2001 Suburban - strange stumbling/missing


#1

Hello all!

I have here a 2001 3/4 ton Suburban. It arrived last fall with 280K miles and a 6.0L V8 that was on its last leg (low to no oil pressure). I dropped in a 5.3L that I had handy, and had been happily driving it throughout the brutal Minnesota winter.

A couple months ago I noticed an issue developing. After a cold start, the engine would be severely lacking in torque for the first minute or so of driving. You could reach any RPM you wished, but the truck could barely accelerate. Once a minute or so had passed in this fashion, the issue would suddenly disappear… you’d be thrown back in your seat a bit if still attempting to accelerate, and the truck would take off. For the remainder of the trip, things would be fine.

Recently I went to drive the truck after it had sat for some weeks. It started fine, but the issue was now more persistent. When trying to accelerate, the gutlessness would continue for a good 15-20 seconds until finally and suddenly disappearing. Let off the gas, wait a moment, try to accelerate again, and it was the same thing all over again. Eventually I also noticed very obvious stumbling/missing when trying to get moving from a dead stop. Idle remained smooth.

The plugs and wires are only a few months old. The fuel filter is new. I checked for vacuum leaks (none that I could find). Fuel pressure is consistently around 50psi, even while the issue is happening.

The MIL was not on. I checked with my scantool, and found that P0101C was set, with P0101 pending. Visually examined the MAF harness and sensor; all looked fine. Swapped in a used working MAF; no change. (I suspect these codes may be due to the fact that I’m running a 5.3 with a stock, unmodified 6.0 PCM.)

My scantool is a Genisys running System 2.0. I’m by no means an expert with it, but I did know enough to try watching the miss counts while the symptom occurred. All cylinders seemed to be missing equally as much as the others, more or less.

Normally I’d be suspecting the fuel pump and/or associated wiring was going bad, but the pressure seems fine. Any ideas as to what I might be missing?

–Keith


#2

Have you checked all the possible sources of the P0101 code?

http://www.obd-codes.com/p0101

Note that the MAP sensor can also set this code.


#3

Bad MAP sensor never crossed my mind. Did check for vacuum leaks, but didn’t suspect that.

Strangely enough, as I was re-checking things a bit ago, I decided to try running it with the MAF sensor disconnected (‘what’s there to lose’ and all). Strangely enough, that seemed to have dramatically decreased the problem. What’s stranger yet is that the motor actually idles fine without it (contrary to what I’m used to seeing on older GM products).

I’m going to keep checking it out, but I think this is a major clue.


#4

After a cold start, the engine would be severely lacking in torque for the first minute or so of driving. You could reach any RPM you wished, but the truck could barely accelerate. Once a minute or so had passed in this fashion, the issue would suddenly disappear.

Sounds like the transmission is slipping when cold. As the fluid warms up, seals swell and pressure builds to normal, or near enough for relatively normal performance.


#5

I’m with NYBo. This high rpms with no power? Did you check the fluid?


#6

I highly doubt it’s the transmission - I would have known the feel of slipping. Fluid levels have always been fine. Once I had the scantool hooked up, I could also see the miss counts rising while the symptoms were happening (I was having difficulty feeling/hearing it miss, but it apparently was).

Since my last post, I took two test drives with an hour of cool-down in between… no MAF sensor and yet it’s running remarkably better. I’m beginning to suspect that my original MAF sensor may have been faulty, and that my junkyard “ran when pulled” replacement wasn’t good either. Never had a problem taking that yard’s word in the past, but I suspect exchanging the questionable MAF sensor will shed more light on the situation.


#7

I had the same thought as NYBo. If the tranny is revving and the vehicle isn’t going anywhere, the problem has to be between the engine and differential. The two should be hard-connected. Whatever the crankshaft is doing should be reflected in the wheels.

If this is a manual, I’d start with the clutch assembly.
If this is an automatic (which I’m guessing it is), I’d start by checking the fluid level and condition. Since it’s an OBDII vehicle, you may have some stored powertrain fault codes too.

If this is an automatic, it’s also possible that you have a brake dragging in the morning. The engine’s energy would be absorbed by the torque converter until the wheel broke free. If you use your parking brake, try not using it for a while and see if the problem disappears.

Post back with the results.


#8

Sound like a fuel/air mixture problem. Could be caused by a host of things. When you disconnect the MAF, the engine computer probably switches to “open loop” mode. This is how computer-less engines worked in the 1970’s. In open loop, the default ECM ignores the O2 sensor and delivers enough gas to give you sort of the approximately correct fuel to air mixture. The normal operation is “closed loop” where the computer measures the O2 sensor and the MAF reading to determine the amount of fuel to inject. You may need to find a shop that has the capability to read real time data, fuel trim etc. from your engine computer while it is running.


#9

Another vote for transmission. Any disconnect between engine rpms and road speed is transmission-related. Of course, there could be other problems, too. But high rpms + no motion = transmission problem.


#10

Started it up bone-cold this morning (MAF sensor still disconnected) and it test drove fine, no hesitation, no stumble, no missing. I understand how it looks, but I really don’t think it’s the transmission. One other thing I noticed, which might help put the tranny doubts to rest: with the MAF connected, the symptoms occur even in park while working the throttle. Unplug the MAF, start it again, and the stumble/miss immediately disappears. Again, MAF unplugged = shifts fine, no slip, no hesitation, drives fairly normally.

GeorgeSanJose: Sounds like you’re on the same track as I am. From what I remember of the EFI portion of high school auto shop (a decade ago, not exactly fresh in my mind), your prediction that unplugging the MAF is keeping it in open loop and thereby side-stepping the actual problem sounds about right. I am, however, surprised that it wasn’t setting any additional DTCs while stumbling/missing.

I do have a scantool capable of displaying real-time data, but I’m by no means an experienced diagnostician. Are there any particular outputs you’d suggest I pay attention to, which might give some additional clues?


#11

See if you can assess the fuel trim parameter. Fuel trim is a measure of how much extra (or how much less) gasoline needs to be injected to maintain the correct O2 sensor reading, compared to the amount of gasoline predicted by the ECM’s calculation based on the airflow measured by the MAF. If quite a bit more gas is needed to be injected than expected by the measured airflow, that means extra air may be getting past the MAF due to an air leak somewhere. Or the MAF isn’t working, or the O2 sensor isn’t working, etc.

I have a 40 year old Ford truck auto-equipped that occasionally develops similar symptoms, stumbling, hesitation, almost stalling during acceleration. It’s fine for months, then one day I start it, and it runs poorly. Every time this has happened, it has been due to an air leak somewhere or another leaning out the mixture. And it does sort at first seem like it might be transmission problem, but it isn’t actually a transmission problem. As soon as I fix the air leak, it goes immediately back to running perfectly, like it was before.