fuel pump pressure

[playsdixie]

weird one....was going to tear down the engine on my 85 and rebuild...decided to take one last run....car started/idled/ but wouldnt go.....threw a code 34....maf....did some diagnostics couldn't get the maf wire to glow with test leads...determined burn off probably bad....BUT....also threw the fuel guage on,,,,o pressure with key on, 20 # after start up...idles /won't drive....did further diagnostics....the fuel pump will turn on with 12 v to the fuel pump test lead...but still only 20#...fsm doesn't cover this issure....how come the pump works but wont come on with the key???....is something in the maf/burnoff circuit affecting this??? why only 20#...fuel pump is new...2 months old...:ugh

 

[gmjunkie]

weird one....was going to tear down the engine on my 85 and rebuild...decided to take one last run....car started/idled/ but wouldnt go.....threw a code 34....maf....did some diagnostics couldn't get the maf wire to glow with test leads...determined burn off probably bad....BUT....also threw the fuel guage on,,,,o pressure with key on, 20 # after start up...idles /won't drive....did further diagnostics....the fuel pump will turn on with 12 v to the fuel pump test lead...but still only 20#...fsm doesn't cover this issure....how come the pump works but wont come on with the key???....is something in the maf/burnoff circuit affecting this??? why only 20#...fuel pump is new...2 months old...:ugh

My guess the pumps bad,it's probly running for 4-6 seconds at key up but not building enough pressure in that time!! When you jump it through the test lead it don't shut off so it's building 20 lb,Not near Enough! 85-91 L98's should build 34-39 lb's at key up!

 

[playsdixie]

My guess the pumps bad,it's probly running for 4-6 seconds at key up but not building enough pressure in that time!! When you jump it through the test lead it don't shut off so it's building 20 lb,Not near Enough! 85-91 L98's should build 34-39 lb's at key up!

pumps only 2 months.....comes up pretty quickly when I jump it..goes to maybe 30 and quickly drops to 20

 

[gmjunkie]

pumps only 2 months.....comes up pretty quickly when I jump it..goes to maybe 30 and quickly drops to 20

It might have a leak in the rubber hose in the tank and be bypassing back into the tank,It happens!

Still could be a bad pump,I seen new pumps not last a week before!

 

[playsdixie]

It might have a leak in the rubber hose in the tank and be bypassing back into the tank,It happens!

Still could be a bad pump,I seen new pumps not last a week before!

bought it from ecklers...probably a piece of crap....i'll get a better one if that's the problem....still don't see why o pressure with key...should be some....the minute you jump it the pressure comes up...:confused

 

[playsdixie]

ok...getting wierder....now with key on...pressure runs up to 40 but quickly drops to 0 with a distict click from the fuel pump relay....that's new too....???? also, no strored codes anymore....:ugh

 

[playsdixie]

got around to pinching return fuel line ....pressure goes quickly to38...then 20, then 0 at the click of the relay...fsm indicates bad injector/cold start valve....that would have to be some leak to make the pressure drop that fast...i'm thinking has to be the fuel pump...if there's a check valve, it must be internal, I didn't see one when I installed this pump 3 months ago...the car will begin to start with the 38* spike, but not run...0 pressure....what do you think???;help

 

[Hib Halverson]

A leak that large to the outside would have been obvious so there's either an injector with a significant leak, a bad check ball in the pump, a defective pump or maybe a bad fuel pressure regulator but....rather than doing a lot of guessing and, perhaps, needless parts replacements, my advice would be to continue to use the FSM's diagnostic information.

Also, just in case the manual you have is an aftermarket (ie: Chilton, Haynes) and not the GM book, my suggestion is to get the GM book.

As for the pump not running when you key-on/engine-off, I'd verify that's true. I suggest that because when you KOEO, the pump only will run a second or two until the engine detects no rpm then turns it off. With a big leak, you may only see 20-psi in that sitch. It is, in fact, non-op with KOEO but runs when the test connection is used, you want to check both the relays which control the pump.

 

[playsdixie]

I have the gm manual...all thats left is to check injectors/cold start....since this happened so suddenly, just don't think an injector could leak that bad. pressure drop with return line pinched eliminates the fpr...I'm going to pull the fuel pump out and check it out. I didn't pinch the pressure line per manaul to test pump because pressure drops before I can pinch the line...this is murphy's law...I have a new manifold/fuel rail/injectors /egr/fpr/plenum/runners/throttle body...all achrome..ready to be installed...wanted to take it for one last run...wouldn't run right.....want it to run right before I start....so I know who to blame if it doesnt run when i'm done.... i'll bet something stupid at the fuel pump or connections there....i'll let you know... weather permitting...s floida...

 

[playsdixie]

It might have a leak in the rubber hose in the tank and be bypassing back into the tank,It happens!

Still could be a bad pump,I seen new pumps not last a week before!

you might be right.....had the war dept turn the key on so I could quickly pinch the fuel pressure line...held pressure until I released.....going right back into the tank....considering the tries at fixing the old sending unit..3..1 fuel pump exchange , i;ve gotten pretting good at pulling the pump and sending unit...it's either the pump or the moron who installed it....wish I could find that guy....

 

[playsdixie]

end of this thread....pump didn't always turn on because I wasn't aware of the 10 second delay...followed the gm fsm to the bitter end...last box/ bottom of the page...loose rubber connector on the fuel pump....car now runs like a champ...

 

[roger longman]

Wasn't that 'gmjunkies' sugestion ?.....Roger.

 

[playsdixie]

Wasn't that 'gmjunkies' sugestion ?.....Roger.

absolutely!!!! he the man

 

[roger longman]

Yup, he certainly is......Roger.