Ok folks here is the story. 96 Z-28 LT-1 5.7L stock.

New fuel filter, new (2 different ones) fuel pressure regulators. When you turn on the key you get 40psi at the rail which drops to 0 as soon as the pump shuts off.

I blocked the output of the regulator and it will hold 65 psi. If you open the return block...wham back to zero. Hence the reason I tried another fuel pressure regulator, the first new one came on the car and this problem is why it was being sold.

What else can make it drop pressure? If I block the regulator it holds fine, can the system evaporater etc. cause this kind of problem. As of right now I don't know enough about how this closed loop system works.

Thanks in advance.

Jmt

 

If you are asking if the EVAP system - carbon canister in the driver's rear fender to collect hydrocarbon vapor venting from the tank - can cause the fuel pressure to drop to "0", the answer is no.

"Closed loop" generally refers to the A/F ratio control system, which is the PCM using feedback from the O2 sensors to tweek the injector pulse width. That also would have nothing to do with the fuel pressure.

If by "closed loop" you mean the recirculated fuel sytem, there's not too much to it. Pump runs the fuel through the rails in series, starting at the back of the passenger side rail, with the FPR on the outlet end of the driver's side rail. FPR is a self-actuated pressure controller. Return line runs to the tank.

Have you considered the possiblity of a leaking injector? Pinching the return line may build enough pressure (65psi) to hold the "ball" in the injector firm against the seat. But suddenly releasing the return line may allow the pressure to drop fast enough to unseat the ball, allowing the injector to leak. Might also have a similar affect on the check valve in the fuel pump. May stay seated at 65psi, then release when you release the return line blockage. Just a thought.

 

Thanks Fred,

I thought the whole problem is why the car would not start. On a wild hair I checked the spark and guess what....no fire. I noticed the optispark had been replaced and when inspecting the coil the plug lock is broken along with the one to the air pump. I reseated them and the car starts now, but the fuel pressure is still acting strange. It runs around 30 psi when the motor is running, and drops to zero when you shut off the ignition.
I did find the hose with the plastic valve going from the canister to the tank was off. I put a clamp on it but the system acts the same. I don't think the injectors are leaking because there is no fuel smell (rich) and the plugs look black and worn out but not wet? There is also evidence that the fuel tank has recently been pulled but I'm not sure if the pump was replaced. I guess I don't understand this check ball in the fuel tank. Are you saying there is a check ball that stops back flow? What's confusing is when I took the return line off the FPR after the fuel pump stops running it leaks the pressure off with gas pouring out. Plugging the return line makes it hold pressure....Thats why I thought it was bad, but two new ones means its not.
I still don't know whats going on, I may compare readings to my other z and see what it does for a comparison test.

PS: Do you know how to adjust the throtle cable slack out of this contraption? It has an electric motor with a bunch of cables going in and out. The problem is it won't yank the throtle closed sometimes and after looking at the gas pedal conection I have about an inch of slack. The original owner put a huge spring on the throtle body and you have to be Godzilla to push the gas pedal down....I don't like amatuer fixes like that....LOL!
Thanks!

Jmt

 

The big black lump on the driver's side valve cover is part of the traction control system. I don't know anything about the linkage from there to the throttle body.

 

The throtle cable runs to it, and then from it to the TB. It says on it to read service manual to adjust cables?