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96 Tahoe Fuel Pressure Issues


mcmyket
09-16-2009, 10:27 AM
1996 Chevy Tahoe, 5.7. About a month ago I started experiencing an issue with starting after the truck was sitting for a while. It gradually got worse taking many tries to start until finally one day it wouldn't start. When turning the key I could hear the fuel pump run for a couple seconds then stop as it should. I replaced the fuel pressure regulator, put it all back together and it started immediately. I parked it overnight and the next day it wouldn't start again. I've hooked up a fuel pressure gauge and when I turn the key the fuel pressure jumps to 60 psi. Usually at this point it will start and while running itís at about 54 psi. After running, the pressure will gradually decline (down to 0 overnight), but it seems to happen over an acceptable time period, not rapidly. This morning I noticed that after turning the key and pressurizing the system up to 60 it dropped immediately down to about 50 psi when I tried to start it and took a couple tries to start. I canít see any visible leaks from the fuel lines and since the pressure regulator is under the upper intake manifold I canít tell if maybe an o-ring isnít set right. Would this indicate the fuel pump is ready for replacement? Iím trying to figure out where to focus my efforts, so if thereís a fuel pressure behavior thatís specific to either the fuel pump or the pressure regulator passing it along is appreciated. It has 100K on the engine and since I bought it used I donít have any history of the pump having been replaced in the past.

Godofwarmike
09-17-2009, 02:24 AM
you may want to replace the fuel filter if it is getting clogged it wont let enough fuel through making the pressure go down.

also there may be crap on the bottom of your tank clogging up the fuel pump strainer.

fuel pressure dropping to 0 slowly overnight is normal usually. All that happens is the warm fuel cools and takes up less room making the pressure go down.

j cAT
09-17-2009, 08:23 AM
the pressures appear normal. the pressure should hold steady for a few minutes key on engine off...when cranking the pressure will drop slightly..

other things to do would be as mentioned the fuel filter...also replace the fuel pump relay,and check the ground connections to the fuel pump in the fuel tank area,,these grounds and relays at this age become a problem...

make sure the battery is good [load test], and the connections are clean and secure..

another possibility would be an injector leaking / dirty...

MT-2500
09-17-2009, 09:09 AM
1996 Chevy Tahoe, 5.7. About a month ago I started experiencing an issue with starting after the truck was sitting for a while. It gradually got worse taking many tries to start until finally one day it wouldn't start. When turning the key I could hear the fuel pump run for a couple seconds then stop as it should. I replaced the fuel pressure regulator, put it all back together and it started immediately. I parked it overnight and the next day it wouldn't start again. I've hooked up a fuel pressure gauge and when I turn the key the fuel pressure jumps to 60 psi. Usually at this point it will start and while running it’s at about 54 psi. After running, the pressure will gradually decline (down to 0 overnight), but it seems to happen over an acceptable time period, not rapidly. This morning I noticed that after turning the key and pressurizing the system up to 60 it dropped immediately down to about 50 psi when I tried to start it and took a couple tries to start. I can’t see any visible leaks from the fuel lines and since the pressure regulator is under the upper intake manifold I can’t tell if maybe an o-ring isn’t set right. Would this indicate the fuel pump is ready for replacement? I’m trying to figure out where to focus my efforts, so if there’s a fuel pressure behavior that’s specific to either the fuel pump or the pressure regulator passing it along is appreciated. It has 100K on the engine and since I bought it used I don’t have any history of the pump having been replaced in the past.

Fuel pressure is to low for that make and model
You need 65 lbs on a cold start.
Specs are 60/66

Even a few lbs low wiil not squirt injectors.

The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold or hot and fuel pressure testing guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak in system.
Post back fuel pressure readings.

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