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Matt
Matt, Engineer
Category: Pontiac
Satisfied Customers: 22005
Experience:  Honors degree in Mechanical Engineering, worked 8 years as a Formula 1 engine engineer.
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2004 Aztek, 150k, son-in laws car. Acceleration past half

Customer Question

2004 Aztek, 150k, son-in laws car. Acceleration past half throttle, sometimes less, around 30-40 mph results in "stumbling". Pulled fuel filter, half clogged, replaced. No change. Checked fuel pressure, don't remember the number but around 50. Felt like possible cat restriction. So, cut out cat and installed temp flex pipe to test drive. Ran good (a little loud) for the 3-4 miles test drive. Installed (welded&clamped) a universal cat on Saturday. Ran really good! Drove the car a little on Sunday (5 miles) ran fine. Monday, longer drive, first 30 miles running as should then 1 "hiccup", "stutter" for a couple seconds. Ran fine for 10 more miles. Drove again with scanner connected didn't see any abnormalities (not sure which data exactly to be watching).
Car idles fine, no misfire history presently, no recorded misfires while "stumbling", no engine codes.
Other problems: RR wheel speed sensor throwing hard code and AWD disable lamp on, LF Bearing has a little play (no ABS code). Don't want to start replacing hub assemblies until I get the engine running right. Really thought the cat fixed it.
No engine codes.Thanks
Submitted: 3 months ago.
Category: Pontiac
Customer: replied 3 months ago.
Couldn't find any solid answers or really any solved problems on the web to this symptom. I've had similar problems and solved with filters or cats or coils even.
Expert:  Matt replied 3 months ago.

Hello
This could be an airleak after the airflow meter, any air dragged in here isn't 'seen' by the ECU and so not compensated for and can lean the engine out and can also allow the engine to rev up when not desired causing rough running.
As its a mechanical fault it tends not to turn on the fault light and you can sometimes hear a 'hissing' noise with the engine running.

Check the hose clips for tightness and inspect the trunking for any cracks or splits and also all the vacuum system, the small bore pipes and fittings for cracks and missing parts.

The best way to locate a leak is to have the engine running and warm and then spray lighter gas /propane or brake cleaner around each joint in turn. If the engine rev's up you've found your leak.
Now you might think that spraying lighter gas around a hot engine isn’t wise, however the flash /ignition point of gas is about 400°C so you need a naked flame or spark to set it off and I’ve used this method for many years without incident.
Work your way through each possible joint one at a time and you should find it. I use a slightly flattened piece of brake pipe and some rubber hose from the can of lighter gas to provide a spraying 'wand' and allow a direct blast of gas into each area, especially those difficult to reach with large implements.

Air leaks are very temperature dependent as gaps can open up as things expand with temperature so depending on how the leak occurs ( on a hose joint with a clamp expansion will help seal and on a vacuum hose expansion ill make a leak worse) so leaks can be better or worse as the engine heats up.

It’s also worth getting the fuel pressure checked as if this is low due to a blocked filter or faulty regulator or even a poorly pump will all result in insufficient fuel being delivered to the engine

Might also be worth cleaning out the throttle body as these get clogged up with carbon and some brake cleaner washed through helps free things off.

Might also be worth checking the wiring and connector to the airflow meter for any signs of corrosion or damage. you can do a quick fault find if you unplug the meter and run the engine without it.
if the engine condition is the same then chances are the meter or the connection to it is faulty

It’s also worth cleaning the MAF sensor wires, as they can get coated with dirt over time which then offsets the reading
Use some contact cleaner or brake cleaner to spray onto the wires to remove the dirt – on no account touch the wires with anything as they are very fragile

Customer: replied 3 months ago.
Well... I did do a visual inspection of ducting and just cleaned the MAF. The fuel filter was changed as a first step as it was partially clogged (usually the first place I look, I've been working on diesels since the 80's). I have used the "spray method" for finding leaks but not yet on this as there is no rough idle or hissing sounds.
New / more info- While driving after cleaning the MAF I see the problem occurs right about 2500 RPM and above.
Just borrowed a fuel pressure test kit (mine's missing) and will use to see what pressure is when symptom happens, just to eliminate a fuel supply problem. Almost feels like it's hitting a rev limiter, which it does do around 4000 or so in neutral or park..
Expert:  Matt replied 3 months ago.

Hello

OK

might also be worthwhile checking the state of the air filter? if its badly clogged then this would choke the engine

also if you remove the 1st O2 sensor you can peer inside the exhaust to view the cat brick, you can then check the state of the brick for any deformation which would indicate the brick is breaking up and choking the exhaust

Customer: replied 3 months ago.
As I stated in my first post, "...cut out cat and installed temp flex pipe to test drive. Ran good (a little loud) for the 3-4 miles test drive. Installed (welded&clamped) a universal cat on Saturday. Ran really good!..." So... I feel the cat was probably not the cause since the symptom returned after about 40 miles. The cat had not broken apart so I also don't feel that the muffler has any debris clogging the baffles. The muffler does have a very small hole/ leak in it and should be changed eventually but didn't/ don't want to spend $500 on a complete new exhaust system with other problems ( RR speed sensor, hub assy's, AWD disable..) until I figured out the engine running problem. That's why I opted to put a universal cat on for $100 for "test".
New info: Fuel pressure 58-60 psi KOEO, 50 psi KOER in park, 58-60psi KOER at speed when symptom "stuttering" occurs. Also, symptom starts to happens between 2000-2500 RPM when TPS% is about 40% or higher. TPS% is smooth through the range up to 99%. At 1/4 throttle vehicle will accelerate to higher RPMs and speed without "stuttering" or "missing". See original post, "...no misfire history presently, no recorded misfires while "stumbling"..."
Thanks
Customer: replied 3 months ago.
I reread your last reply and first reply and noticed the air filter fuel pressure remarks. The air filter was dusty but not really clogged at all.. A couple of leaf pieces. I blew it off while it was out when I cleaned off the MAF. The fuel filter was replaced first as stated in my first post and pressure listed in my last post.
Temporarily set that job to the side while I work on other paying customers vehicles.
Thanks
Expert:  Matt replied 3 months ago.

OK

thanks for the extra information

in that case I'd say to remove the air intake pipe so you can see the throttle and then with the ignition on and engine off your assistant slowly presses down on the accelerator whilst you watch it, as it should open smoothly with no sudden jumps or steps

Customer: replied 2 months ago.
Matt,
I don't have an assistant to watch the throttle but, the throttle is cable actuated and feels smooth while moving either pedal or lever on engine.
More info-
I did some more data checking and both the TPS volts and TPS % were smooth from no throttle to WOT.
the symptom occurs at almost exactly TPS volts 2 and TPS% 40. up to 1.96v = steady acceleration, 2.02v and above to WOT = "stuttering".
Also I cut the exhaust pipe ahead of the muffler to eliminate any possible muffler restriction/plugged cause and no change.
So... new cat, fuel filter, good fuel pressures at all times, smooth idle, smooth acceleration to 40% throttle (approx 2000-2500 RPM under normal driving), engine will go higher in RPM's if allowed to do it under 40% TPS. if going slow and floor gas to WOT stuttering starts almost instantly.
Will pull of intake hose and check throttle plate to insure smooth movement and not loose.
Expert:  Matt replied 2 months ago.

Hello

OK well done for finding this out

I'd go ahead and replace the TPS with a new part as the sensor is used to provide acceleration enrichment and if its not changing smoothly this will result in poor fueling control

Customer: replied 2 months ago.
I have no problem changing parts when defective and as a last effort.
Maybe you mis read my post
The TPS voltage is smooth throughout the pedal range. No drop in TPS (V) .65v to 4.55v or TPS (%) 0-99. These readings are from the scanner presumably from Engine computer.
Correct me if I'm wrong about the scanner readings being what the computer "sees"
Expert:  Matt replied 2 months ago.

hello

OK you're correct I misread your last post so if the TPS is fine then I'd next check how much the long term fuel trim is moving as anything more than 10% and something is up

Customer: replied 2 months ago.
Matt, you've left me hanging.
Some of your suggestions, although sound advice, had been addressed in my earlier posts.
The latest reply about changing the TPS, "...if its not changing smoothly this will result in poor fueling control...", although true I observed and posted that the voltage and percentage were smooth throughout the range of pedal travel without any "hiccups". Only when the throttle is over 40% or over 2 volts does the engine act up.
My post "...symptom occurs at almost exactly TPS volts 2 and TPS% 40..." could have stated more clearly " the symptom starts to occur... and continues through WOT"
thanks.
Expert:  Matt replied 2 months ago.

Hello

OK is the TPS voltage steady at WOT?

if it is then thats probably OK

hence my suggestion of checking the long term fuel trim

is the coolant temp and air temp signals on the scan tool also making sense?

Customer: replied 2 months ago.
Matt, looks like our last replies crossed in the mail. So we're in agreement the TPS is functioning as it should.
The next suggestion was the long term fuel trim:
What readings am I looking for under what conditions? When the symptom starts to happen? The long term fuel trim numbers should be moving which way under acceleration?
Am I not concerned with the short term fuel?
I can set the scanner for just the fuel trim data and watch what's happening.
Trying not to sound "snippy" in the last post but this car's "killing " me.
Thanks
Customer: replied 2 months ago.
the son-in law wants to know how much it's gonna cost to fix it.
Hard to answer when I don't know what the problem is yet.
Customer: replied 2 months ago.
Coolant temp and air temp were making sense. But will check again. Going to collect more data in a couple minutes. Not looking forward to test drive with muffler pipe open but only have to drive a few thousand feet to observe symptom.
TPS voltage was steady at all points. I'll check WOT again also.
Expert:  Matt replied 2 months ago.

hello

don't worry about the short term trim they move all over the place

the long term trims should be no more than 10% in either direction

if you have 20+ Long term trims then this shows something is up and as you probably know if its positive then its adding fuel ( too lean) , negative trim is removing fuel ( too rich)

Customer: replied 2 months ago.
Air and coolant temps are correct for cold engine and hot Midwest Summer.
Air up to 100 now was 93 when started.
Coolant up to 180 was 100 when started.
Long term now at idle -13. 2500rpm in park 0.
Will drive and try to see what's happening at problem area.
Customer: replied 2 months ago.
Well, first shot down the road car ran perfect for 1-2 miles. WTF! After that, temps are accurate. Long term fuel under accel is about 22, then stuttering it drops to 0.
Stopped now to send this. Will do some more data looking on the drive back to shop.
Thanks
Customer: replied 2 months ago.
Well, long term rising under acceleration until it's at 22 then more throttle and it drops to zero. If I let off throttle it comes back up. Also at higher speeds 45+ doesn't seem to act up. Gonna record symptom area to get a better picture.
Customer: replied 2 months ago.
Didn't get to record symptom area data as I wanted to.
Welded the muffler pipe back as I eliminated a possible clogged muffler.
What can cause long term fuel trim to drop to 0 from 22 under throttle.
Expert:  Matt replied 2 months ago.

Hello

if its a positive 22% then this can down to:

low fuel pressure

faulty O2 sensor

leak or crack in exhaust before O2 sensor

faulty MAF sensor

the fact it seems to get better at high load rules out fuel pressure I'd say and you've already measured this

and as you've already been round looking for vacuum leaks and cleaning the MAF

then this leaves a faulty O2 sensor or a crack / leaking exhaust

Expert:  Matt replied 2 months ago.

Hello

if its a positive 22% then this can down to:

low fuel pressure

faulty O2 sensor

leak or crack in exhaust before O2 sensor

faulty MAF sensor

the fact it seems to get better at high load rules out fuel pressure I'd say and you've already measured this

and as you've already been round looking for vacuum leaks and cleaning the MAF

then this leaves a faulty O2 sensor or a crack / leaking exhaust

Customer: replied 2 months ago.
Well, after many many hours of troubleshooting and experimenting with fuel filter and cat another mechanic approached it from a different direction.
We unplugged the abs module,as there was a malfunctioning rr wheelspeed sensor, and the stumbling symptom while under half throttle disappeared. Astonished by this I pulled over, plugged it back in and problem was back. I did this a few times and every time the abs module was plugged in the symptom was there.
We knew then that the problem was in the traction control/ ABS. So I put a new RR hub assembly in and power problem was gone! Since then I've had to bypass and replace a few electrical junction plugs and a wheelspeed sensor harness as we also discovered corrosion and rotted terminals inside the plugs causing high resistance and abs / awd disable lamps to come on.
So... the problem was not engine related at all but abs!!
Still have more electrical gremlins to hunt down but overcoming them slowly.
Expert:  Matt replied 2 months ago.

glad you got it sorted and thanks for letting me know