So today i decided to test the fuel pressure with a battery boost so i parked the camaro beside the truck and hooked up the cables...before hooking them up i was getting the usual 48psi..then once i had the boost going for about a minute, 2000rpm on the camaro i tested the fuel pressure and it was 60psi...the truck started fine. Battery still 12.4 off and 14.4 running. Fuel pressure around 50-52 running, raises when you get on the throttle. Took it for a drive... ran fine. Pump sounds even and strong from kneeling under the car running.

Turned it off... pressure stayed in the 50's.

Key back to run, pressure up to 62, key off, presure held.

Truck starts fine.

i'm going to try to start it and read the pressure every hour for the rest of today if i remembe rto see when it starts to get below 50 at key run.

Is there any other tests I can do or is this just the pump on it's way out?

 

See post #3 for 1999 CSFI fuel pressure specs. Fuel pressure and leakdown must be checked when the engine is cold. GM does not publish any "engine running" fuel pressure specs for the CSFI system, there are too many variables involved.

 

Okay so each time I went out after an hour... turned key to run, 50 psi then key off 55psi truck started. Next hour, 48 psi at run turned off and 55psi. Truck started. 2 hours later, 40 psi at run turned truck off 50 psi...truck no longer starts.

WTF????

 

If the pressure with the key in the 'on' position (not 'start') was showing 50 then 48 then 40, I'm going to have to say that the pump is bad.

 

I just dont' get why a minute with a battery boost will get it running fine....even though the battery in the truck tests out fine.

It couldn't be a ground to the pump or something creating resistance??

 

Have you checked the voltage at the pump? Or the ground continuity? It is quite possible that a bad ground is the cause of your problem, but you will not know without testing.

 

Honestly no, I wouldn't know where to look or what to do. I am useless troubleshooting electricl issues

 

Don't worry about that, we all were/are when it comes to that stuff. Electrical issues are one of the most complex and tedious (at the same time) things to troubleshoot given the number of things which can cause them.

You really need to have some back probes to stick into the connector while it is connected so that you can read the voltage. If it were me, I would back probe the ground and power. This will give you the actual voltage that the pump is seeing. If less than battery voltage, you have to determine if it is in the positive supply side or in the ground side.

If you disconnect the negative lead and put it to a known good, clean ground source and the voltage pics up, you have a bad ground. Alternatively, you can leave the ground attached and take the positive lead back to the battery positive post. If you do not read full battery voltage, you have a compromised ground.

 

Okay that makes sense...can you give me an idea where the connector to the pump is? I can get at it without dropping the tank?

thanks!

 

Bad part is that the one you should be testing is what is at the pump. If you have a body lift or VERY skinny arms, you may be able to get at it without dropping the tank a bit. If not, you will have to drop it down to access the connector.