2003 Yukon issues - SOLVED

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ic455

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Hi all, new here. I've searched and not found anything, so here's my situation: 2003 Yukon 5.3 flex fuel. Wife complained of near stalls at stop signs intermittently with hard starts after shut down. Was already about to do valve cover gaskets, so did those, pcv valve, plugs and wires. Later her problem got worse. Replaced fuel pump. Replaced fuel filter and regulator. Found that new pump cracked and was sucking air. Replaced fuel pump again. Started seeing p0342 and p0343 codes. Replaced cam sensor and plug, replaced crank sensor and plug. Later found loose starter cable and broke solenoid on tightening, so replaced starter. Replaced injectors. Replaced PCM due to the cmp codes and having tested everything and found all functioning properly. Has new MAF (I stole the old one for my 04). Replaced ignition switch. Using Snapon Solus Ultra I'm finding that once the engine warms up, fuel trims go off the rails and it starts attempting to stall. I moved the grounds from back of heads to good ground on front of head. With the Solus, if I change fuel from closed loop to open loop during the event, it smooths out. If I let it go back to closed loop it begins rough idle and will stall in gear. Any insights would be appreciated!
 

Fless

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Welcome to TYF.

Lots of questions. Mileage? Is the work you listed all very recent due to the stalling problem? Throttle body cleaned recently?

Do the fuel trims go positive, or negative when in closed loop? Numbers on both banks? Positive trims (and stalling) could point to a vacuum leak. Does it run better with the MAF disconnected?

Monitor the upstream O2 sensors to see if they are moving between 0.1 and 0.9 volts (100 and 900 mV). Graphing the PIDs can help visualize.

When the crank sensor was replaced was a crank correlation run? If not, do one to rule that out.
 
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ic455

ic455

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232000 miles, all new parts done while chasing this problem. Throttle body not cleaned. Short term fuel trim goes from a low and flat baseline on bank 1 to high and flat, bank 2 similar. I should've recorded the event but didn't. Haven't tried disconnecting the MAF. I've tried to run CKP relearn multiple times after initial replacement but my Solus never recognized the WOT event. It did succeed once, but I think I did it with the engine cold. Upstream O2 readings remained mostly normal, bank 2 was pretty spiky, while bank 1 had flatter peaks. I have a hole drilled in each cat before the substrate in effort to bypass a possible obstruction. Thanks for your input!
 

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What are the long-term fuel trim values/numbers, and the short-term trims when it goes to closed loop? Is it adding fuel or subtracting?

I'd probably doubt a clogged cat if it has power above idle, maybe set that aside initially.
 
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ic455

ic455

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The short terms are in closed loop, it appears to be adding fuel. I don't recall the long term numbers, they were fairly steady though. Also of note, the fuel pressure is about 44 at idle(higher than my 04), but most interesting is after running about 30 minutes I shut the engine off and attempted to release the fuel pressure using the relief button on my gauge, it just kept pouring pressurized fuel out. I duplicated this situation on my 04 and there is only a short squirt of fuel and done.(this has also been the case over the years testing other vehicle's fuel pressure). I would almost suspect a stuck regulator even though it's new.
 

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Hard to tell much without actual short term numbers. Suspect vacuum leak (do standard vacuum test), exhaust leak around/before upstream O2s, or unmetered air entering before throttle body (obviously vacuum leak would be after). Check condition of air intake after MAF for sealing and leaks.

When hot starting and difficult, floor the accelerator to put it in clear flood mode. If it starts quicker than before, suspect fuel dumping from a stuck injector or bad fuel pump regulator.
 

afpj

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Need actual values for fuel trims as has been stated a few times (I love to pile on) High positive or high negative? Also, what brand maf, cam, crank sensors. Gotta use Delco or gm, maybe Delphi on the maf, otherwise expect to continue to chase your tail. Change the map sensor?
 

MassHoe04

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If ******* fuel trim at maximum... Something is making the computer think there is too much air (or not enough fuel) in the mix.

If the upstream O2 sensors were ever replaced, do you know what brand?
Upload photos of the live data for O2 Sensors (B1 S1 and B2 S1) and photos of the fuel trim readings (LT and ST on both sides).

Seeing what the live data shows might be helpful.
 

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