Giving up, need help with fuel pressure problem

[Jason]

So....I had a problem where the car would turn over and then immediately die. Happened for a couple days, tried again, car started and ran great all day. Next day, car started and then died about 10 seconds later. After that I continue to get 0 psi on my fuel pressure gauge. Changed the pump, didn't fix the problem. Pump doesn't prime and I know there isn't a blockage in the fuel lines because I can disconnect the fuel line from the pump and no fuel comes out at all. That tells me the fuel pump isn't even coming on. It should be noted that I have the gt500 upgraded fuel system. I measured voltage to the harness going to the pump. It's getting 7.3 volts from both power wires going to each pump. From my understanding it's a variable voltage pump so 7.3 volts should be enough to power the pump correct? I would say next would be the FPDM but I'm getting 7.3 volts to the pump......

 

[JeremyH]

Pumps could be dead. Not likely though, pull the hat and look for leaks and bench test the pumps.

 

[Jason]

I didn't bench test it but I just put in new pumps which didn't fix the problem. I bypassed the aftermarket wire harness and plugged in the oem connector to the FPDM. Then I measured voltage to the old pump harness, presto, I'm getting 12v.

My question really is, would 7.3 volts be enough to activate the pump and prime the system? If not then I believe I'm getting some sort of impedance through the aftermarket wire harness that was provided with the kit.

 

[JeremyH]

So at key on you initially get 12v to help prime the system then after starting and at idle 6-7 volts is normal to maintain rail pressure. Replace the relay on the gt500 harness for starters and if you have zero fuel pressure at the rail then you have a leak in the hat somewhere. Do you have your stock fuel hat to swap in and test as well?

Also check your frps.

 

[Jason]

No, currently the car does not turn over at all and I read 0 pressure with key on, engine not running reading 6-7 volts from the harness. Pump does not prime. I just put in the new pump yesterday and problem was not solved. I plugged in the old pump to the harness and nothing happened. The power from the gt500 relay reads 11.xx volts with key on to both the red wires going to both FPDM's so I know the relay works. Grounds from the gt500 harness going to both FPDMs check out good.


Here is the kicker, when I disconnected the oem FPDM harness going to the gt500 harness and plugged it into one of the FPDM's I read 12volts going to the old plug that hooks to the old pump.

 

[Jason]

I bench tested the old pump, works just fine.

 

[Jason]

Also, if anyone can understand this data great, I measured the voltage with everything plugged in from the module monitor and the module control wires (how I did that, long story). Anyways, module monitor wire, the voltage was jumping all over the place. The module control wire read a constant 4.5ish volts.

One interesting fact, I had my wife turn the key on and off. The drivers side FPDM would have a click noise but not the passenger module. Can someone let me know if a click is heard from one, or both FPDM's when power on is first applied? I think maybe my passenger side FPDM is shot.

 

[JeremyH]

Your readings all look normal something may be bleeding off pressure, if you don't have fuel leak outside of the tank then its a leak inside the tank at the hat. Even a hairline crack in a hose on the hat can bleed off all pressure, path of least resistance. I have seen guys scratch them up during install and then over time pressure cycles break it at the weakened spot.

The relay from the gt500 harness should click (wether it works or not). The fpdm's do not, just buzz. Replace the relay first off, cheap and easy. The realy coil can fail and still show voltage but not enough contact strength to allow enough current through to run the pumps.

Check oem relay and fuse underhood as well.

Also have you confirmed no pressure or fuel at the rails? I would disconnect the feed line at the rail and key on to see what happens. More ways to positively half split the problem. This will also let you know your gauge is indeed reading correctly.

Some trouble shooting tips.

Even with only one pump or only one fpdm working causing only one pump to run you would still have full pressure at the rail and be able to drive the car.

Chances that both pumps fail at the same time = slim to none

Chances that both fpdms fail at the same time = even more slim to none (I have never acutally seen one fail it was always another issue)

FRPS failure is more common and this sends the signal to the pcm to tell it what to do with the fpdms. If the frps sees no pressure it should ramp up voltage to the pumps very high to try and compensate, it doesn't seem to be doing that so frps could be bad. Worth a check for sure.

Without getting my hands on the car and seeing stuff for myself my diagnosis.

1. Leak in the plumbing on the fuel hat bleeding off pressure.
2. Clogged pprv valve on the hat.
3. Weak coil in the gt500 harness relay.
4. Bad frps.


Hope this helps, good luck!

 

[Jason]

That definitely does help. Was wondering if the frps had a connection with the system. I'll replace the harness relay and then the frps. I don't think it has to do with anything hat or pump related. Correct me if I'm wrong, but it should still run even with a leak or clog correct? I verified the old pump to work, then I hooked it to the fuel pump harness, turned on power but nothing happened.

Also, if it is the frps then why is it when I plugged the original wire harness to the FPDM I read the 12v I need from the old fuel pump plug.

 

[Jason]

Ok, narrowing it down I think. You were right, the clicking was from the relay. So I disconnected the plug going to the relay and jumped the "hot at all times" wire with the two wires going to the FPDM's. With key on and the harness plugged into both FPDM's I'm reading 6 volts coming from the jumper wire. With both harnesses unplugged from both FPDM's I'm reading 12 volts. What.....the.....f*#$. Any ideas?

 

[Jason]

So to give you a visual, here is how I jumped the wires to bypass the relay. Like I said before, when the harness is plugged into the FPDM's the grey jumper wire picked will read 6 volts. When I unplug both FPDM's the voltage from the grey wire reads 12 volts. I'm clueless why it is doing this.

 

[Jason]

Also, if it was the frps shouldn't it at least prime? Or go into some sort of limp mode?

 

[JeremyH]

That definitely does help. Was wondering if the frps had a connection with the system. I'll replace the harness relay and then the frps. I don't think it has to do with anything hat or pump related. Correct me if I'm wrong, but it should still run even with a leak or clog correct? I verified the old pump to work, then I hooked it to the fuel pump harness, turned on power but nothing happened.

Also, if it is the frps then why is it when I plugged the original wire harness to the FPDM I read the 12v I need from the old fuel pump plug.

It could run or not depending on how bad the leak or clog is. I developed a leak from a tear about as wide as a dime and hair thin on my hat and my car held no pressure and wouldn't run.

But I can hear the pump running and priming the whole time.


Don't get too hung up on the voltage difference from the oem setup to the gt500 it uses a second fpdm that mimics the first and shares the load between the to pumps to get the same result the car wants so readings will be different.


I would still positively half split the problem a few times. By doing mechanical checks first. Physically make sure you can get fuel through the pumps to the rails.

I would get some wire and run 12v from the battery and a wire to chassis ground straight to the prongs on the plug at the fuel hat to each pump and make sure it moves fuel and creates pressure at the rails. if the pump kicks right on you should have 40psi plus in seconds if everything is good in the hat, line and filter. If you do, you now know the hat plumbing, pumps, line and rails and your fuel pressure gauge is good.

If your not getting anything at the rails and you just bypassed all the oem wiring and controls you have a leak/clog.

Depending on the result from this test you now know 100% where to focus on. Either looking for the physically problem or jumping into the electrical side, ie frps, relays, fuses, wire runs for the harness, inline fuse. Look for damaged/frayed wires, burnt contacts on fuse receptacles etc.

 

[Jason]

Made a wire harness out of some spare female terminals I had from some pigtails left over from my wire tuck. Anyways, applied power from the battery, pump ran and gauge shot up to 90psi but then leveled out at about 45 after I disconnected the power. Only applied power for maybe a couple seconds.

Now, I have two fuel pressure gauges, one going to my gauge pillar and the second on my fuel rail block. I havnt been checking the pressure by data logging. I usually dont do the configurations as Lito usually sends me them but I decided to give it a shot. Not sure if I did it right because I get a message "Failed to setup PID"fuel_rail_pr_psi" Description: fuel rail pressure in PSI" after it connects to the ECU.

Anyways, I discarded the message and did the datalog. This is the data I got. The first picture is with no power to the pump and my gauge reading 0psi. The second picture is after I applied power to the pump for a couple seconds. My gauge leveled out at around 45psi.

 

[saleen836]

Your problem sounds almost the same as I had on my Saleen SC with GT500 dual fuel pumps!After the numerous times of the car stalling on me or starting but running rough and dieing if the throttle was pressed I had enough, solved the problem by removing the GT500 pumps putting the standard pump back in and adding a KB boost-a-pump, not had a problem since!

 

[Jason]

It had something to do with the BEC. I took it out, looked everything over, put it back in, and the car fired up and has been working fine ever since. Probably was just a loose connection in the back of it. I have a feeling it wasn't making a good connection with the PCM relay, or what ever haha.