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Engine hesitation at low RPM

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jgbrown

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Hi --

I have a 1987 Cavalier Z24 with the 2.8 MultiPort FI V6 engine and automatic transmission. This has been my daily driver until recently when I became leery of it's reliability. The car runs smooth at idle and at speed. But when first starting out, the engine will hesitate / stumble as if not getting enough fuel.

I can start the car up anytime and it will idle and rev appropriately in park. As soon as I put it in gear (reverse or any forward gear) and apply throttle, the engine will stumble as if it wants to quit. All the sudden, it will 'fix' itself and take off with a jolt. If I'm driving down the road and have to slow down, I will get off the gas and slow accordingly (coasting or braking). As soon as I apply the throttle again, the engine will stumble and then take off again.

The Service Engine light does not display. We have not been able to get any codes from the system. We have replaced plugs and wires, and checked fuel pump and replaced fuel filter. Everything seems to check out. The car passes smog.

My next step is to take the car to the GM dealership to see if they can figure out what's happening. Before doing so, I thought I would post a message to see if anyone here might have some thoughts or ideas. Anyone?

Thanks -- Jeff
 
Thanks for the quick response Zora. Not sure what all tests were performed a the garage, but the OS sensor was just replaced during my last smog test. I'll look over the other information and troubleshooting steps you offered up this weekend and post back my results.

Thanks -- Jeff
 
Ok, finally found time and weather to do the tests.

1. Thoroughly cleaned the throttle body and MAF sensor.
2. O2 sensor was replaced recently. (Issue existed before and after replacement.)
3. TPS tested okay. Was a little out of tolerance so adjusted to .425v.
4. I didn't find any other tests to run. Coil and electrical test indicated car wouldn't even start if had issue so didn't try.

I did do an additional test in disconnecting the MAF sensor. At first it appeared the car ran better...not perfect but maybe close to tolerable. I had an extra MAF sensor so replaced it. Same results as before. I didn't find a way to test the MAF sensor to confirm they were functioning correctly so I'm just assuming one of the two I have is okay.

A couple other symptoms to report:

1. On startup and during idle, the car is definitely running rich. You can really smell fuel out of the exhaust.
2. The hesitation appears to exist any time there is an on-throttle action...at any speed.
3. Sometimes...not always, but more often than not...I can rev the engine in Park at idle and the motor will stumble when returned to lower revs. Get on and off the throttle quickly, the engine will drop below idle RPM and acts like it's going to actually stall before catching itself and returning to normal idle.

I was pretty confident the MAF sensor was the culprit with my initial tests because I have had a LOT of issues with MAF sensors since I got this car. If I can't test the ones I have, maybe it would be worthwhile to purchase a brand new one?

I think I'll double-check the vacuum hoses.

Any other thoughts or ideas with this additional information?

Thanks -- Jeff
 
Hi Zora --

Yes, I'm sure it's the MAF sensor. I have a LOT of experience with that thing failing. Why else would I have an 'extra' laying around. :)

I tested the MAP sensor and all values were within a couple tenths of those posted. The vacuum line also looked dry.

Another symptom I'm noticing now is that the car is not starting up right away. It requires additional cranking. Don't know if I would associate that to the underlying problem or to the plugging and unplugging of sensors I've been doing while testing. I tend to lean towards the latter.

Thanks -- Jeff
 
Hi --

Wanted to give an update. I purchased a new MAF sensor just to be certain to rule this out. The new MAF made no difference.

Of all the testing, all of the sensors seem to be functioning as specified. I'm now leaning towards an EGR issue. I tried to pull a vacuum off the EGR valve and it wouldn't hold anything. I tried unplugging the wiring harness and the vacuum line from the valve. Naturally, the car would not even start without the wires connected. The car initially felt like it was not hesitating on acceleration with the vacuum unplugged, but it started hesitating after the car warmed up a little.

I don't think this test really rules anything out since the valve won't hold a vacuum anyway. Is there a specific way to further diagnose the EGR valve? Can anyone think of any other areas I might consider?

Thanks -- Jeff
 
I would say if the valve wont hold vacuum it is not functioning and it would not be a waste of money replacing it.
 
Ok, finally got a new EGR valve and found time to install it. As expected, no real difference with regards to the hesitation. The engine sounds like it might be idling a little smoother but that's about it.

I tried to make a comparison between when the car is cold (first started) and when it was up to temp. I can't say with any amount of certainty that there was a difference. It sort of felt like things were better when cold but it still hesitated. I'm not holding much on this temp analysis.

I also verified that that hesitation only happens when the car is in gear. At idle in park or neutral, slowly applying throttle simple revs the engine smoothly. As soon as the car is in gear (forward or reverse), slowly applying throttle causes the engine to hesitate and sputter immediately. Eventually the engine will work through this condition and then accelerate 'normally'.

I can't think of a system that would fail depending on the transmission setting. Vacuum maybe? I don't think it's injectors because I would think the hesitation would exist regardless of gear selection. Coil packs...again...I can't imagine electrical to be affected by gear selection. CO2 sensor...I ruled that out because it was just replaced. And I figured that an issue with the CO2 sensor would show a difference between when the engine was cold and when it was up to speed.

Any other thoughts or ideas?

Thanks -- Jeff
 
how about the engine temperature sensor reading too low causing the rich condition
 

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