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6.0L 1500 RPM Hesitation

zealer

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zealer Enthusiast

zealer

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Hi all, I'm back for some more advice.

My 6.0L Silverado 2500 has a hesitation at 1500 RPM when pulling away from a stop. If I accelerate slowly off idle, you don't really notice, but if I floor it the RPM will climb smoothly from idle to 1500 RPM, hesitate there for about half a second, then continue powering on.

 

I have 3 codes stored, which no doubt will lead to me figuring out the problem. I have a P0101, P0171 and P0174 - MAF volume circuit, lean bank 1, lean bank 2.

Unfortunately, no freeze frame data was stored when these codes were thrown (at least not yet), nor has the CEL come on.

 

I "checked" the intake manifold gasket via propane around the perimeter and I did not note any changes in idle. I'm not ruling the gasket out yet, but I feel if there was a leak there I should have been able to detect a noticeable change. IMO, I don't think it is idling rough or abnormally to begin with (but as this is my first GM V8, I don't have any baseline experience to compare with). I don't have a fuel pressure gauge at the moment, but if the stores are open tomorrow I will pick one up and check the pressure and regulator.

 

I have read that plugged cats are often the cause of this time of symptom and similar codes. A way to verify this is to unscrew the upstream O2 sensor and observe for changes, right?

 

Also, my air filter is stock and clean. I don't have MAF cleaner but will pick some up tomorrow as well.

 

Any other suggestions? I'd like to do all the free/cheap diagnostic tests before I start throwing parts at it.

zealer Enthusiast

zealer

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It is a 2000. Finished my last exam today, will commence trouble shooting tomorrow. The CEL finally illuminated today, and I noticed it took longer to start than usual, and when it finally took it was a 'weak start', struggling to 400-500rpm and then finally caught and revved to the usual 1000-1200 rpm before dropping to idle.

zealer Enthusiast

zealer

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Ok just cleaned MAF and took truck around the block. So far no codes, but I don't think I drove long enough to really put her through the paces.

 

The real issue is acceleration from a stop/idle. If I floor it, the engine stumbles for a few seconds between 1000-1500RPM and then starts to take off. At 2000+ RPM there is no trouble. I also have noted a harder start issue, where it takes longer to crank and when it finally catches, it is a soft start (coughing at 400-500 rpm then starting). I pulled the vacuum line off the regulator, it is dry. I really need to find a gauge. They're $112 new from the auto parts store, I'm trying to find someone to borrow one instead.

 

I also connected a timing light to observe for misfires. It appeared there were many (on all cylinders on driver side, 2 cylinders on passenger side). I never heard a distinct misfire or anything, so I'm lead to believe the issue lies within the timing light. I pulled off a few wires to test resistance, and they are in spec. Pulled a few plugs and they looked fine and were gapped correctly.

 

Any ideas? I really want to know the fuel pressure... Will either confirm or eliminate fuel problems.

zealer Enthusiast

zealer

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Still struggling to locate a fuel pressure gauge. I guess I will just bring it to a garage.

Is there anyway to test the MAF apart from using a frequency enabled multimeter? Mine does not have that setting and of course the stores around here do not sell one.

zealer Enthusiast

zealer

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Brief update. Weak start issue disappeared, 1000-1500 RPM WOT stumble hasn't. Interestingly enough, I've seen a large array of varying codes, of which P0101 is always the common factor.

I've had:

P0101, P0171, P0174

P0101, P0300, P0157

P0101, P0157

P0101, P0300 (today)

 

I also have some data from a scan tool, if anyone can help me make sense of it (or notice any abnormalities). I think the fuel trims are way out. This data is freeze frame data saved when the P0300 code was set.

Fuel System (1&2): CL (Closed loop)

Calc Load: 9.4%

ECT (F): 195

STFT B1: 45.3 %

LTFT B1: 24.2 %

STFT B2: 52.3 %

LTFT B2: 24.2 %

MAP (in-Hg): 18.0

Engine RPM: 2008

Veh speed: 51 MPH

MAF (lb/min): 3.486

TPS: 25.0 %

 

From the minimal amount of experience I have, it seems to be those fuel trims are way out of spec. However, at low speeds the short term trims would range from +/- 5% of which I gather is the acceptable range. I should finally get a fuel pressure reading tomorrow.

  • 3 weeks later...

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