Intermittent Engine Stumble

[55trucker]

It's not very often that I find myself in a position of not knowing the cause of an engine irregularity but I have now. The engine is seeing a stumble/miss at near all rpm (but it's most noticeable at speed when the TCC is locked up and the converter can't absorb any irregularity from the engine so you feel the problem.
My car sees very little use and last July a problem reared its ugly head that saw an intermittent stumble/miss begin to occur. Seeing as the plates on the car soon expired and I did not renew them, the car basically sat until four weeks ago when I pulled the car out of it's storage and once again I am facing this problem. Miraculously the car passed it's emission test. I've dived into the problem. My scanner reveals no codes at all. It's not emission related. At first I thought the problem might be a sticky line pressure valve in the trans TCC circuit that is causing the TCC to lock & unlock repeatedly seeing as the TCC solenoid is doing what it is supposed to do & is not to blame. Then I noticed that I could actually hear the tailpipes sputtering & missing as the car is driven at a relatively mild speed. This led me to think that I might have a ICM situation, pulling the plug wires at idle told me next to nothing except that all cylinders are firing. So I went to the trouble of getting the ICM & coil packs off the block (wonderful job) and replaced the entire assembly with a known good set. Problem remains. Next stop...Fuel Pressure diagnosis...
I plugged in a fuel pressure gauge to the rail and as it should be the rail is seeing 50lbs at idle so this leads me to think the the FPR hasn't failed. At this point there is the obvious left and that would be the injectors. I have discounted them because if an injector (or all) were miss firing then this miss/stumble would be all the time especially at WOT but that is not happening. It happens when the throttle is feathered, or just slightly off the throttle. So now I'm wondering what I have missed, as stated the scanner reveals nothing out of the ordinary. All one sees on the scanner screen display with regards to mixture is *lean* when on the highway, not *lean/rich/lean/rich* just lean. This makes me think that the ECM would cause this. So, if anyone wants offer any possible solutions...I'm all ears.

Steve

[pocket-rocket]

If the mixture isn't swinging, my first stop would be the O2 getting replaced, then I would go from there. How old are your spark plug wires? Was the rpms steady on the scanner and not sporadic?

[55trucker]

Nothing of a sporadic nature where rpm is concerned, the rpm will stay where it is according to where the throttle pedal is.
The O2 sensor has approx 6,000kms (not miles) on it. When motionless and at either engine idle or between the throttle limiter the scanner displays the O2 constantly sending info to let the ECM adjust mixture as it should do. The wires are about 9 years old with less than 10,000kms on them.

[robertisaar]

since you have a scanner, what kind of O2 voltage are you seeing? if the O2 does stay lean all the time, the ECM will fire off a DTC.
what AFR is being commanded in that situation as well?

[55trucker]

Rob:
In park or neutral & on the limiter (approx 3100rpm) the scanner sees more or less .37v (it will fluctuate to a degree). No codes are set at all.
The AFR is constant at 14.6 when the throttle is held stationary. (keeping in mind that under this situation there is no load on the engine so the AFR should be leaner than when under load)
At idle the AFR is also at approx 14.6 as well.
At speed I wasn't actually looking at the AFR, I was noting the timing curve & the coolant temp.

[robertisaar]

if the O2 doesn't switch go above .55 volts or under .35 volts within 25 seconds of the last crossover with throttle above 4%, that should trigger DTC13.
if the O2 stays under .2 volts for 60 seconds after entering closed loop, it should set DTC44.
O2 above .7 volts for 50 seconds after closed loop with TPS between 3 and 45% should set DTC45.

[Schmieder]

Checked for a leaking head gasket? Bubbles in radiator filler neck at idle, milky oil under fill cap, exhaust soaks paper held near the tailpipe.

A weak ground or burnt fusible link can cause a weaker spark. Bad battery terminals, loose connections.

Used a thinner oil viscosity? Maybe aging valve stem seals are blowing oil into the intake.

Plugged up catalytic converter putting excessive back pressure on the exhaust.

Failing crank sensor, cam sensor or bad wiring.


What are your fuel trims like? Are they skewing fuel dramatically richer or leaner?

[Schurkey]

EGR???


Curious about block learn and integrator numbers, while "sputtering and missing".

What is the scan tool showing for O2 crosscounts?

[55trucker]

Well, my problem has been solved. Now, I'm not the sort of individual who allows themselves to *throw parts at the problem*, I always rely on my tools & the service manual for diagnosing problems. In this case the scanner kept telling me you have a lean situation ....*solve it*....everything the display showed led to a possible fuel lean problem with fuel delivery. So, after doing all of the steps for a fuel delivery situation ( plug in the fuel pressure gauge/ pinch off the return to increase pressure/do an injector balance test) the problem still remained. The O2 readings were not out of the ordinary. The *swing* went from 0.01 all the way to .98 (this is expected), no sluggishness, but when this intermittent problem happens the AFR goes to 12.8 and the *lean* indicator lights up solid. So now, I'm at the point where I'm going to *throw* an O2 sensor at the problem. Keeping in mind that the O2 in there has only 6,000kms(not miles)on it and I've never had a new one fail before irregardless the manufacturer. Out goes the old? .....in goes a new........no change. Same intermittent problem...only now! ...code 44 sets. I know there is not a fuel delivery problem......now I'm really scratching my head.......Ok...pull off the exhaust crossover again/remove the EGR piping.....remove that new O2 I just installed and get another new one....this time I get a Delco. Put the exhaust back together........road test.......problem SOLVED. Damn.....if it hadn't been for that fact the the original O2 was so *young* and was responding on the scanner as it should do I would not have dismissed it. I'm generally not biased against any part manufacturer but I for one will NEVER use a Standard/Bluestreak part again. What a headache.

[pocket-rocket]

I've read Bosch O2's aren't the best either, but I've never seemed to have a problem with them.

[TGP37]

Okay, now you should make sure it wasn't a pre existing condition that killed the O2 so young. Or else the new one will die just as fast.

There are a few things that will kill a new O2 sensor fast, and I mean FAST. Coolant leaking into the combustion chamber, WD40 on the sensor or used anywhere ahead of the sensor and silicon products.

Example, using WD40 to clean a throttle blade or runner, etc. The drawn vapors will spray the O2 sensor and kill it. Probably get away with minor amounts but is still bad for it.

You may have a small coolant leak killing your o2. A good test would be this, warm up the car after a good spirited drive. Hold a piece of paper up against the tail pipe while engine is running. Don't touch the tail pipe, just let the exhaust gases impact the paper. If there is moisture on the sheet, you have a coolant leak.

Fix the problem before the O2 dies again. But, it very well could have been a faulty product or a combination of both.