to 20-25% (depending on gas, driving characterstics, etc.). So basically the car "thinks" it is consuming only about 10% more gas, but it is "missing/leaking" 50% more than before, for an overall increase in consumption of 25-30%... Is it normal to "lie" constantly and by that much, in the first place? I haven't noticed it in other cars, but they were not turbos. Could this "liyng" plus the increase in it be related to issue?
Anyway, I will make the purge valve suggestion to the service this Friday. The car is under warranty still, so I will let do the testing and thank me for doing their job of investigating, on my own expense...



PS: While I am at it, having access to the ScanGauge and the paratemeters it can read (air tempareture, manifold air pressure, ignition timing, engine load, etc.), is there anything in particular I could/should be looking for?
Expert:  Mazda Ed replied 4 years ago.

I read through the scangauge manual online about the correction factor. It seems that this should only need to be set one time in the beginning. I would start by resetting the scangauge and performing the correction procedure for the tank fillup.


Also, I would trust the onboard computer more because it uses information directly from the PCM to gather vehicle speed and amount of fuel used to calculate the average consumption. If I were you I would be comparing the onboard computer values for the time periods in question. The scangauge is subject to too much user error and accuracy problems.


Another thing when calculating the average consumption. I am not sure how the computer calculates for fuel used when not moving. This is after all consumption of fuel! It could be that your hand based calculations (distance traveled/fuel used) is showing a discrepancy with the onboard computer. This may be because the onboard computer only calculates while the vehicle is being driven. Again I don't know for sure the calculation methods but this is something to think about.

Customer: replied 4 years ago.
Hi Ed,

Thanks for the feedback!

I have studied the ScanGauge user manual, principles, user experience etc. quite a bit before buying and installing one. SG takes the values given by the PCM, same as the BC. The correction factor is simply a percentage being added to the consumption reported by the PCM, to reflect actual consumption. I know the SG manual says it should only be calibrated once, but that is for ease of use (otherwise users would complain about the tool being a pain to use). Now I am doing the equivalent of the calibration procedure virtually with every fillup (i.e.: drive until empty, then fillup), albeit not always at the same pump (but I worked in the gas marketing business and I know the standards for pumps, and the pump-to-pump differences are really negligible). What SG says when you do a fillup is: "I think you used x liters of fuel since the last fillup (previous correction applied)". What I noticed is that whereas before the y liters that I actually did fill up were sometimes more sometimes less than what the ScanGauge thought, after the cosumption increase, for several fillups they were constantly and significantly more, so I had to adjust the correction factor from ~15% to ~20+%.
Regarding the BC, I cannot speak for Mazda, but from experience it does take into account gas consumed when idling (average consumption goes up), so that is not a justification for the difference vs real consumption...
Ultimately, as stated in my first post, I noticed the worst consumption increase while crusing at constant spped, so idling is not really a factor there...
Anyway, on Friday I will hopefully have a feedback whether the purge valve is responsible for the problem or not.

Thank you,

Expert:  Mazda Ed replied 4 years ago.
Just wondering if they found any problem with the purge valve??
Customer: replied 4 years ago.
Hi there Ed,

It's been a while, I have been away on holidays. The answer is no, they have not find anything wrong with the valve, or so they say... In the same session I also did a consumption test with one of the dealer representatives, where the extra-urban consumption ended up being 7.9 L/100 km instead of the rated 7.5 L/100 km. This does not sound too bad, but I only later found that the standard extra-urban consumption test is based on a cycle which is by no means the constant 90 km/h which I tested with the deler representative. I still have to organize myself to do the proper consumption test... Besides that, I am still stuck in the same condition as before...
Any other ideas?


Customer: replied 4 years ago.
In an unfortunate recent fender bender the purge valve was physically damaged and then replaced (together with the admission box, but nothing else in the engine bay). Unfortunately the dealership proved right: it was not the purge valve, because the new one changed nothig...

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