5.3 LS fuel pressure drops/ hard cold starts!

[Sonar Chief]

I would like some advice on my truck 5.3 based 1500 GMC seems the fuel pressure is inadequate to start the truck after it sits overnight! I rented a fuel system pressure tester. With key off, engine warm, fuel pressure is 59 psi and it starts right up w 55 psi running at idle. Turn the truck off and the pressure drops right away, within 15 minutes (went in the house and came back out) the gauge read 0 psi. GM bulletin says if it loses 5psi in 3 minutes to TS pump assembly ... or vice versa!

Cold starts ... fuel pump kicks on, and the starter just turns (like no spark) off/ on/ start multiple time. FIGURED OUT if I prime the system on/ then start w the accelerator pressed down the truck would start (blow excessive fuel out the exhaust) and idle normal and run just fine for the rest of the day warm. hmmmm!

Knowing now this is a return-less fuel system seems the fuel pressure is regulated by the ECM via the pump and there is a check valve inline that is supposed to keep the pressure at the rails. There are no codes on the SES and the truck runs just fine, no stumbling, or rough idle ...

There is no leaking externally, and based on the pressure drop rate I wouldn't expect fuel injectors, assumption good or bad? Seems GM put everything associated with the fuel system in the Fuel Pump Assembly ....grrrr.

Anything else I should check before I drop the tank and replace the pump assembly? Is the fuel pump filter part of the assembly?

Any help would be appreciated!

Michael

[NAPA 68]

It sounds like the check valve in the fuel pump is bad. You've got to replace the FP module.

Happy trails!

Tim

[Sonar Chief]

That's exactly what I came up with ... just wanted another opinion!

Thanks,

Michael

[Sonar Chief]

Replaced the fuel pump assembly yesterday and the pressure at the fuel rail is good and there is no pressure drop when keyed off! The truck starts just like its supposed to ... guess I did something correct!

The good news is I was offered the use of a lift at the garage that ordered the pump ... I took advantage of the offer, as there is 6 new inches of snow on the ground and its cold here, which I didn't want to lay in to drop the tank!!!

Go figure ... the part number the rep listed for my truck was wrong (engine size, VIN # and bed length metrics), the good news is the garage ordered another pump from CA for an ext cab w the longer bed and it was exactly like the part we took out.

Michael

[Che70velle]

That's a win! Good for you!

[Sonar Chief]

Well evidently the problem still exists!!! Thought I'd fixed the problem w the new fuel pump assembly but it's still acting up ... same problem on cold starts. If I don't pump the accelerator the engine just turns over like there is no spark ... gonna take it back to the garage Tuesday and check fuel pressure at the rails.

Any one have any thoughts? I am gonna let it sit overnight and run a video tomorrow of the cold start up ... see what happens!

Michael

[Efi69Cam]

Sticking open injector? I know you said returnless, but not all 5.3s are. The older ones have a failure prone regulator on the rail. What year is the truck, I know my 2005 is returnless, but my bro's 2000 has a return.

[Sonar Chief]

Sticking open injector? I know you said returnless, but not all 5.3s are. The older ones have a failure prone regulator on the rail. What year is the truck, I know my 2005 is returnless, but my bro's 2000 has a return.

Efrain ... the truck is the new body style 2007, there is no regulator on the rail. I have had several mornings where is has failed to start w just touching the key to start, just turns over and no start unless I give it some gas ....BUT now it has started every time for the last week ... grrrr.

I did have the engine light come on last weekend for the day but then cleared, don't know the code yet. The garage has been busy and I don't want to press him for using the garage as a favor ... you know.

Suggestions?

Michael

[Efi69Cam]

Efrain ... the truck is the new body style 2007, there is no regulator on the rail. I have had several mornings where is has failed to start w just touching the key to start, just turns over and no start unless I give it some gas ....BUT now it has started every time for the last week ... grrrr.

I did have the engine light come on last weekend for the day but then cleared, don't know the code yet. The garage has been busy and I don't want to press him for using the garage as a favor ... you know.

Suggestions?

Michael

Try swapping the fuel pump relay.

Auto zone can pull the codes. Look for any codes related to fuel rich or lean

[Sonar Chief]

Efrain ... I listen for the relay every time I start the truck, I don't turn the key until I hear the relay cut off ... it's always come on and clicked off!

I just thought of Auto Zone myself, I really need to leave it at the garage and let it sit overnight and then check the fuel pressure at the rail again.

Michael

[Efi69Cam]

Efrain ... I listen for the relay every time I start the truck, I don't turn the key until I hear the relay cut off ... it's always come on and clicked off!

I just thought of Auto Zone myself, I really need to leave it at the garage and let it sit overnight and then check the fuel pressure at the rail again.

Michael

I had one go bad, it would click, but had high resistance across the switched terminal. It's an easy check, swap it for another in the fuse box that's for a non crucial function.

[Sonar Chief]

I had one go bad, it would click, but had high resistance across the switched terminal. It's an easy check, swap it for another in the fuse box that's for a non crucial function.

Thanks for the info ... I'll do that in the am and see what happens.

[Sonar Chief]

Just an update ... ordered a new relay (it's sitting in the glove box) filled the truck w premium gas and ran it on the highway to Denver and it's never acted up again .....hmmmm! :relax:

[cjsgarage]

Michael,
Those throttle bodies get gummied up a bit too. I clean them with a rag and a little solvent and it makes a huge difference. Be sure to shoot some solvent through the idle bypass hole.

[Sonar Chief]

Michael,
Those throttle bodies get gummied up a bit too. I clean them with a rag and a little solvent and it makes a huge difference. Be sure to shoot some solvent through the idle bypass hole.

I haven't touched it in quite some time, seems some good gas did the trick, she fires up like she always did ... no hesitation. I just made the trip out here to CA 1280 miles or so hauling the Camaro on a U-haul and she never missed a beat.