redrummer

Here's the rundown...
Car starts, runs for roughly 5 seconds, then dies. It will keep running if I give it gas. It will sputter and die 5 seconds after taking my foot off the gas. Once in a blue moon, it will fire up and run like there's nothing wrong at all. It's only been driven once in the past year and probably twice in the past two years.

Here's everything that's been replaced in the past three years...
fuel pump (replaced with Holley 255)
pulsator (just did it while I was in there)
fuel filter
oil pressure switch (replaced for the heck of it)
fuel pressure regulator (old diaphragm was torn)
spark plugs (short plugs for headers)
plug wires
knock sensor (connector on old one was broken)
ESC module (just because)
fuel pump relay (just because)
MAF relay (just because)
MAF burnoff relay (just because)
MAF (was bad. car wouldn't run with it plugged in)
distributor (magnets were cracked)
pickup coil
ignition module
ignition coil (just because)
IAC
TPS

I get spark, fuel, and air. The only problem I can find is the fuel. Before the fuel pump swap, the fuel pressure would jump so fast between 40-60psi that you couldn't see the needle moving and it was constantly moving. After the swap, it holds steady when the motor runs for the five seconds it actually does run.
Now the problem is that when the pump primes, the gauge reads about 80psi. When it's done priming, it reads about 45psi, which bleeds down to less than 20psi in under two seconds. I pulled the fuel rail tonight and put white cardboard under the injectors and pinched the return line shut. It still bleeds down at the same rate. The injectors, cold start valve, and pressure regulator are all holding shut like they should. I'm not getting as much as a drop out of anything. I also can't find where gas is leaking out of anything. The rear of the car is up in the air still, so I am able to check all the hoses and connections.
When I hold down the gas, the gauge reads 45psi and doesn't budge. As soon as I let off, it sputters, dies, and immediately drops down to 20psi again.

The computer was swapped out with another known working one about a year ago. I'm wondering if I should do this again since I got the new fuel pump in. Some people are suggesting that the computer is bad or the PROM is messed up somehow.

Can anyone give me any more insight? Is there absolutely anything else that would make a car run then die after only running for five seconds?

Nixon1

You definitely shouldn't be dropping pressure like that.... You mention 20 a lot...does it drop to 20 and stop?? Or does it bleed all the way down to zero.... There should be some kind of one-way valve in the return-side of the system to hold pressure I believe.....but I'm not 100% sure. Just trying to brainstorm with ya. It'd be awful odd for it to bleed down and then just stop at a given pressure.

redrummer

Yeah. It bleeds down to 20 and holds. That's what I can't figure out.
I keep reading about a fuel tank check valve, but I can't find any info on it whatsoever when I do a search. If you pinch the return line shut and it bleeds down, all my info says the check valve is bad.

RednGold86Z

iTrader: (1)

I'd try another fuel pressure regulator if I were seeing the same data. Those pressure readings sound a little fishy at low MAP. Not because of the bleed down, but because of the low pressure at closed throttle. If you have an adjustable one, I'm going to suggest that you just raise the pressure by at least 10psi, and check again, because your gauge may be as accurate as a sundial.

redrummer

This is a brand new regulator.
I'm using two different gauges and getting the same readings, so sundial accurate it isn't.

RednGold86Z

iTrader: (1)

I think I read your message wrong, it doesn't go to 20 *while still running, does it? That means it's probably not a fuel supply problem, more like a fuel delivery problem, or perhaps it's just the throttle is too closed and the new IAC is stuck shut or something. Does it run with just a little throttle, or do you have to have it floored?

It's strange that it reads 80psi while priming though.

Which year computer did you swap with? Did you use an '89? If so did you disconnect the cold start injector wires?

Red1992V6Rs

If your car has a Vats Key it could be the ignition switch

John 89 Formula

I could be wrong here, but I believe the fuel line check valve is built in to the pump.
You may also have a defective pulsator valve, or maybe it is just misaligned and the fuel presssure is bleeding off through it.

Is there a long crank time before it starts?

redrummer

Nope, it starts on the first turn of the starter.

kevinc

The pumps I've seen had a check valve in the outlet to hold line pressure when the pump shuts off.

Given your findings thus far, I wouldn't say the pressure loss on shutdown is a problem. It will take longer to build pressure at the rail given the bleeding, but that's about it.

Get your pressure dialed down to 45psi or whatever during priming, very likely that 80# you're seeing is what it's running at and it's loading the engine w/ fuel.