Hi guys,

After a chilling no-start incident on a labor day weekend, 100 miles away from home, I decided to learn more about my fuel system and check out what's going on. Sorry for the long post, the next paragraphs summarizes it all, I just want to give you as much data as possible.

As soon as I shut my '92 TBI Camaro off (see sig), my fuel pressure drops to 3 psi. A minute after that it's virtually 0.

The fuel pump primes it to 15 psi, but if I don't start the car immediately, the pressure drops as soon as the fuel pump is off.

When the car idles or cruises the pressure is at 15 psi. On a WOT it drops to 11 psi and stays like until I ease the throttle.

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To measure the fuel pressure I did the following.

1. Got the Actron TBI fuel pressure gauge kit at PepBoys ($45).

2. I installed the Schraeder valved T-fitting (provided with the Actron kit) where the rubber hose on the driver's side connects to the steel fuel line by the engine.

I assume I am to measure the pressure on the feed line, before the fuel enters the meter and the regulator.

I assume that the feed line is the one closer to me on the driver's side (the upper one).

3. I installed the gauge. The long tube that attaches to the Schraeder valve is pretty nifty. First I checked the fuel pressure with the car in Park and hood open. For the pressure under driving conditions, I routed the gauge tube around the driver's side of the master cylinder and up through the cowl to the outer side of my windshield, where I taped it. Electric tape. Yes, it's temporary.
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I messed with the throttle body a long time ago, basically did some stuff from Ultimate TBI, changed an injector, rebuilt using Borg/Warner carburetor rebuild kit.

 

IIRC, form what ive heard, there is an antidrainback ball valve in the pump thats supposed to help maintain fuel pressure when its off. If thats dirty it can leak and cause the fuel pressure to fall when the pump is off. It could also be your injectors as well, although this is unlikely. If you prime the system and see fuel leaking from the injectors, then you know theyre leaking. Of coarse, there is also the possibiliy of a leak somewhere else in the system.

 

well i changed the fuel pump 3 times to correct this situation... i have also changed the regulator. and by the design i think is normal meybe im not sure but i sure havent fixed it yet

 

I don't think the stock pump is supposed to drop pressure when the engine is off, pretty much like dimented said. However, badxs, if you are using a high performance pump, then it sould releave the pressure (safety reasons).

The problem could be as simple as a dirty fuel filter, or more complex such as low voltage to the pump or a busted line/hose in tank (that was my problem)

 

If it is dropping off after the car is off, then it's more likely to be like dimented said, or the busted line/hose in tank

 

Guys,
TBI systems are designed to be self-relieving. The pressure should drop to zero after turning off the ignition. Also, when you turn the key on but don't actually start the engine, the pressure will drop back off. 15 sounds high for a stock system but most TBI pumps will not maintain 15 at WOT. Dropping to 11 sounds low, though, and it would be worth while to check the filter. After that, you may have a pump problem.

 

Paul, are you sure the pressure is supposed to be 0 after shut-off? Where did you hear about this? This seems a little strange to me, since I am not sure how this is supposed to work...

The way I see it working now is the FPR provides the resistance to fuel flowing further up to a certain pressure (in my case 15 psi). I am pretty sure 15 psi is because Pat (88'camaro305TBI) & me messed with the FPR during the fuel meter rebuild. So, when you shut the car off, the fuel pressure from the feed line to the regulator should maintain 15 psi, since there is no additional force to push it beyond that. Unless there is a leak in the fuel system or somehow it drains back into the tank.

The fuel filter hasn't been changed in a year, I'll check it to make sure.

I'll keep you guys posted on the progress of things...

BTW, is it dangerous that I left the Schraeder valve in the feed line. It doesn't seem to leak and I think it would be annoying to attach it and deattach it all the time...

 

Marin,

I didn't just "hear" about it. I drive it. Pressure is built between the fpr and the pump. When the pump shuts down, no more pressure. There is no check valve. Not all the fuel drains back to the tank but pressure is not held.

I have a fp gauge installed for my vafpr and watch pressure daily.

There is nothing wrong with keeping the Schrader valve installed. That's what I did before the gauge install. A brass 1/4" NPT or 3/8" NPT (forget which) cap will fit over the valve end to keep crud out of it. Yes, I know the threads are not NPT but it fits fine.

 

Thanks!!! This has kept me worried for days... Sorry that I asked in such a tone, but there's all sorts of advice on this forum sometime, so I was a little cautious.... I keep learning things about my car every day. And thanks for the tip on the sizes for the valve cover, you saved me some time here.

Have you heard of any in-line fuel boosters worth pursuing? According to my research the stock TBI injectors can hold 30psi comfortably and I might need more fuel when I got a little more flow moving (prolly after headers installation, or an intake swap)...

My BLMs are higher in the higher RPM range and lower in the lower range, this perhaps corresponds to variations in the fuel pressure.. When I get my next paycheck, I'll get an EPROM burner...