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Old 06-02-2014, 07:48 PM   #1
kthelen
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2001 Suburban 2500 strange stumbling/missing

Hello all!

Have here a 2001 3/4 ton Suburban. It arrived with 280K miles and a 6.0L 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 using it all winter.

About a month 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, but the pressure seems fine. Any ideas as to what I might be missing?


--Keith
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Old 06-02-2014, 10:03 PM   #2
Special-Ed
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Re: 2001 Suburban 2500 strange stumbling/missing

problem 1.. you are running a 5.3 on a 6.0 ecm....
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Old 06-02-2014, 10:34 PM   #3
kthelen
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Re: 2001 Suburban 2500 strange stumbling/missing

I know it's far from ideal (to say the least), but until I'm willing to sideline it to get a mail-order tune, it is what it is. And it DID run fine for months prior, so I'm not sure that the incorrect flash is entirely to blame.

Strangely enough, as I was re-checking things a bit ago, I decided to try running it with the MAF sensor disconnected. 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).

After a bit of testing, I'm beginning to suspect that my original MAF sensor may indeed 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.
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Old 06-03-2014, 07:33 PM   #4
clinebarger
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Re: 2001 Suburban 2500 strange stumbling/missing

I can guarantee it's NOT the tune, A 5.3L will run fine on a 6.0L tune & vise versa.

Possible causes.
Bad Mass Air Flow senor.
Check for holes/crack in the intake tract.
"Open" in the wiring between the MAF & PCM.
Clogged converter/s. (Not likely on an intermittent issue).
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