Losing fuel pressure

[Chipofsc]

Been having trouble starting my cobra when hot. Thought I had a leaking injector because it would stumble when it started. My mechanic ran a test today and found I was losing fuel pressure from 60 to 6 lbs in six seconds after turning the engine off. No engine miss fire in any cycliner histor in computer. He feels it's the check valve in the fuel pump. I was told my car has ford gt pumps but no way of knowing without pulling the tank. Pump is working good other than losing pressure. Just thinking outside the box why not install check valve in the fuel line ??? I'm sure someone will set me straight why this won't work. If it won't where can I get ford gt pumps ?

 

[Bdubbs]

I would data log fuel components first. If you've never done it you'll be amazed at how many things you can log.

 

[Chipofsc]

My guy used his ford computer to check the fuel pressure. He check the compression and miss fire from the computer also. Don't know what a data log is.

 

[jeason15]

My guy used his ford computer to check the fuel pressure. He check the compression and miss fire from the computer also. Don't know what a data log is.

Even though we can decipher what you mean, it's really tough to read. I'll try to address your statements.
1. You never said what the fuel pressure was when checked...
1a. What scanner did he use?
2. What misfire codes was it throwing? Single cylinder? Random?
3. What kind of compression did he pull? We're there any low compression cylinders?
4. A data log is a running capture of the engine performance sensors and other engine data while running. It can give you very good clues as to what is wrong with your engine depending on what you see...

 

[Chipofsc]

Fuel pressure would run like the RPMs. Low was 30 and high was 70. When he shut the engine down it would not hold pressure and drop to 5 lbs. quickly.
I don't know what the program he used but he said it was a ford software.
The car data did not show a misfire.
The compression data in the program does not tell you what the compression is but that all cylinders were the same. Two cylinders were lower than the others by 2 %.
He wanted to do a injector leak down test but the fuel pressure would not hold long enough to complete the test.

 

[TRBO VNM]

Sounds to me like the pprv was removed when doing the ford gt pump install. Which is common. That pprv is the check valve in the tank and causes hesitation issues. So you need to cycle the key a few times before you start it so it can build pressure in the lines. Dropping pressure fast after shut down sounds normal with no check valve.

There are some things you can adjust in the tune for hard starts when the car is hot

 

[Chipofsc]

Sounds to me like the pprv was removed when doing the ford gt pump install. Which is common. That pprv is the check valve in the tank and causes hesitation issues. So you need to cycle the key a few times before you start it so it can build pressure in the lines. Dropping pressure fast after shut down sounds normal with no check valve.

There are some things you can adjust in the tune for hard starts when the car is hot

I keep reading about the PPRV. What is it exactly

 

[jeason15]

Positive pressure relief valve. TRBO explained it pretty well...

 

[Chipofsc]

Never mind I just read all about it. I think that is my problem. Going to try the key on and off and see if that helps. Thanks

 

[Supperior1]

Not to high jack the thread but, Im dealing with the morning hard starts....excessive cranking until finally it starts...then starts fine all day until the next morning.....I like the inline check valve idea. lol I just turn the key to the on position and have to leave like that for like 30 seconds for it to build pressure until it finally starts.....its horrible but I dont want to spend the money on a new fuel pump(s)/and labor when it works fine other than when it sits for over 12 hours....is there any other remedy?

 

[jeason15]

Your pumps only prime for 1 second before the ccrm cuts off iirc

 

[Supperior1]

Yeah but leaving the key on will allow it build pressure, even though it only primes for a sec....thats why if I turn they key on and off several times it does nothing, but if I turn it on once and leave it for 30 seconds it starts...

 

[Blueline]

In my aftermarket hat with a -10an feed line I turn the key on, it primes, I pause for a sec or two and turn the key the rest of the way. No issues, just give it that pause in between prime to crank

 

[TRBO VNM]

Yeah but leaving the key on will allow it build pressure, even though it only primes for a sec....thats why if I turn they key on and off several times it does nothing, but if I turn it on once and leave it for 30 seconds it starts...

That makes absolutely no sense. I have seen it on a gauge. One cycle gets a little pressure, few cycles get full pressure, then start without issue

 

[Chipofsc]

That makes absolutely no sense. I have seen it on a gauge. One cycle gets a little pressure, few cycles get full pressure, then start without issue

The pump only runs for one sec. I have to do it three time to build pressure. It starts fine cold

 

[cj428mach]

The pump only runs for one sec. I have to do it three time to build pressure. It starts fine cold

log your FRPS while trying to start the car, if you see pressure when you hit the key it should start.

 

[jeason15]

That makes absolutely no sense. I have seen it on a gauge. One cycle gets a little pressure, few cycles get full pressure, then start without issue

LOL. Thanks. I wasn't gonna call him out...

 

[blk02edge]

Yeah but leaving the key on will allow it build pressure, even though it only primes for a sec....thats why if I turn they key on and off several times it does nothing, but if I turn it on once and leave it for 30 seconds it starts...

Just think, how will it keep building pressure when the pump isn't running? Lol

 

[bigmoose]

Just think, how will it keep building pressure when the pump isn't running? Lol

I can see his point but probably not why he was thinking. In the returnless system when the system is not pressurized its possible for air to get in. The key on primes but maybe the extra time has some effect on any air pockets, therefore it's easier to start.

 

[cj428mach]

I can see his point but probably not why he was thinking. In the returnless system when the system is not pressurized its possible for air to get in. The key on primes but maybe the extra time has some effect on any air pockets, therefore it's easier to start.

It could be or it could be that his tune has too much fuel at start up so by letting the car sit its losing pressure and he is getting less of a fuel shot.