99 5.7 4x4 Yukon Intermittent Run Issue

[Cousin_J]

Hey all,
So I have read through several threads and see similar issues but nothing quite the same so appreciate any help.

The truck will start cold just fine, and will run for 15-20mins while parked and then just cut out. It doesn't sputter at all, its just a clean cut out. Once the truck dies it takes about 45 mins and will then start up again. In the mean time it will crank but not start.

I was getting P0300 & P0303 codes so started out by changing the spark plug in #3 which was bad, but still am having same issues. I cleared the codes after replacement but both codes have resurfaced. I have checked spark once the truck cuts out, and have spark to all cylinders. If I shoot some ether into the intake after it has cut out, the truck will start for a few seconds then die.
Also, when I first start up the truck(within first 5 mins or so) I am able to turn of the truck and restart no problem, but after about 10 mins of run time if I shut it of it will not restart. The issue seemed like it could be linked to a sensor/relay that was requiring a cool down period, so I replaced the cam & crankshaft sensors and the fuel relay hoping for an easy fix - no luck.

Its a pretty consistent cycle; starts fine, idles for 15mins, dies, and then requires a 45 min cool-off period prior to restart.
If I rev the engine it does reduce the run time before shut-off, but during the time it runs I am able to rev without issue.

I am going to pick up a pressure gauge today to check fuel pressure, but was skeptical of the pump being the issue since when the truck is running I am able to rev the engine without any hesitation or seeming loss of power. I have tried cycling the fuel pump several time once the truck dies to see if I can build fuel pressure, but it does not seem to have any affect. I replaced the fuel pump 4 years ago with a GMB pump and also just replaced the fuel filter. I'm hoping someone might have some suggestions, and I can avoid having to drop the fuel tank again. Are there any sensors that would inhibit fuel during a cool off period?

Thanks in advance,
Josh

 

[east302]

Hey,

Just to make sure, do you have the 5.3 or 5.7?

Reason I ask is that your title notes it is a 99 with a 5.3 (stock would be a 5.7L in that case, the 92-2000 GMT400 body style for Yukons). The 5.3 came out for the next style in 2000.

For the 5.7, fuel pressure should be 60-66 psi with key on, engine off and hold for a few minutes - say three at least but it may drop a couple of pounds after that. If it starts dropping off a cliff after a minute or two, then I’d suspect an injector leak (which somewhat points to your #3 misfire, were there misfire counts on other cylinders?), regulator leak or fuel pump check valve leak.

Key on, engine off, you should be able to hear the pump run for a few seconds to prime.

Is there any coolant loss and when were the lower intake gaskets last replaced?

The 350 is pretty dumb when it comes to fuel pressure - not a long list of usual suspects and I cannot think of anything that would stop injector flow short of the above or a Passlock issue (Security light doesn’t go off after a few seconds) and that happens within a second or two of turning over.

I wonder if you have a bad connection at the pump. Did your replacement pump require splicing the existing harness? Maybe it’s a bad connection, overheating and shorting out. I know it’s a pain to see, but how do the splices look? Seems like a pump/injector/regulator issue would manifest at first start, not just quit in the middle of things.

Thinking out loud, there.

If you have the 5.3, I’d try posting in the 2000-06 section as they’d be more familiar with what the pressure spec should be.

 

[OR VietVet]

Welcome to the forum. Need to get the fuel pressure readings when this is happening because it is something you are able to duplicate easily.

 

[Cousin_J]

Thanks for the responses guys. 5.3 was a typo it is indeed a 5.7.

So I hooked up a pressure gauge this evening, and was getting around 52psi while at idle. At the point of cut out, the pressure dropped to 15psi and stayed there for the first 10 minutes regardless of if I was cranking or not. I turned on the ignition a few times over the next 30 mins and the pump was gradually building pressure, 15psi, 20psi, then 30 psi. I checked voltage at the fuel relay and have the same voltage as at the battery, I also jumped my fuel relay(pin 87 to 30) just to be sure it was not an issue with the new relay, but same psi. After about an hour I checked again and sure enough pressure was back up to 52psi and it started back up.

I have tried listening for the pump with key on engine off, but cant really hear it running. I just see the pressure gauge instantly jump from 0-50psi when key is turned on. Once primed, the PSI seems to hold well - it only dropped a few psi over the 10 mins I watched it with everything off.

I couldn't really see anything regarding the fuel pump, so I tired to reach up and feel around but nothing was obvious.
I believe the pump I installed 4yrs ago was plug and play, I don't recall splicing. The pump is a GMB aftermarket pump I got of amazon - great reviews but it was cheap. Maybe I get what I paid for.

As for coolant loss, yes it does go through some coolant. I have had the truck 8yrs and the lower intake gaskets have not been changed in that time, I am not sure about the history prior to that.

If PSI is supposed to be at 60-66psi then the fuel pump is probably the issue right? Just seems strange how it runs strong for a short period and then just cuts out.
Assuming I change the pump, any thoughts on the ACDelco MU1777, Delphi FG0089, or?

 

[OR VietVet]

A fuel pump change is in your future.

More Information for GM GENUINE MU1777

 

[exp500]

Surprised it ran at 52#. May also want to scan coolant temp switch (cold start) when new pump installed. Check pump wiring closely.

 

[Cousin_J]

The new pump went in today and that solved the shut off issue. Something in the old pump must have been been causing pressure loss once it got to a certain temp.

Still trying to track down the cause of the p0303 code. After installing the new pump I am now also getting a p300 code in extended diagnostics. Going to change cap, wires, and plugs and see how it does.

@east302 you had asked about coolant loss and lower intake gasket replacment. Would that be a possible cause of the p0303 and now p0300 code?

Thanks for the input guys, guess i should have just waited to post until after i put the pressure guage on it - Ihad convinced myself it wasnt the pump and was something electrical because of how it was cycling.

 

[OR VietVet]

Yes, the coolant loss could be explained by head gasket probs or intake. Time for a compression test at least.

 

[east302]

@east302 you had asked about coolant loss and lower intake gasket replacment. Would that be a possible cause of the p0303 and now p0300 code?

Yes, it could be. How does the oil look?

If you have a good scanner, you can see the misfire counts by cylinder. That would be more telling than the #3 misfire code since a cylinder might show a misfire but not necessarily throw a P030# code.

Try swapping the 3 plug with one on the opposite bank and see if the code follows. Compare plug appearance as well. Then swap the wire if that doesn’t change anything, you know the drill. If it stays at that cylinder, I’d probably toss a new Delco cap on it since it’s easy.

The intake gaskets were probably done just outside of the 36k warranty but if they made it 100k miles I’d be surprised. The problem is that two replacement gaskets could have been used - one is similar to the stock and doesn’t have the metal reinforcement like the better ones so it wasn’t much better. Too, they still sell both today.

See if you can spot any coolant leaks coming from the intake seams at the front/rear and in the valley between the valve cover and intake.

 

[Cousin_J]

So I did a compression test today and went ahead and replaced the plugs and wires since i had them off - So far no new codes getting thrown after about 1hr of run time. It does seem to be running a little smoother.

My scanner is a simple bluetooth obd2 scanner that i view with "incardoc" app, so not very high tech, no specific misfire counts just codes.

If it ends up throwing a misfire code again Ill try what you said about swapping wires and see if it follows.

Its the 1st time I have done a compression test, so I am not sure exactly what to make of the results.
#1 - 208
#3 - 140
#5 - 175
#7 - 200
#2 - 198
#4 - 130
#6 - 165
#8 - 187

#3 & 4 are low compared too, but from what I was reading on some other threads it sounds like 1,2,7,8 may be high?I put a little oil in #3&4 but the compression results stayed the same

As for coolant leaks or coolant in the oil, i havent seen either. The oil has looked clean the last few oil changes.

I picked up some K-seal and was planning to change coolant monday and add then. Hoping compression improves after a few days on 3&4 - any thoughts?