View Full Version : Hesistation, Bucking, Miss Firing

09-18-2007, 04:27 PM
Hello All,
I have had my Pontiac 6000 for a year now, and for the past six months of that year it has been hesitating, bucking, and miss firing whenever it is under approximately 2500 rpms. I have tried replacing many parts related to what could possibly be causing this (fuel filter, air filter, new coil pack assembly, new plugs and wires, seafoam engine treatment (before plugs of course), different IAC valve, MAF, and oxygen sensor). The car throws no check engine lights, but whenever slight throttle is applied (even in park), the car has a slight miss fire and feels sluggish. What is causing this, I have tried everything! Could it be a vacuum leak somewhere? Please advise!

09-18-2007, 05:55 PM
Vacuum leak. It sounds more intimidating than it is. Follow the lines coming off the plenum. I'm sure one or more are broken.

As for 2500 RPM, maybe the injectors? You would need to ohm test them to be sure

09-18-2007, 11:35 PM

09-19-2007, 03:34 AM
Look at the underside if the "boot" going from the throttle body to the air cleaner. Its possible there is a rip or tear in it, letting air in. This is especially probable if your motor mounts are weak and letting the motor "rock" back and forth under a load.

09-19-2007, 03:58 AM
Air leaks BEFORE the throttle body (thereby the MAF and IAC valve) do not matter in terms of engine driveability. It will however allow unfilitered air in. Inspect all vacuum lines for leakage. After the car has sat overnight, before starting it, change the position setting for the vents. like if you turned it off last night, switch it over to Vent. you should hear it hiss, since the controller works on vacuum. If no vacuum leak is present, it should still have residual vacuum in the system. If you don't hear a hiss, you have a leak.

Checking for a vacuum leak:Get a stehoscope and with the engine running, run it across all vacuum lines listening for a hiss. Be careful of the serpentine belt and radiator fan.

Injectors are a possibility. Ohm them out. I don't know the values, but someone here does.

Every once in awhile I'll disconnect the MAF sensor and start the engine. The Service Engine soon light should illuminate, meaning that the ECM isn't totally braindead.

Strong spark on all plugs? Blue spark is ideal

OH! 2nd on the EGR, but I doubt it. Disconncet it's vacuum line and plug it with something. Electrical tape works fine. Or channel locks.

Is the engine idling normally? Not too high or low?

09-19-2007, 08:40 PM
They usually rip AFTER the MAF, right around the hose clamp that holds the boot to the throttle body. But.....being that its an '88, the MAF probably is not used by the computer anyway.

09-20-2007, 02:25 PM
Thanks for the input guys. I have replaced the MAF, and I have disconnected it to see if it was the cause (it will use preset values if the MAF signal is not present, and if the MAF is bad it will run better without it connected), and found that the car still runs badly without the MAF connected. I have replaced all spark plugs, wires, and coil packs, and it still runs the same. The intake tubing is intact. I took it to get inspected yesterday and the mechanic said he did not know why it was acting the way that it does. It did pass emissions though, so I was happy. When the engine gets revving it accelerates basically like a healthy 2.8 at WOT (I have had two other versions of this motor before). As for idle speed, it usually idles a little higher than normal. According to my ear (no tach), I have heard it idle as high as 2000 before, and it was not that cold out (ruling out the fuel injection making the car idle higher due to cold weather). I dont know whats going on, I want to call this thing the mystery machine. Thanks.

09-20-2007, 02:26 PM
Oh, and I have looked and cannot see any vacuum leaks or ripped hoses, cracked hoses, hear any hissing, etc. Maybe I'll just drive this thing until it dies and see what happens, Im stumped.

09-20-2007, 03:40 PM
Did you check the voltages on the TPS? I have mine set at .5 volts w/no throttle right now, which is where it should be. If I had it any higher, the car would idle very, very high, among other weird things.

09-20-2007, 03:43 PM
The WOT thing leads me towards what Tongle said first. EGR. There is no EGR at WOT as far as I have learned.

09-20-2007, 03:53 PM
Hmm, this is all very interesting. The car doesnt only function well at WOT, but basically at anything after 2500 rpms. It gets up and goes when the pedal is pressed down, but is extremely lazy at crusing speeds of 35 mph. I tried the air conditioning vents after I turned the car off, and there was no hissing or vacuum sounds, so I believe we are low on vacuum. My car also takes a while to change from vents to floor while running. I don't know if that has anything to do with the way the engine is running (its vacuum related thats for sure). Well, theres my story for now. Would disconnecting the TPS give me any sign of whats going on, or would it just run worse?

09-21-2007, 01:42 AM
Slight vacuum leak, or major. Mine takes three or four seconds to switch from vent to heat, but I have strong vacuum. Losing vacuum overnight is not normal (I.e vents not working w/o engine) (I can turn it off tonight and hop in tomorrow morning w/o starting it and switch from OFF to vent and hear a hiss and the doors moving.

There should be a vacuum diagram on the firewall or strut towers. Check all lines. Get a rubber hose and hold one end to your ear, and with the engine running (Careful of moving parts) run the other end along the path of each vacuum hose. If they leak, you'll hear a hiss.

Do not forget to check the vacuum resvoir (large ball shaped object, behind or near front bumper)

09-21-2007, 12:12 PM
I am going to check the TPS voltage in addition to checking my vacuum leaks. The vacuum reservior was destroyed when my friend wrecked it (the previous owner). All that I found was a plastic/rubber line going to the front of the passenger fender. I soon found the ball underneath, and it was mangled. I rigged up an old aerosol can as the new vacuum reservior by pulling off the top (spray part), and jamming the vacuum line in there. It works good because I first tried it with a water bottle, but it sucked the water bottle flat. I am going to try the 2+2 method where you spray 2+2 on the vacuum lines to see if it fluctuates the idle. Thanks guys.

03-13-2008, 06:15 PM
a little trick i just learned for finding vacuum leaks. when you first start it up spray carb cleaner around the lines and intake, just a little at a time, if the engine speeds up you have found a leak.