Fuel pressure drops over night

[o2sys]

I recently noticed that it takes about 2-3 cranks to start the car.

I have a 2007 GT with GT500 dual pumps.

So after reading my fuel pressure before trying to do the first crank, my fuel psi was at 16. Read via an obd2 scan gauge so I I think it's reading off the frps? Im not sure.

After turning the key on and off (acc) and watching the fuel pressure gauge come back to 39-40psi it starts right up.

Then it has no problem all day until the next morning.

What could it be??

Other mods worth mentioning are fore fuel lines from hat to rail and fore inline filter.

 

[Wes06]

try keying the car on/off/on/off/on/start?

sounds like the check valve could be letting the pressure bleed out

 

[skwerl]

It's either leaking back into the tank or leaking out. If you have no signs of an external leak then it's internal.

The standard starting procedure for my big truck is to turn the key on, wait until you hear the fuel pump cycle (it does it 4 times) and then crank.

 

[o2sys]

That's what I'm doing now to start the car in the mornings, key on/off couple times them starts right up. Not seeing the fuel gauge drop overnight so could be internal.

Where's the check valve?

 

[Wes06]

pretty sure its in the fuel pump assembly, not 100% though need a double check to confirm

 

[o2sys]

pretty sure its in the fuel pump assembly, not 100% though need a double check to confirm

Wouldn't fuel pump assembly cause other symptoms while driving? Pressure is fine after starting.

 

[Wes06]

well i mean i think its connected in the hoses leading out of the assembly.

not 100% cant remember exactly what it looks like in there

 

[skwerl]

I've heard of issues with the fuel pressure regulator sensor which is on the driver's side fuel rail where the fuel line comes in. Make sure there's no fuel leaking out there.

 

[chandlern]

While I have next to zero experience with the gt500 set up in my car, my experience with check valves is that they are placed in line post pump discharge and are there to prevent backflow from tearing up a pump when it slows down or shuts off. They almost never completely hold pressure over any considerable length of time (not what they are there for). Unless you have a leak somewhere I would say the gradual loss of pressure is perfectly normal. I think the Haynes manual I have for the car says letting the car sit overnight is one way to bleed fuel line pressure to change the inline fuel filter.

 

[Eel Mit]

Most pumps have a pressure relief valve built in. If you look at the top of the pump there is a small hole and you can see a spring and ball bearing in it. Other thing that might cause loss of pressure is a leaking injector.

 

[boll58]

Could be the fuel pressure regulator leaking down. Not too familar with the set up but if it has a vacuum line it could be leaking into the intake causing hard starting.

 

[BruceH]

Could be the fuel pressure regulator leaking down. Not too familar with the set up but if it has a vacuum line it could be leaking into the intake causing hard starting.

It's a returnless system. There isn't a regulator. Fuel pressure is determined by a sensor on the rail and modulated via feedback from the frps by cycling the pumps on and off.

OP, just keep the key on for a couple of seconds prior to starting. No need to cycle the key on and off. Like El Mit said it's probably a leaky injector but it could be a few different things. In the end it's not affecting much other than having to leave the key on for a few seconds prior to turning it over.

 

[JeremyH]

While I have next to zero experience with the gt500 set up in my car, my experience with check valves is that they are placed in line post pump discharge and are there to prevent backflow from tearing up a pump when it slows down or shuts off. They almost never completely hold pressure over any considerable length of time (not what they are there for). Unless you have a leak somewhere I would say the gradual loss of pressure is perfectly normal. I think the Haynes manual I have for the car says letting the car sit overnight is one way to bleed fuel line pressure to change the inline fuel filter.

Correct.



OP, its completely normal, the gt500 pprv is not a true check valve and will relieve pressure slowly over time and let fuel back into the tank after a day or so.

And your setup is not stock anymore so it will behave a little different from what your used to. With the larger 8an line and filter upgrade you have more volume to "refill" at key on so it may take a second or two longer to bring pressure up so the injectors give the proper amount of fuel when cranking.

At key on the pcm/fpdms turn the pumps full on to prime the system, so wait a few seconds you can hear the pumps prime the system to tell when its ready or use a pressure gauge etc, then crank away.

 

[o2sys]

Ok I think its safe to say it's my fuel pumps that are going.

This morning I went to fill up the tank and the car sputtered to life then stalled.

Fuel pressure dropped to like 6psi. Wouldn't come back for a while so just started unplugging and plugging all the connections till eventually I got pressure again.

Now do I have to replace the full assembly or can I just get the actual pumps only?

 

[o2sys]

Maybe get these as an upgrade?

http://www.deatschwerks.com/news/pr...40-lph-ford-specific-direct-fitment-fuel-pump

 

[JeremyH]

It could be many things other than the pumps. Could be a loose connector or relay going bad in the gt500 harness. Hence messing with things fixed it. Its easy to blame the pumps but from my experience 9 out 10 times is something else and a lot cheaper and simpler to fix. I have troubleshot a lot of fuel setups for suspected bad pump and its almost always something like a split hose, loose hose clamp, bade connection or wire, relay, damaged or clogged fuel socks etc.


Those pumps are a nice upgrade for the gt500 hat. Have built a few with them.

 

[o2sys]

So my problem ended up being the relay that powers both fpdms melted.

The power wire that splits into two started melting the relay connection (not the relay itself) so the pumps were malfunctioning due to a bad connection.

Not sure why it failed here, guess too much heat for the relay to handle?

 

[pics06gtstang]

nice , could of been worse , .
@ JeremyH . he hit that one right on the money !

 

[JeremyH]

Yeap not the first time I have seen it. So the melting from too much heat is normally excessive current draw from the something failing and shorting. It is one way a relay can go as it starts failing. Get a new 40amp relay and swap it out also I would clean up and redo those connections.

 

[Eel Mit]

Heat = too much resistance or too much load through that wire. Either there is a poor ground, you have a pump on its way out, or the relay used is rated at too low amperage for the application.