Hard Starting - no fuel pressure

[iroc343]

I have a 94 caprice with the 5.7 mpfi engine. The problem is fuel pump related. Through investigation it appears that whatever momentarily runs the pump when the ignition is first turned is not working. If I use the test plug and run the pump prior to cranking it starts every time. If you let it set overnight and the pressure bleeds down you have to force the pump to run, using the test plug, or crank it until the oil pressure switch closes and energizes the pump. What tells the pump to run when you first turn the ignition key? Also does the pump run continuously? I thought it only ran until pressure reached 40 or so psi. If it doesn't, what electrcally monitors the fuel pressure? Oh yeah, the fuel pressure is 40-45 psi. and the relay is new and all fuses are good. Any help would be appreciated, this is driving me nuts.

[j cAT]

Originally Posted by iroc343

I have a 94 caprice with the 5.7 mpfi engine. The problem is fuel pump related. Through investigation it appears that whatever momentarily runs the pump when the ignition is first turned is not working. If I use the test plug and run the pump prior to cranking it starts every time. If you let it set overnight and the pressure bleeds down you have to force the pump to run, using the test plug, or crank it until the oil pressure switch closes and energizes the pump. What tells the pump to run when you first turn the ignition key? Also does the pump run continuously? I thought it only ran until pressure reached 40 or so psi. If it doesn't, what electrcally monitors the fuel pressure? Oh yeah, the fuel pressure is 40-45 psi. and the relay is new and all fuses are good. Any help would be appreciated, this is driving me nuts.

when you place key to ignition on,,,,the fuel pump runs 2 sec.,,,unless the engine sends to the PCM a signal,,,then the pump continues to run...the pcm does not monitor any pressures,,and because of this when the fuel pump goes bad no DTC's...

with the engine running 45 psi is good,,,when you shut the engine down how long does it stay at 45 psi...if it falls rapidly this is very much possible that the fuel pump is bad,,,or your injectors are leaking down,,,the fuel regulator can do this as well as any other fuel leaks...

when the pressure guage is connected that pressure should rise very fast ,when first starting,,if the pressure takes too long the filter may be plugged,,if that is new then the pump/poor voltages to fuel pump...connections to pump/ground as well..

[iroc343]

That's the problem, the fuel pump doesn't run for the 2 seconds when you first turn the key on. From what you have explained it must be something in the pcm. The fuel pump holds 40 psi for several hours after you shut it off, so I don't think it's the pump. So does the fuel pump run continuously while the engine's running? Thanks

[silicon212]

Originally Posted by iroc343

That's the problem, the fuel pump doesn't run for the 2 seconds when you first turn the key on. From what you have explained it must be something in the pcm. The fuel pump holds 40 psi for several hours after you shut it off, so I don't think it's the pump. So does the fuel pump run continuously while the engine's running? Thanks

First, the pressure should be in the 55PSI range. Second, there's a fuel pump relay circuit that is what runs for 2 seconds until the PCM shuts it off (in absentia of a tach pulse from the distributor). If this relay is bad, then you can have problems getting it started, but if the fuel pump is putting out 40-45PSI then this is where I'd look. Check all grounds, including the ground wire coming from the fuel pump, to make sure they're solid.

[disignition]

Hello, I looked up the fuel pressure spec in several databases, key on engine off shows 40--47, engine idling 30--44. There is also an oil pressure switch in the fuel pump circuit, once you crank it enough to bring up oil pressure and the switch closes, the pump will run regardless of the relay. With no initial prime, you may have to check the relay circuits. You don't need any rpms to get the initial prime. Do you have good spark at the plugs and injector pulse, engine cranking?

[j cAT]

Originally Posted by disignition

Hello, I looked up the fuel pressure spec in several databases, key on engine off shows 40--47, engine idling 30--44. There is also an oil pressure switch in the fuel pump circuit, once you crank it enough to bring up oil pressure and the switch closes, the pump will run regardless of the relay. With no initial prime, you may have to check the relay circuits. You don't need any rpms to get the initial prime. Do you have good spark at the plugs and injector pulse, engine cranking?

ya I agree my 96 has the same pressures you have given key on engine off its been 45 psi since 1996..and the book says 41-47 psi...

I don't have the wiring diagrams to the 94...so someone may have this to help locate the problem...

the fuel pump with engine running ,,,runs continuously...excess fuel is returned to the tank...

[iroc343]

From what I'm reading I've come to the conclusion it must be something wrong with the pcm. I can force the fuel pump to run with both the test wire and by jumping the oil pressure sending unit. Once it starts it runs fine until you let it set overnight and the fuel pressure bleeds down. I can get a reman'd pcm for about $100, if I put the prom in from the original do I have to have any additional programming done?

[j cAT]

Originally Posted by iroc343

From what I'm reading I've come to the conclusion it must be something wrong with the pcm. I can force the fuel pump to run with both the test wire and by jumping the oil pressure sending unit. Once it starts it runs fine until you let it set overnight and the fuel pressure bleeds down. I can get a reman'd pcm for about $100, if I put the prom in from the original do I have to have any additional programming done?

with the original e prom you should have no programing required ..


before replacing make sure the connections at the PCM are clean...recheck all grounds in the engine compartment ,,,then if its all good based on my 1996 diagrams and troubleshooting guide the PCM is suspect...

[manicmechanix]

Yeah it sounds like the problem is in the wiring between the fuel pump relay and ECM, or the ECM itself.

[CD Smalley]

Isn't there a ground post that can melt on these cars near the fuel pump relay and cause these problems????

[iroc343]

Originally Posted by CD Smalley

Isn't there a ground post that can melt on these cars near the fuel pump relay and cause these problems????

Already been through that. I think you're referring to the positive terminal located by the junction box that houses the fuel pump relay. I had to replace that years ago when it wouldn't pass enough current to run the starter.

[CD Smalley]

That's it! Have you looked at it again? Just a thought.