2008 Yukon dying.

[SpareParts]

My friend's Yukon has developed a problem and i can not quite pinpoint the problem. I'm almost thinking FP regulator or bad intake gaskets.

It will randomly die like it's out of gas. Restart it and it runs ok (ish) slightly rough. Sometimes runs perfect, hot. When cold it will remote start and within about 10 seconds dies. Restart, Sometimes runs good sometimes slightly rough.
Fuel trims ST hover around 0, LT will vary from -5 to -14, KOEO fuel pressure holds steady at about 54PSI.
With a fuel gauge hooked to it, it will read 54PSI. Spec is 52-62. But i have seen it at 80PSI and drop down to 15PSI when running. I think the FPCM is varying it like that? or a malfunctioning FPR?

I did find a badly cracked rubber hose at the PVC and replaced that. Seems to have helped the LTFT slightly.
Cleaned the MAS. No leaks in the intake tube/resonator.

Map sensor shows Test 1 Fail. Test 2 pass. We are putting a new Map on it tomorrow but i doubt that's the entire problem.

Some days it just runs perfect. Only codes are Bank 1 Bank 2 Lean. o2 sensors appear to be fluctuating good.
I checked my FP gauge on the wife's truck and mine. It is working properly.

Anyone experienced this before? Any suggestions?

 

[Marky Dissod]

If 'MAS' is 'Mass AirFlow sensor', good that you cleaned it, IFF cleaned with MAF cleaner.
Next, clean the throttle body. Then clean the PCV system as best you can, including cleaning the valvecovers out.

The Charcoal Carbon Canister / EVAP system is the next most likely culprit;
even if the vehicle was never 'over-topped' at the pump.

 

[SpareParts]

Would have never thought to look at the canister.
I'll give it a look over. Other than pellets coming out of it, what else would i be looking for?

 

[j91z28d1]

if he is one that does a few more clicks after full at the gas pump, I've seen that st work. the canister gets liquid fuel in it and then when it opens the solenoid it would suck in enough either fuel or vapor to run rich and die. worth a check.


there's a trim or offset number in the fuel pump control module data. that might help you see what's going on. a theory was the more it has to offset to hit the target might help see if the fuel pump is going out.


I once had a fuel pump that ran great but every now and then after a long run time it would keep running but not pump fuel. cool down a few mins and run again for months. finally had a scanner on it and saw the o2 go lean the the fuel trims try to add max fuel and die. changed the pump out and never happened again. after that I probably change pumps a little more often than needed. I'd probably throw a pump at his and see what happens. especially if it's got say 150k on the oem.

 

[SpareParts]

The MAS was cleaned with the proper cleaner.
I replaced the fuel pump with a Delphi unit about 5 months ago. It for sure could be going out but seems unlikely.
I am really leaning to intake gaskets leaking after reading many many posts about lean bank 1 and 2 problems being unmetered air. Seems about 90% of the time it was intake gaskets. The other 10% was varying other problems, including bad FP
Wish i had a smoke tester.
It has 249K on it and no way to know if they have ever been changed. AFM/DOD is turned off in the tune. No way to know if the engine was apart when ever that was done or just turned off in ECM.

I just remembered at hot idle the MAS was reading 4.6g/s slightly low. Could be reporting incorrect reading or unmetered air making a false reading.

Seems like there is enough pointing to intake gaskets it would be worth changing them.
Also, im not going to toss at pump at it without a firm belief it is the problem.

@j91z28d1
Once had a Ford Taurus that would not start after driving about 1/2 hour. Wait a half hour and it would start and run fine till turned off, then no start again for 1/2 hour.
Found when the fuel in the fuel tank got warm the fuel pump would not run till the fuel cooled off a little. Replaced pump and never a problem again.
That problem is one of the reasons manufactures changed to a returnless system.

 

[mikez71]

80psi is about how high the fuel pressure gets when you shut down a hot motor and let it sit a couple minutes.
Are you sure it was at 80 running? 15psi also way too low. Should drop to about 43 psi at idle.
I wonder if your fuel pump or control module is dying..

 

[rdezs]

The dropping down to 15 psi fuel pressure while running, is very indicative of a fuel pump on the way out. (And when it drops that low, that should trigger an extremely lean condition on both banks as there's no way to compensate the mixture)

I would replace the fuel pump, and while you're at it the carbon canister, evap vent valve solenoid, and the purge valve.

Considering the mileage, I would also throw a new intake manifold gasket set in, and since you're in there go ahead and replace the oil pressure sending unit with a genuine GM ACDelco unit.

I've seen more fuel pumps showing symptoms like this when they go bad, compared to ones that just flat out die. I'm assuming it has more to do with the internal fuel pressure regulator then the pump itself.... But it's an all-in-one module.

 

[OBSandaNNBS]

I would replace the fuel pump, and while you're at it the carbon canister, evap vent valve solenoid, and the purge valve.

Considering the mileage, I would also throw a new intake manifold gasket set in, and since you're in there go ahead and replace the oil pressure sending unit with a genuine GM ACDelco unit.

At this mileage, these are all about due anyway.
I'm also voting fuel pump as the main issue, 15 psi seems horridly low. Maybe you can get it warrantied after only 5 months.

 

[SpareParts]

New Purge valve and evap vent valve within a month of the fuel pump, Canister appeared fine. New GM oil pressure sending unit about 8 months ago.
Im going to wait on the fuel pump (for now) It is controlled by the FPCM and maybe the one time i saw it dip down to 15PSI for a second something told the FPCM to do that.
I have had a gauge on it yesterday and all morning today driving around and it has not done it again. The FP varies a little around 54PSI idling or wot.

Looking further i found the oil fill tube O ring not sealing at the valve cover so fixed that.
After changing the MAP sensor it now tests ok. I don't know if changing the map sensor had anything to do with it but it started giving a TPS code. Removed and cleaned TB did the relearn procedure and still got the TPS code.
Swapped it out with a spare i had here and did another reset. All good now no more codes after driving it a little more.

The MAS is showing about 4.7 g/s idling around 575 rpm. From what i know that should be over 5 g/s

The HOT, LTFT is around -14 (varies a little)idling and when giving it heavy throttle driving it moves to 0 + - a little.

 

[SpareParts]

Thanks for all the suggestions guys!
I'm just not ready to condemn the FP (yet)
With the fuel pressure remaining steady around 54PSI and still having the same symptoms, i don't think the fuel pump is the entire issue if at all.
The next step we are going to take is change the intake manifold gaskets. Probably needs it anyway.
I'll also get a used MAS from the wrecking yard and see if that changes the reported reading up any. If it does, he will get a new one.

 

[Fless]

You can use unlit propane around the engine compartment to search for vacuum leaks. It doesn't take a lot of flow from an unlit torch.

 

[SpareParts]

I tried carb cleaner but i did not like the fire risk. Soaked it with water, tried propane. No change. Problem is i can't get to the bottom side very much and that is where the leak could be.

 

[rdezs]

A Tech 2 would be incredibly useful here...

 

[rdezs]

I tried carb cleaner but i did not like the fire risk. Soaked it with water, tried propane. No change. Problem is i can't get to the bottom side very much and that is where the leak could be.

Injector o-rings are another source of vacuum leaks. Are they original? Just like the oil pick up tube o-ring, as well as the ones on the oil filler cap and the dipstick.... They are a maintenance item once you get some miles on.

 

[j91z28d1]

I didn't realize the pump was that new. still possible but I'm like you, hate to do it again so soon.


for the air leak test. I have never had any luck spraying around and hearing the engine idle different. efi is to fast to compensate. for me, I gotta have the fuel trims pulled up and watch them. they will change ****** when it seen brake clean. I've never had any issues spraying brake clean around an intake. I do at least try to do it before the engine is blazing hot, but run enough to get out of open loop.

 

[j91z28d1]

oh and yes the maf sounds low. my 6.0 idles right around 6.0 up to temp and without any load in the engine. anything less than engine size would lead me to think vac leak or drifted maf sensor.

 

[SpareParts]

Tech 2 for what? I'm pretty sure my scanner will work.

 

[strutaeng]

Good holding off on the FP. I almost missed you mentioned you had already changed that. I don't own a newer vehicle that has a FPCM, but I understand those use pwm controls to run the fuel pump based load Not sure how you go about checking that? They do seem to sometimes act erratic.

Does you scanner have live data? The way to determine if you have a vacuum leak is this:

1. check and note the fuel trims at idle. Should be kinda high on the lean side of course.
2. Rev the engine to 2k, then 3k and see if the fuel trims become less lean
3. If they do, vacuum leak is possible.

Reason is because at idle, the vacuum "hole" is a percentage of the intake CFM, typically a high number. As you increase RPM (and intake CFM), that number gets smaller, since the "hole" size isn't changing.

Another thing is that vacuum leaks usually affects only one bank on a V8. Fuel delivery issues almost always affects both banks, causing fuel trims to be almost exact, or nearly so.

 

[rdezs]

Tech 2 for what? I'm pretty sure my scanner will work.

Depends on what sort of scanner you have. Not too many equal the Tech 2, or go in as deep. Getting all the live data, and being able to send commands to all modules, is what dealer level diagnostics is about.

For example, if your scanner won't program and troubleshoot your driver door Master control window switch.... You're missing out on a lot. Most of the widely available scanners just hit the major items , and are somewhat limited even at that point. With the proper device and a small fee for the subscription, you could plug in and check for an updated file for your fuel pressure control module. Send commands to it as well to troubleshoot.

My old 72 Nova with the slant 6 was so much less complicated.... And so much more user friendly.

To avoid shooting the parts Canon at it, the logical thing to do is make sure you have a 100% accurate diagnosis. Rather than investing the money in the equipment, one option is to pay the dealer there one hour flat rate to plug in and diagnose it.

 

[SpareParts]

I have the capability to program a module with the TDS website.
I can not plug in and do dealer level diagnostics im sure, but I can test all modules for function. I can tell when a window switch is in the up or down position, I can command it up/down
I can activate the shocks/struts individually. I can/have reset ALC. I can see live data from the suspension. I can adjust the TPMS pressures to what the tire is rated for. I can see live data from the FPCM and command it to increase or decrease pressure, On/Off.
I am no expert for sure and a lot is interpreting the available data to come to an informed decision. In the end, you're either right or wrong.

After much research I'm making the decision, intake gaskets are leaking unmetered are in causing the negative LTFT fuel trims.

I totally understand what you are saying and i get it.
After a lifetime of success and fails with everything that runs on gas and diesel i have to think i am a little more capable than most but i do not deal with it every day like a real Tech does. In the end, im either right or wrong. We all gonna find out though!