Fuel pressure question
 

Hey guys, still working on the 94 LT1.....

My uncle is bringing by a good fuel pressure gauge in a few days, but I may be able to save him the trip if I get a little more information

My car hasn't been able to start for about a week now, but after troubleshooting everything electrical I turned the key on......pump buzzed.......I took my screw driver and depressed the schrader valve and fuel dribbled out. I got it started a few days back and did the same thing and fuel SPRAYED out like crazy, shouldn't it do the same once you turn the key on to fire the engine?

Like I said.....fuel pressure gauge is on the way, but I figured I'd ask. I ordered a new regulator for the heck of it since my pump is running.....still may be bad, but I figured go cheap first and then tackle the pump if that doesn't fix the issue.......I replaced the filter about 2 weeks ago, and the relay is clicking.

I am hoping I'm right and the pressure is there before the regulator which is why when I push the schrader the pump doesn't re-prime from the pressure loss. But the pump does prime everytime you turn the key on.

Re: Fuel pressure question
 

The pump will not "re-prime" on loss of pressure. The PCM turns the pump off after a couple seconds because it does not have an indication the engine started. The PCM cannnot see the fuel pressure... there is no sensor.

You really need a fuel pressure gauge. System should prime to at least 40psi, and when the pump shuts off, should hold that pressure, dropping slowly. If pressure drops rapidly, it will make the engine harder to start. But once the engine is running, rapid pressure loss on shutoff is not a factor.

Rapid pressure loss could be:
-faulty check valve in the fuel pump
-leaking injectors
-leaking fuel pressure regulator
-leaking lines anywhere in the system, including the line inside the tank

Re: Fuel pressure question
 

Ok, thanks.....the gauge is on the way, I had my neighbor turn the key on for me....pump buzzed, but it was just a very small dribble coming from schrader. I'll just wait on the gauge, but I thought I may have at least found a clue as to why I can get it to run for about 10-20 seconds like a charm......but once it shuts down theres no re-start for a couple hours.

Havent found any leaks yet at least none under the car, or under the hood, and no smell of gas. The leak from inside the tank could be possible though. I need this thing on the road bad!

Thanks for all the help guys......I'm just waiting for relatives and friends to help me out bcuz some of this stuff requires you to be in two places at once and I can't.

Plus it doesn't help that I'm a diesel guy.....:) No start on diesel = loss of prime, loss of pump, loss of glow plugs.......I've become lazy....lol. Not use to all these sensors and such.

Re: Fuel pressure question
 

Ok guys, I got my fuel pressure gauge, and with key on, pump primed, it goes up to 44psi, but drops to zero after 5 seconds.....literally. I wish I could see what its running at idle, but I can't get it to run.

I have a new delphi regulator coming in either today or tomorrow, figured I can install that and at least eliminate that being the potential problem. No leak by from the diaphragm or anything, but hell.........why not?

I have seen in another post that a bad "check ball" can cause the bleed off?
I know I need to check the lines on the pump, but I think I just answered my own problem.......:)

When I bought the car for some reason it would stutter at about half tank of fuel......runs like a dream when full......holy crap......

I wish I remembered that. I bet the one of you is right and I have a busted line in the tank, but when the tank is full it has the broken straw effect.....full drink shows no sign of broken straw til the drink gets past the hole in the straw....
This actually makes a lot of sense to me, but I guess I could still be wrong........hope not.

Man that sounds like a conspiracy.......hopefully understandable though.......I'm going to keep high hopes for this evening and pull the pump and see whats going on in there.

Re: Fuel pressure question
 

The "check ball" would be the check valve that is an integral part of the fuel pump, which I already mentioned.

Pull the rails up with the injectors intact, pressurize the fuel system and look for drips.

The straw analogy is not valid. Vacuum in the straw will continue to exist as long as the hole in the straw is covered with liquid. Hole uncovered, vacuum lost = no flow. But the line in the tank is pressurized. It's going to leak fuel out of a hole whether the hole is covered or not.

Re: Fuel pressure question
 

OK, I'll try that this evening. So the check ball is part of the fuel pump, what exactly will pulling the injectors up prove? Bad injectors?

Re: Fuel pressure question
 

Prime pressure can be lost for the four reasons I indicated in post #2. One of those is "leaking injectors". You pull the rails up, turn the key to "run" (NOT "start") to pressure the fuel system, and look for drips from the injectors. That would be an indication of "leaking injectors". If they leak, you have found a cause. If they don't leak, you have ruled out one of the four possible causes.

Re: Fuel pressure question
 

Excellent! I still haven't done it, its been storming for the last 3 days here in VA, so I've gotta wait. My regulator came in, but I haven't installed it. I'll get back with everyone once it dries up a bit.

Another question while Im waiting though.......I can get the car started randomly, so once its actually running can this "bleed down" or "pressure loss" be the reason it won't run for long? After reading your posts I'm becoming a little skeptical. Once the car is running......if it runs for some amount of time......you can go out and smoke the tires and blast down the road. Thats actually where this most recent "no start" issue came from. I was cruising trying to do some test runs and everything was fine until I hit the train tracks "bump" and down she went and that's where I'm at today. MAN! This car has demons!

Re: Fuel pressure question
 

Already answered that question.....

Re: Fuel pressure question
 

Well damn! Back at square 1..........

Re: Fuel pressure question
 

I would qualify what I said by adding "depending on the problem". If you have a leaking check valve in the pump, it won't affect how the engine runs. If you have a small leak in an injector, it may cause problems on that cylinder when it runs. If you have a large leak in the in-tank hose, or if you have a problem with a leaking diaphragm in the fuel pressure regulator, it's going to affect the way it runs.

You have to measure the fuel pressure, under heavy engine load.

Re: Fuel pressure question
 

Yeah, gotta get it running. I have noticed.......when the engine dies it doesn't matter if I go full throttle or not. Basically when it stutters I floor it out of habit and sometimes it will rev, die, rev back up, and finally die for good.

Also, as I mentioned before if your cruising down the highway it will die....all gauges go to bottom and car is coasting, but it will come back to life on its own with out any driver interaction. Dies like you switched the key and comes back.....very weird....

Re: Fuel pressure question
 

The ignition switches are so old they are starting to show signs of melting and deterioration.

Re: Fuel pressure question
 

4th Gen F-body Ignition Switch Replacement

Re: Fuel pressure question
 

So before I rip this beast apart to put another ignition switch in it.........the car turns over fine all day long, just doesn't start. I was under the impression that a bad ign switch was just dead car, no turn over, etc. Am I false?

So far heres whats new:
ICM
Coil
Fuel pump relay
Fuel pressure regulator

The ignition switch was "supposedly" just replaced before I bought it. Owner said GM did the replacement.