My Dad and I were troubleshooting a fuel pressure problem in my '88 L98. We had a fuel pressure gage on the fuel rail to monitor fuel pressure. We noticed that the engine won't hold ANY fuel pressure when the engine is stopped. The gage drops like a rock to 0 PSI. It used to sit at about 40 PSI and creep down over a period of a few hours. So, we got out the GM service manual and followed their troubleshooting procedures. We came to a step where it says to pinch off the fuel return line, turn the key on to run the fuel pump, then turn it off, then observe whether or not the fuel pressure still drops.

Well, here's where things went really bad. The GM procedure does not take into account the NOS fuel boost pump that is in series with the factory pump. Neither did we. We pinched the return line, turned the key and whammo....the fuel pressure gage slammed all the way around to 100 PSI - pegged. Actual pressure was probably more. We could smell gas everywhere.

We un-pinched the fuel line and tried to start the engine. It was flooded real bad. It would start but blow tons of white smoke out the exhaust. Then we saw raw gas dripping out of the place where the headers join the exhaust pipe.

Here's my theory: The massive pressure spike blew out the diaphram in the stock fuel pressure regulator. That allows the fuel pressure to go directly from the fuel rails - past the regulator - and into the intake plenum - where the regulator gets its vacuum signal.

I know this is probably a very unusual problem, but do any of you have any additional comments on what else may have been damaged and what else I should be looking at?

Thanks in advance for your help.
Greg S.

 

whoa...well...the fuel press. regulator may be shredded like you said. is it possible to blow out seals and rings in the injectors themselves with that kinda pressure being forced through them? could the pump itself be subjected to an electrical spike because of mixed signals and fry itself?
one thing that i know i would be concerned with is washing down the cylinders because of all the raw fuel. if that occurs, the oil coat is "washed down" and all you are left with is piston rings scrapping on the cylinder walls. do compression and leaks down tests to be sure. there are no problems there. cuz if there are, every turn on the crank makes it worse and worse.
let us know what you find. and good luck

 

I wouldn't really worry about washdown, since you didn't run it very long. Since everything has to come off for the FP regulator repair, remove the injectors and have them serviced while they are out. Rich at cruizin performance does excellent work from what I hear. He will be able to evaluate any potential damaged done to the injectors. Also, don't forget to changed your, oil, it has become fuel contaminated.

When you get it all buttoned up, see if the pressure holds like it should. If it doesn't, you'll have to run the diagnostic procedure again, with the secondary fuel pump disconnected. Good luck.

 

It sounds to me that your problem you have is a leaking injector(s). One thing to check is to make sure that the diaphram from the OE pressure regulator is not defective. All you have to do is to remove the vacuum line to the regulator and see if its wet with fuel. Or for that matter just key cycle the ignition without starting the engine and see of fuel squirts out of the vacuum fitting at the regulator. Be sure to hold a rag around it to catch the fuel.

The injectors and the vacuum line is the only way that fuel can get into the engine like it did. One thing to remember is once its flooded out like that, you need to check the engine oil. Pull the dipstick and smell the oil, if it smells like fuel its time to change the oil and the filter. Thats one thing that happens when it gets severly flooded as it was.

 

Along with all the good advice above, I must ask why the NOS boost pump comes on when the NOS isn't being activated???

RACE ON!!!

 

It could be that the NOS pump is in series with the stock in the tank pump. Therefore it has to be running otherwise it would be a restriction in the line. I don't like such arrangements because if the second pump fails, or if the relay that fires the pump fails, the stock pump can and will pump enough fuel to run the car, but during high demand the pump that is not running will become a restriction and fuel pressure will drop. I know because it happened to me one day. I now run a single large in tank Bosch pump with a boost a pump device to give me additional flow during boost.

A better arraingement would be to run the second pump in parallel with added NOS safety interlockings. That way the second pump only comes on if the NOS system is armed. Mind you the second pump in this arrangment has to be one prior to the actual firing of the NOS gas. That is why I said "on" when the NOS system is armed that way its already running and there will be adequate fuel flow when the system actually fires.

 

Thanks to everyone who responded. I carefully checked the dipstick. The level had not changed at all and I could not smell any gas. I do plan to inspect the injectors after installing the new regulator (on order).

The NOS pump was installed because the stock fuel pump could not feed the engine with sufficient fuel pressure when the nitrous and fuel solenoids opened up during WOT. The low fuel pressure safety switch would activate and cut the nitrous flow off. As most of you know, if your fuel pressure drops too much during a full throttle nitrous activation, the pistons get destroyed - due to the cylinders "leaning out". The NOS pump was insurance against that. The NOS pump is wired in parallel with the factory pump...that way it does not become a restriction during normal driving conditions. The setup works very well. The fuel rail pressure stays pretty much locked at 40 to 50 PSI - regardless of fuel demand.

Until this current problem cropped up, that NOS fuel pump has worked flawlessly and never impaired reliability or driveability. We created this problem because we grossly underestimated the force that pump could generate if the return line was clamped off.

Thanks again.
I'll post a follow up after I make the necessary repairs.

Greg S.

 

What ended up being the problem? Same situation

 

Mine did that. It was the Fuel regulator

 

10 year old thread.

Some advice to posters.....send a PM to the original poster.

 

LOL oh well answer is the same.

 

It was the fuel pressure regulator. My memory is a little fuzzy on the specifics from that disaster 11 years ago. As I mentioned in the original post, we were following a troubleshooting procedure in the GM manual to see why the pressure in the fuel rail would bleed down so quick after turning off the engine. The manual said to pinch the fuel return line and then turn the key on. Of course, the manual didn't know about the NOS boost pump that was plumbed in series and wired in parallel with the factory pump. With the return line pinched off, I turned the key on. Both pumps did the 2 second pump up. As I recall, we could smell raw gas everywhere. Although it was too late, we unpinched the fuel line and tried to start the engine. No go. I looked under the car and saw gas dripping from the exhaust pipe where it connects to the catalytic converter.

We ended up pulling the intake plenum to remove the fuel pressure regulator. We got it out and determined the diaphragm was blown apart. I'm not sure of the exact path that the raw gas took to get to the exhaust pipe. The crankcase did not appear to have raw gas in it. Maybe somehow the gas went through the EGR system to get to the exhaust pipe.

Nothing else was damaged. We replaced the regulator and everything returned to "normal". I can't remember how we resolved the fast bleed down on the fuel rail. I know I didn't touch the injectors. I might have replaced the fuel pump. The fuel pump in a C4 is so easy to change.

BTW...I still have that Vette. It won't pass California smog due to very high NOX. It reads about 1200 PPM and the limit is around 400 PPM. So, for the last three years it sits in my garage getting very lonely. I start it about once a month and I keep a charger on the battery. Once in a while I take it out for a quick spin. It still runs like a raped ape. It still chirps the tires on the 1-2 upshift (automatic).

I hope that answered your question.
Are you having a similar issue or just curious?

Take care,
Greg / Canyon Lake, CA.