Need some GM 5.7 help

Topic: Need some GM 5.7 help (Read 1250 times) previous topic - next topic

Need some GM 5.7 help

August 01, 2012, 10:12:58 AM

2 days of hard to start, after 6-8 cranks can get to start most of the time. Has worsened now and takes 10 or so, if even then.

My fuel pressure at priming is 52-54 psi. When I crank it drops to 44-45 ish.

I dont think that is enough to keep the poppets open and fire. How do I troubleshoot the problem, as in is it FPR or fuel pump.

Both pump and regulator are original on a 96 vortec with 158K.

Need some GM 5.7 help

Reply #1 – August 01, 2012, 10:08:38 PM

Make and model of car please. Sounds like FP is to low. Monitor the pressure and see if it climes then starts. Also does it pre prime while keyed and time out. Try cycling the key a few times. If this makes it easier to start either the bypass timeout pump circuit is defective or the pump is shot. The FP relay can also be shot and the engine is running on the oil pressure bypass ckt. Some were wired this way. Also some of those 5.7 use the oil pressure switch for shut down safety and when the relay fails you get long cranking times till the oil pressure builds and activated the pump. That is why i need the make and model of the vehicle!! Make sure the pump PRE PRIMES and the FP is correct. Good luck

Need some GM 5.7 help

Reply #2 – August 02, 2012, 08:35:11 AM

Thanks Tom.

It is a 96 Silverado 5.7 4x4

Pump primes fine. When it primes pressure is 52-54, stays at that no matter how many times I cycle it.

It doesnt help the start condition to cycle it. (pressure doesnt build)

Pressure drops to 44ish as soon as the engine cranks.

I logged my pressure about a year ago and it would be 61 psi when priming, so its slowly tapered off.

Should I buy a relay and throw at it?

Need some GM 5.7 help

Reply #3 – August 02, 2012, 07:24:29 PM

If the FPR is bad you usually have fuel leaking out when you pull off the vacuum line connected to it. Quick and easy enough to check.

Need some GM 5.7 help

Reply #4 – August 02, 2012, 09:06:36 PM

Quote from: Sinista Chicken;395291

If the FPR is bad you usually have fuel leaking out when you pull off the vacuum line connected to it. Quick and easy enough to check.

On this truck, you have to pull the manifold to get to the FPR. It under the manifold.

Need some GM 5.7 help

Reply #5 – August 03, 2012, 04:53:48 PM

Quote from: mcb82gt;395297

On this truck, you have to pull the manifold to get to the FPR. It under the manifold.

My bad, been a while since I have looked under the hood of a chevy truck. I guess that's not so quick to check.

Need some GM 5.7 help

Reply #6 – August 04, 2012, 10:05:24 PM

This is what all data says the pressure should be. It seems like your FP is low.

Wrap a shop towel around the fuel pressure connection to absorb any small amount of fuel leakage that may occur when installing the fuel pressure gauge. Turn ON the ignition, the fuel pressure should be 415-455 kPa (60-66 psi). This pressure is controlled by a spring pressure within the regulator assembly.

Need some GM 5.7 help

Reply #7 – August 04, 2012, 10:53:23 PM

So... your telling me. Pull the manifold and replace the regulator?

Need some GM 5.7 help

Reply #8 – August 05, 2012, 07:03:29 AM

It seems to have low FP as described by ALL DATA. You also confirmed that you had a higher reading some time back?? So if the pump and (pulsator if used in this car??) are OK and their are no internal leaks on the supply side the regulator may be an issue. Their may be a place in the systems return line that has a length of rubber hose you can slightly pinch off and see if the pressure rises as a test!!! But that has to be dun very carefully. Or you can spike the pressure way to high. Question? Is the pump new and is it getting the proper voltage. And if new was it installed recently. If so i would think the regulator is shot. What else could it be!!

Need some GM 5.7 help

Reply #9 – August 05, 2012, 04:13:27 PM

Put new pump assembly in, fired right up with a psi of 62. Thanks everyone.

Need some GM 5.7 help

Reply #10 – August 05, 2012, 07:07:24 PM

I'm glad you got it all lined out, Good Job!

Need some GM 5.7 help

Reply #11 – August 06, 2012, 07:26:17 PM

Ok that is good. Those engines need 60Lbs or more. Just a few pounds shy and bingo hard or no start!!