I just stepped up a few tire sizes, wouldn't mind a controller to better suit, or a possible re-gear, if that's even possible.

I've NO IDEA what is causing this engine cut out during the warm up cycle, strangest thing I've ever seen. About 2 minutes after it reaches full temp, I get power cut-out as though an ignition wire is coming lose or fuel is cutting, It feels like someone flicking off/on your ignition switch on your dirtbike while you're riding. Sudden total power loss, then sudden full power, random intervals for random lengths of time, sometimes a couple per second, sometimes for a couple seconds long. You can sort of rev through it but not always, it starts occurring, worsens for 30 seconds sometimes killing the engine (often actually) Then a restart and it fires right up, still stumbles heavily, then in 30 seconds improves greatly and then suddenly another 30 seconds later drives totally normal again for the rest of the day. This routine happens in the morning only on a cold start. So odd.... I'm going to try to track using the Torque App ODBII and see what's happening. I'm not sure if it's ECU power cut out or a sensor or what. Tomorrow morning I hope to catch it in the act.

Any grounds that are known to be weak by design I should consider improving? I've set my ODBII app to monitor voltage via ECU and TPS, I wish it had fuel pressure sensors I could watch.
-Greg

 

The issue repeated again this morning in the exact same manner, growing worse then getting better, all said and done in 3 minutes, it died once. Now it'll be fine for the day no doubt unless it sits about 8hrs. I stayed plugged into the GM ODBII port and watched fuel/temp/TPS/ECU voltage. It didn't seem the ECU lost power, the issue happens right as it comes into around 175-190 degrees. The transmission does seem to jump around with it sometimes but not always, under load while driving as it stumbles it'll sometimes upshift or downshift but perhaps that normal given how badly the motor is stumbling and trying to recover. The TPS didn't seem to read incorrectly. The voltage stayed normal and fuel-gal/hr did indeed drop but, perhaps just as it would with an engine that's stumbling for any number of reasons. I guess I can try some more things tomorrow...... what a hunt this is becoming. It's as though some temp differential is the issue.

UPDATE> This maybe affect other folks too. Looks like with google searches, all of our chevy motor options have this issue for many folks out there. 4.8 / 5.3 etc. in these years suffer from the issue. Many are pointing to MAF/TPS and this service bulletin regarding a cold morning warm-up issue just as I've been describing. Info response below:
Document ID# 1899786
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Subject: Cold Engine Idle Misfire Or DTCs P0171 P0174 - keywords 4.8 5.3 6.0 air cold crack hesitate idle intermittent internal intake leak lean
misfire miss P0171 P0174 P0300 power rough surge vacuum #PIP3232B - (02/07/2007)

Models: 2002-2005 Cadillac Escalade
1999-2005 Chevrolet Avalanche, Express, Silverado, Suburban, Tahoe, Trailblazer
1999-2005 GMC Savana, Sierra, Yukon
2003-2005 Hummer H2
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This PI was superseded to update parts information. Please discard PIP3232A.
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The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.

Condition/Concern:
99 through 05 GEN III truck engines may experience any of the following:
• A cold rough idle that diminishes as the engine warms up.
• Possible DTCS P0171, P0174, or P0300.
• Random misfires at idle.

Recommendation/Instructions:
The intake gasket seals may be leaking around the intake ports, causing a lean condition. The intake gasket seals may leak around the bottom
side of the port, making it a challenge to detect the vacuum leak. The technician may also notice lower than normal MAF readings if the intake
seals are leaking.

If leaking intake gasket seals are found, replace the intake gaskets. During the repair be sure to use the current GMSPO part number for
replacement intake gaskets. The current GMSPO part number for intake gaskets will have a teal green intake gasket seal. The old version of the
intake gaskets were orange in color. Use only the teal green colored gaskets for a repair.

The intake plenum should also be checked for excessive warpage that may cause an uneven clamping pressure of the seal. If the plenum is
found to be warped it will need to be replaced. Service Information states, "An intake manifold with warpage in excess of 3 mm (0.118 in) over
a 200 mm (7.87 in) area should be replaced". Intake warpage is measured at the gasket sealing surfaces, and should be measured within a
200 mm (7.87 in) area. This measurement is across only two of the intake runner port openings. Measurements taken across the entire distance
of all four intake runner ports will lead to unnecessary manifold replacements.

>UPDATE
Well, not sure if this issue is solved but I did solve the issue of having to add a little throttle when starting it up, turns out the IAC was just a little sticky and cleaning it out made a world of difference. Nice to have that solved or so it would seem thus far.

 

Two things I would check in terms of grounds / power. 1. The main ground running from the battery and body ground to the engine. Make sure it is mounted somewhere solid on the engine. The bolt I picked originally loosened up on its own. I ended up moving to a different part of the alt bracket. 2. Battery terminals. The factory terminal design can be an issue - once they are well used, they can be hard to tighten up or keep tight. Still looking for a good solution to that besides shims.

 

Seems fine on my end but tomorrow I'm going to improve the grounds anyway. I replaced the battery and they seem very snug (terminals that is) so I don't think that's the issue in my case.

~166 degrees - 190 degrees it just hesitates, bogs, fully cuts out until it warms past that. It seems to be worse if I try to accelerate harder. I still have the cold start issue of needing to add throttle on startup for a few seconds, sometimes up to 10 seconds. I just replaced IAT which cures this throttle on startup issue but only for the very first time I put it in. Each time I remove and reinstall it, it starts perfect ONE time, then goes back to the same normal startup so maybe it's not the issue but creating a high rev which coincidentally solves the issue, reinstall the old IAT made no change. I don't mind ignoring that cold start issue but the cutting out issue 3 min later when it starts to warm up, THAT is a real safety issue for the wife when it cuts out sometime so bad it stalls. Something about the warm up cycle is causing this it seems, can't imagine what. O2 sensors perhaps? I guess I can throw some at it and see what changes if anything. I also got new cam/crank position sensors on the GM side and i'll install those next too and see if those change anything.

I do still have a motor oil leak, not sure from where, it seems to go all around the oil pan (which is new with a new gasket) generally I can track leaks well but with the fan blowing everything around it's hard to say at the moment. My power steering unit at the common top adjuster **** is wet (very slow leak) too so I'll be doing the cheesy RTV fix there after I snug up that steering box as I think it has more play than it should. That'll bring me down to I think just the one oil leak. I don't THINK the oil leak is in any sensors but I'd prefer to rule it out and my driveway would appreciate it too.

Issue of cutting out during warm up does indeed appear to worsen under heavier engine load. Considering ignition related issue or ECU as culprits. I bought a new coil pack, I'll swap it from each cylinder 1 day at a time and by the 8th day I should know if I found the issue there, or move on to other potentials.

UPDATE> Problem has gotten worse, now does it sooner and much worse, I wouldn't dare take in but a few minutes from the house at this point.

Not Battery
Not Alternator
Not spark plugs
Not spark plug wires
Not IAC valve
(Probably not ground issue)
Not physical connection to ECU plugs
Not O2 Sensors
Not LS Crank sensor
Not TPS

 

Do you have the GM EVAP hooked up? Is the plumbed to the Rover EVAP hoses? I can imagine a scenario where, if plumbed in, the Rover EVAP could open up at hot idle and let in more air, basically making a temporary vacuum leak, and the GM ECM would not compensate for it correctly, making the engine stumble.

I wonder if something is off with your startup fueling tables in the GM ECM?

 

I don't believe I plumbed any of that in, ditched most all the Evap stuff on install. Very fitting scenario you described though.

I too figured the startup table might be off and for the first 30 seconds of running or just a little longer, I could believe it and was planning on going to get a touchup tune when this bigger problem started up. For a while this bigger problem never existed, just the having to add a bit of throttle to keep it running for 10-30 seconds on a cold morning start.

Much appreciate the thinking on it!

 

One more thought -- have you checked for fuel pressure? Could be a weak (but not dead) fuel pump?

Did you check further into that leaking intake manifold gaskets issue? I can imagine those flexing as it warms up and creating a vacuum leak that comes and goes.

I would say swap the ECM and see if it goes away, but that is easier said than done without a tuner app.

 

I did just do that actually. Fuel pressure shows 50psi on the dot. It doesn't seem to ever move from that regardless of the state of it running. It falls off a couple PSI and holds when the engine is off. I'm not sure what was expected for pressure. I think I recall the GM wanted closer to 60 but I would expect it to at least idle and mope around decently on partial throttle at 50.

I checked the throttle body gasket a couple times. Honestly have'nt looked into the intake manifold further. I suppose I can rip it apart and do that at this point.... Like you said, it could make sense.

The ECM swap and paying for $250 for another tune is a bummer, had I known I'd have to mess with it further I'd have bought the software by now. Really a downer there but I'm running low on ideas to try. I'll ty the intake manifold as you stated and all new coils, then perhaps I'm onto the ECM.

 

It almost sounds to me like its running rough after going into closed loop. After you first start it (cold), it will run in open loop using a base map until it reaches a certain temp. After that, it goes into closed loop and takes O2 readings and adjusts fuel appropriately. I replaced my transmission recently and had accidentally hooked up my O2 sensors backwards (i.e bank 1 and bank 2 switched). It would start and run fine in open loop and once it hit closed loop it would run horribly. Once I realized this and switched them back properly, it ran great again. You could try disconnecting your O2 sensors and forcing it into Open Loop mode as a trouble shooting step.

 

I think Rackolamb’s advice is good — if the issue is closed v. open loop will narrow down a lot of potential sources.

My fuel pressure sits at 53 psi and does not move. GM wants 58, but 53 has been fine. I wouldn’t necessarily replace all the coils since it runs clean otherwise. Maybe look at one of the injectors if you have the intake off? I had a questionable one in the beginning. Probably from the engine sitting at a junkyard for who knows how long. Ended up replacing all of them with a known good used set. World of difference.

HPTuners is great, but pricey at $300 for the dongle + $100 per ECM to write. You can read with just the dongle. I can pick up ECMs at my LKQ for $50, so $150 per really isn’t bad once you are past the dongle cost. You could always try the free LS Droid software if you have an Android.