Abeyance

I did not try testing the fuel pressure while holding at a higher RPM, but it was rock solid while momentarily running the RPMs up and down. I'd need to go rent a test kit again to try testing this way. While I can see that maybe leading to an explanation for why the dropoff at WOT at speed, I don't see any correlation with the poor cold startup behavior or the odd fuel tank pressure sensor data. Thanks for the replies, and please let me know if there are other forums where you think I might get better/more responses.

1 - I have my own bidirectional scanner, an Autel MK808K, bought specifically for this project. No current codes, pending or active. 2 - ...1F168440

I have a 2001 GMC Sierra 2500HD with the 6.0L engine that I'm trying to bring back to life. I have little to no history on the vehicle because I "inherited" it from a deceased family member and he, obviously, can't tell me anything about it now. Shot the parts cannon at it and still couldn't get it to run right, so I went ahead and tore down and rebuilt the engine as it had 213k+ on it. Turns out it had some bent valves, so there's why it wouldn't run right. Now that it's all back together, it runs a lot better, but it's still not right, especially at cold startup. Never starts on the first crank (when warm), always starts on the second attempt, easily. Idles fine after startup, but the moment you rev it any, it stumbles and "backfires" through the intake. Even once warm, under WOT it's stumble, then recover, then stumble, etc. around 50-60 MPH, but drives just fine when being more sedate. - New plugs and plug wires - New injectors - New intake manifold gaskets - Intake thoroughly cleaned when it was off the engine - MAF sensor cleaned a couple of time with MAF sensor cleaner, but not replaced, and seems to be working correctly based on scanner live data - New knock sensors (was throwing a code) - No codes, pending or active, in the computer (since replacing knock sensors) - O2 sensors are all functioning within spec - Have run the CKP (crankshaft position variation learn), twice - Have tested the fuel pressure at the rail a couple of times now, it's good, jumps to 60 the moment you turn the key, and doesn't drop below 55 when running. - Fuel tank pressure sensor is returning a value of 14 mmHG, which is outside of the stated range of [-32...10]. This is the only thing I can see anywhere in the scanner that is out of spec. This is particularly odd as I'm RPO FE9 and over 8600 GVWR, so my truck does not have a fuel pressure tank sensor. Can anyone explain this and if it means anything or should be ignored? Ok, what am I missing? Any ideas? I'm a bit stumped at the moment and looking for educated opinions/guesses as to what my problem may be and what I should be looking into next. Thanks in advance!

Well, looks like we were all wrong. Gave up and took it to a mechanic for diagnosis... just heard back, engine is bad. Will have more details once I pick it up. Not happy. Buying this truck was charity for a family member down on their luck and my reward is thousands out the window and nothing to show for it.

Following the intake gaskets, it starts and idles well enough that I thought it might be fixed. The second you put it in gear, it gets really rough.

Wasn't the intake manifold gaskets (or the knock sensors) either. After doing those, it now idles better, but the second you put it under a load (as in the second you put it in gear), it's just as bad as ever. Pretty discouraged here, and out of ideas.

213,000

Well, wasn't the fuel pump, zero change after putting in a new one.

Yes, you can tell. Whether the light is flashing or not doesn't seem to make any difference, it always feels like it's misfiring (shuddering, low on power, slow to accelerate, stumbles and almost dies when you come to a stop, etc). I assume there are instructions around here somewhere on how to do this relearn? Or is it a dealer only procedure? At this point I'm willing to try just about anything as I'm running short on patience and thinking about fire-bombing the thing.

Ok, before I order any more parts for my parts cannon, including intake manifold gaskets, I checked the fuel pressure, which I'm told can also be the issue behind a P0300 code. Here is what I found, at the schrader valve on the fuel rail: Key on, not started: pressure went to 50, then slowly dropped back to zero. Not sure if it was supposed to, but it did not hold pressure with the key in the on position Starting and idling: pressure was consistent at 50 Running, feathering the throttle: pressure bounced between 48-52 Those numbers seem a bit low, but I don't know if they're low enough to indicate the issue is there. Hoping someone can tell me if my test results indicate anything, if not I'll probably just move forward with the intake manifold gaskets and a bit of prayer.

I am currently fighting this exact same issue, which seems very common based on all the posts about it all over the internet. Check engine light blinks, mostly but not exclusively, at idle. It also idles around 500 RPM, which I believe is a bit low and at times seems like it's going to stall but never quite does. Computer shows a code P0300 (Random/Multiple Cylinder Misfire Detected). So far I have done the following, which has changed nothing: New spark plugs New spark plug wires Cleaned throttle body New idle air control valve New PCV valve New fuel filter Added a can of Sea Foam to fuel tank (unknown quantity of old gas in tank) Next up, I'm going to borrow a fuel pressure gauge and check that. If the numbers don't look right, then I'll probably try a new fuel injection pressure regulator and/or new fuel pump. If the numbers do look right, then... I dunno, maybe an EGR valve or MAF sensor or new coil packs or...?