Persistent stumbing during engine idle

[Dave111]

Have a 2002 Monte Carlo SS 3.8L with 83K miles. For the past few months, have noticed a slight wavering in the dash tach when sitting at a light. When listening under the hood, you can hear the engine subtly stumble once every few seconds. Otherwise engine seems fine, and has good power. No hesitation on acceleration.

Scan tool shows no codes. Scanner tach shows idle rpms varying between 750-900. Long term fuel trims sit between zero and neg 1%. Ignition advance sits around 20 degrees at idle.

Attached a vac gauge to the intake manifold. Shows 20 inches of vacuum, with an brief increase to 21 inches, once every few seconds. These vacuum increases are timed with the engine stumbles.

Changed the PCV and cleaned the MAF, made no difference. Changed out the original plugs and wires, no difference. Looked under the EGR valve, passages clear, and egr pintle moves okay. Cleaned the throttle body and IAC, which were pretty dirty, but still no change.

Not sure where to go from here. I was thinking that the engine stumbling, since it was timed with the brief increases in manifold vacuum, meant that the air intake was intermittently being restricted, as might be from a bad IAC, but everyone I ask says they doubt it.

Anyone here have other advice? Thanks.

 

[oldhp]

Well you've done about everything except change the oxygen sensor. 83K it's "probably" time to replace it. Fuel pressure ok?

 

[Nissan101]

Had a similar problem, did all that you did. Ended up being the coil packs were going bad. Once changed no more surging at idle

 

[Trav]

Well, you have been barking up the air/fuel tree now try the electrical one.
These engines are notorious for bad crank sensors, they have 2 hall-effect switches in the one housing, one going a bit flaky can cause what you describe. Its worth testing if for nothing else than eliminating it.
Ignition Control Module and coils are also known to cause issues when they are getting tired. Don't start throwing parts at it that can get expensive fast with this stuff, test and diagnose.
Do you have a misfire count for all 6?

 

[BigD1]

Test the MAP sensor.

My 92 Oldsmobile was doing the same thing for about a month randomly, and the MAP finally bit the dust. Finally set an OBD1 code when it completely failed. No codes when it was messing up randomly.

 

[North to Alaska]

Weak valve springs may be the cause. That would account for the small jump in vacuum.

 

[Johnny2Bad]

Just in case it hasn't been considered yet, you are sure this is not the Air Conditioning Compressor kicking in and out (the engine management computer will compensate by raising the idle RPM when it kicks in, and your vacuum gauge will reflect the increased load at idle). Note that A/C will run in winter to dry the defrost air.

 

[Dave111]

Wow lots of good advice here. I'll answer in order. I did some more investigating today.

As for O2 sensors, I can graph them on my scan tool. They both show what is reported to be normal waves pre and post cat. Can one of them be bad anyway?

Did a fuel pressure check. The gauge reads 50 psi with fuel rail pressurized and engine off. Service manual lists 53-59 psi. Interesting thing though is that the gauge often goes up to just 44 psi, then over the following minute, with key/pump on or off, slowly drifts up to 50 psi, as though fuel is seeping past an obstruction. If I bleed the pressure down to zero, and leave the gauge connected, the pressure will go up to 10-12 psi by itself. Fuel filter was just changed though not more than 10K miles ago. Fuel pump is original. Fuel lines are not corroded at all. Hadn't been thinking this could be a problem though since the engine runs better under load, and never hesitates. Also, fuel trim runs toward the neg, not positive, which is the opposite of what it should be if starved for fuel, correct?

So this is a problem, but is it THE problem?

As for the coil packs, the only test for these in the service manual was a basic spark test. Is there another way to check if one of them is starting to fail?

I wasn't able to find a test for the CKP. Does it need to be cleaned/replaced to see if it was the problem?

Neither of my scan tools will list individual cylinder misfires, even under the $06 readout. It just gives a Pass on misfires.

Checked the MAP sensor waveform. It reads atmospheric pressure correctly with engine off, and correct pressure when running, but every second or so, the reading blips very briefly from baseline, in a way that looks electrical, like electrical interference. The movement is randomly either pos or neg. Should it be flat and steady at idle?

Valve springs could be weak, though on a throttle snap, manifold vacuum responds briskly as it should in both directions, not what is described with weak springs.

AC has been off for all these tests.

Thanks for any input.

 

[Trav]

Check the fuel pressure regulator by pulling the vacuum hose of it while the engine is running, you should see a pressure rise with the hose off. IIRC there is also a fuel pressure sensor on this engine but I will need to look tomorrow for exact specs.

 

[North to Alaska]

I'm convinced the issue is coming from the combustion chamber area. How negative is short term and long term fuel trim running?

 

[Dave111]

Originally Posted By: Iowegian
I'm convinced the issue is coming from the combustion chamber area. How negative is short term and long term fuel trim running?


They both tend to be around zero, but occasionally neg 1-2% for both. Rarely, positive 1-2% briefly, after higher rpms.

Also, I just did a compression test on all cylinders. They're all in the 165-180 psi range.

 

[North to Alaska]

I'm not a mechanic however, so don't take anything I write as "great" advice lol.
I just try to do alot of online research for answers from diy articles.
My thought is that one of the chambers is running rich so the computer adjust to run all of them lean. Thus one or more chambers are running lean and causing misfire.
Or one chamber is still running rich and carbon buildup is causing high compression in that chamber and causing predetination Wich could also explain the vacuum jump. Although I just realized that the vacuum should drop a bit due to excessive gasses backing into the intake.

 

[ejes]

My money is on a simple vacuum leak somewhere else. Check with propane bottle and some rubber hose around possible leak points. After that and everything else you've done, maybe the TPS.

 

[Merkava_4]

Usually with driveability problems without a check engine light, you've got to start testing sensors with a lab scope. When they hiccup, the wave will change or they'll be a quick spike in the wave.

 

[North to Alaska]

Wow, the fuel trim and the compression are really good.
I have no ideas now ☺

 

[Dave111]

Originally Posted By: Merkava_4
Usually with driveability problems without a check engine light, you've got to start testing sensors with a lab scope. When they hiccup, the wave will change or they'll be a quick spike in the wave.


My scan tool will graph the sensors as a wave. Is that what you mean? The only one that acts erratic is the MAP, which has brief shock-like blips from baseline, every 1-2 seconds, sometimes pos, sometimes neg. But the blips aren't necessarily timed with the stumbles. They both occur about once every few seconds though. I guess I could replace it and see, since it's not that expensive.

 

[Lubener]

Originally Posted By: ejes
My money is on a simple vacuum leak somewhere else. Check with propane bottle and some rubber hose around possible leak points. After that and everything else you've done, maybe the TPS.


Fuel trims are too good to indicate a vacuum leak. I suspect crank sensor.