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Gm Tech (Cam)
Gm Tech (Cam), GMC Mechanic
Category: GMC
Satisfied Customers: 1685
Experience:  GM Grand Master Technician 2007. 14 year experience.
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It is very apparent that the fuel pump is not cycling on

Customer Question

It is very apparent that the fuel pump is not cycling on for 5 seconds when the key is turned on, in a 1999 BRAVADA. Starts and runs fine with a little starting fluid in the intake. New fuel pump relay.
QUESTION: What powers that function, and what times it?
Is it in the pump itself? the ECM? or ???
Submitted: 1 year ago.
Category: GMC
Expert:  Gary replied 1 year ago.

I am sorry for the long wait

once it starts with starting fluid the truck runs fine ?

no codes are being set ?

Customer: replied 1 year ago.
It runs fine after starting. It will start on occasion with 2 or 3 short tries once warmed up, but long cranking does no good. Don't know about codes. The biggest question is could replacing the fuel pump assembly take care of the problem of the pump not running when the key is initially turned on. I've heard of people banging the tank to get it going, with success, but have not tried that. I would like to know exactly how the system operates. When the key is turned on, the fuel pump is powered for X seconds HOW? Once running, I assume that it is powered through the fuel pump relay activated by an oil pressure switch (which shuts it down when the engine is stopped). Somehow, this is bypassed for initial start up. How? Is there a second power feed to the pump for this? (which means the problem could be a bad connection or something else)
Or is the fuel pump relay activated by something other than oil pressure for those X seconds providing power to the pump? (Which means replacing the pump will do no good). And what regulates that?
I am not looking for analysis of the problem, and how to solve it, as much as getting the details of how the system is designed to work.
Thanks, Jim
Expert:  Gary replied 1 year ago.

as soon as the the key is turned ON the PCM will prime or run the pump for 2 seconds then shut off then run when cranking over the engine

when it is acting up with the key OFF if you put your fingers on the fuel pump relay then turn the key ON do you feel the fuel pump relay click on for 2 seconds then click off ?

Customer: replied 1 year ago.
Hi, just got your answer...
Expert:  Gary replied 1 year ago.

no problem