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GMC 5.3 stalling - hard starting

posted Jun 08, 2016 23:59:47 by DAVE HALVERSON
I've got a 2000 GMC Sierra Z71 5.3, hard starting, stalling for the last couple years. about 130,000 on it. I've put 3 new fuel pumps in the last year, 2 cheap ones and finally a AC Delco a couple weeks ago. new pressure reg, cam, crank , MAF, temp, TPS, idl,sensors,new alt and battery. new heads at 120,000 and still have this problem off and on. it might be fine for 3 or 4 months, then out of nowhere starts all over again. everyone kept telling me bad pump and reg. so when it still did it with the ac pump( i also got a new computer or a reprogramed one) i put in a after market pressure reg but fuel presure was still only about 43psi. so i thought i would put a s.s. ball valve i had off one of my bikeson the return line to throttle the flow going back to the tank. it works great, i have it set at around 68 psi. So what i havent fig'd out is why. I mean i know how it's working, but what is the problem causing it to do this in the 1st place. Thanks, Dave
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6 replies
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RobBrown said Jun 09, 2016 15:01:37
Sorry for your trouble. Sounds like you've found a temporary fix, though.
Were you getting 43psi with the engine running? Or was that the pressure after pump prime, engine off? Were you seeing any pressure bleed down after the KOEO prime? What was pressure with engine running with vacuum to the regulator attached and then unattached?

I think some more specifics would help narrow down where the issues lie.
Just a dude who found a sudden and unexpected passion to bless folks who can't afford auto repair with free help. I never, ever imagined this is how'd I'd be spending my free time:-)
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AndyMacFadyen said Jun 09, 2016 16:37:24
To my eyes it really points to the regulator sticking or leaking.
"Rust never sleeps"
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RobBrown said Jun 09, 2016 18:56:38
That's what I wonder about, too, Andy. And a pressure leak down test and test with vacuum hooked and unhooked from the regulator would make that clearer. Don't know what the exact specs are for this vehicle, but in general you should see a 10psi pressure INCREASE with the vacuum line unhooked from the regulator, right? If the regulator were stuck open, you wouldn't see any increase in pressure (or a very small increase) when the vacuum supply was removed from it. Of course you'd have to remove the check valve you've placed in the system in order to get accurate readings on this test.
[Last edited Jun 09, 2016 18:59:14]
Just a dude who found a sudden and unexpected passion to bless folks who can't afford auto repair with free help. I never, ever imagined this is how'd I'd be spending my free time:-)
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RobBrown said Jun 09, 2016 18:57:30
I suppose I'm also concerned about the power and ground feed to the pump and any extra resistance that might be in that circuit causing it to be underpowered.
[Last edited Jun 09, 2016 18:58:05]
Just a dude who found a sudden and unexpected passion to bless folks who can't afford auto repair with free help. I never, ever imagined this is how'd I'd be spending my free time:-)
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DAVE HALVERSON said Jun 10, 2016 00:13:33
I'm going to double check everything and get back to you. that was 43 psi with eng running and now about 62 with eng running. I've checked and cleaned all grounds, new connectors etc.That was just a ball valve i put in not a check valve, so i can adjust the flow on the return line. I'll get back to you guys, its just been a long day haven't had time to get back to it. thanks so much for your input guys.
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RobBrown said Jun 10, 2016 14:50:15
Hey Dave,
If you get a chance, this video is very helpful.
https://www.youtube.com/watch?v=RyEPOeQB0Zg
At around the 25 minute mark, Danner goes into the process for checking for a stuck-open regulator. You probably don't have the relay bypass tool he has, but you could perform the same test from the driver's seat with somebody watching to be sure fuel isn't spraying everywhere.

So the numbers that will be helpful are (sorry if you already have this down!):
1. KOEO max pressure (probably will take a couple key cycles to reach this after you've attached your gauge)
2. Bleed-down rate (does the pressure hold steady with KOEO or does it drop?)
2. Idling pressure (with regulator vacuum line attached)
3. Snap throttle pressure/behavior
4. Idling pressure with regulator vacuum line UNattached
5. Dead-head pressure (max koeo pressure with return line completely pinched or blocked (can your ball valve totally cut flow? If not, just pinch the line with some vice-grips like in the video.) If the pump is good, it should be able to produce 2x running system pressure.

On a side note, have you recently changed the fuel filter? Is the sock strainer on the fuel pump new and clean? Any concerns about fuel line obstruction?

And to be honest, the fact that you're essentially doing a modified "dead-head" test by increasing pressure with a ball valve in the return line would suggest to my un-trained mind that the issue is with the regulator...it's obviously allowing fuel to pass through itself well before desired system operating pressure is reached.

[Last edited Jun 10, 2016 14:56:33]
Just a dude who found a sudden and unexpected passion to bless folks who can't afford auto repair with free help. I never, ever imagined this is how'd I'd be spending my free time:-)
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