06 STI starts hard or not at all, fuel pressure drops quickly after shutoff

[norcal123]

2006 WRX STI, 105k miles, all stock. Cranks fine but does not start from time to time. Often, priming the fuel pump before cranking helps. Also, started stalling on idle.

So far, replaced fuel pump, fuel filter and fuel pressure sensor. Changed spark plugs and cleaned MAF.

After replacing the FPS discovered that the fuel pressure quickly drops all the way towards 0 after the car is shut off. When the car is running, the fuel pressure stays at 40+ PSI.

We have NOT checked the fuel pressure on shutdown before replacing FPS. Pressure while running was OK back then as it is now. Also, I only noticed that the car started stalling after replacing FPS. Doing it on 06 requires taking off intake manifold.

No fuel smell, no visual on vacuum leak, no CEL or codes in the computer.

What is the right procedure to check if the fuel goes back to return line vs. leaks elsewhere (injectors?).

Is there a separate fuel return check valve or is it integrated into the pump?

Does anyone have other suggestions?

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[norcal123]

Sorry for spamming with a duplicate message, got confused by the interface. Is there a way to remove a thread?

[the suicidal eggroll]

Fuel pressure sensor? I'm assuming you mean the fuel pressure regulator since you mentioned removing the manifold...I've never heard it called a fuel pressure sensor before though.

A quick drop in fuel pressure after shutting off the car would be indicative of a failing FPR, failing pump, or a leak...you've replaced both the FPR and the pump, so other than a leak I'm not sure what else it would be.

This might help you get a better understanding of the layout of the fuel system:
http://forums.nasioc.com/forums/show....php?t=2339842

The return from the FPR is routed straight to the tank, the only check valve in the system is in the pump outlet. You could get a length of hose and a separate check valve and install it in a few key spots in the system to check where the drop is coming from.

[holyjoetart]

Did you check the o-ring on your fuel pump?

[norcal123]

Originally Posted by the suicidal eggroll

Fuel pressure sensor? I'm assuming you mean the fuel pressure regulator since you mentioned removing the manifold...

Yes, sorry, I did mean FPR.

Originally Posted by the suicidal eggroll

A quick drop in fuel pressure after shutting off the car would be indicative of a failing FPR, failing pump, or a leak...you've replaced both the FPR and the pump, so other than a leak I'm not sure what else it would be.

This might help you get a better understanding of the layout of the fuel system:
http://forums.nasioc.com/forums/show....php?t=2339842

And by the leak do you mean fuel leaking from the injectors, fuel line leak or vacuum leak?

I am assuming that by sticking a pressure gauge into the return line after FPR I can tell if the fuel bleeds off back into the tank after shutdown vs. leaking elsewhere. Am I correct? Is there a point in disconnecting the vacuum line from the manifold to FPR when doing this test?

Suppose that this test shows that the fuel does NOT go back into the tank via the return line. Does it immediately mean that it leaks out via the injectors or elsewhere or can it go back via the high pressure feed line by some reason? What is my next step under this scenario?

Originally Posted by the suicidal eggroll

The return from the FPR is routed straight to the tank, the only check valve in the system is in the pump outlet. You could get a length of hose and a separate check valve and install it in a few key spots in the system to check where the drop is coming from.

I wanted to know if there is a separate fuel check valve outside of the pump assembly. This goes back to my question on whether if it is possible if the fuel can flow back via the feed line because the fuel check valve is faulty. Sounds like the answer is no.

[norcal123]

Originally Posted by holyjoetart

Did you check the o-ring on your fuel pump?

Not yet, but will do, thanks.

[kev20]

Originally Posted by holyjoetart

Did you check the o-ring on your fuel pump?

the 06-07 have no oring

[norcal123]

Originally Posted by kev20

the 06-07 have no oring

I was reading about o-ring related aftermarket fuel pump issues on this thread: http://www.iwsti.com/forums/how-inst...i-wrx-lgt.html.

Thing is, we installed the whole brand new OEM fuel pump assembly, with the lower filter sock and fuel filter already attached to the pump. The o-ring in question, whether or not it exists, is on the inside of the assembly, right?

[the suicidal eggroll]

Originally Posted by norcal123

And by the leak do you mean fuel leaking from the injectors, fuel line leak or vacuum leak?

Most likely in a fuel line, probably inside the tank (unless you can visibly see or smell fuel leaking from somewhere else)

Originally Posted by norcal123

I am assuming that by sticking a pressure gauge into the return line after FPR I can tell if the fuel bleeds off back into the tank after shutdown vs. leaking elsewhere. Am I correct?

The fuel pressure in the return line will always be zero, or very, very close to it. A pressure gauge here won't really tell you anything whether or not the reference line to the FPR is connected.

Originally Posted by norcal123

can it go back via the high pressure feed line by some reason? What is my next step under this scenario?

The only outlet for pressurized fuel is the FPR, injectors, backwards through the fuel pump, or through a leak somewhere between the pump and the FPR.

If you rig up a setup with a line -> check valve -> pressure gauge -> line, and install this at several key points (in the trunk after the pump, in the engine bay before the injectors, after the injectors before the FPR), you should be able to see where the leak is coming from.

[norcal123]

Originally Posted by the suicidal eggroll

The only outlet for pressurized fuel is the FPR, injectors, backwards through the fuel pump, or through a leak somewhere between the pump and the FPR.

If you rig up a setup with a line -> check valve -> pressure gauge -> line, and install this at several key points (in the trunk after the pump, in the engine bay before the injectors, after the injectors before the FPR), you should be able to see where the leak is coming from.

Want to confirm the reason for having the valve. Test the pressure at key points without and with the valve in the rig. If the pressure bleeds off WITHOUT the valve but stays constant WITH the valve, the leak is "upstream". Did I get it right?

[the suicidal eggroll]

Originally Posted by norcal123

Want to confirm the reason for having the valve. Test the pressure at key points without and with the valve in the rig. If the pressure bleeds off WITHOUT the valve but stays constant WITH the valve, the leak is "upstream". Did I get it right?

Right

So right now you know the pressure bleeds off. If you install the gauge+valve immediately after the pump and the pressure still bleeds off, you know the leak is further down the line. You then install it immediately before the injectors. If it still bleeds off, you know the leaks is further down the line, etc. As soon as you install it somewhere and the pressure STOPS bleeding off, you just jumped over the leak.

[holyjoetart]

Originally Posted by kev20

the 06-07 have no oring

Good to know!

[scby rex]

my fp always bleeds off. Just takes maybe a day to do it. Maybe theres something wrong with mine too.

[the suicidal eggroll]

Originally Posted by scby rex

my fp always bleeds off. Just takes maybe a day to do it. Maybe theres something wrong with mine too.

A day is fine, a few seconds/minutes isn't.

[norcal123]

Originally Posted by the suicidal eggroll

Most likely in a fuel line, probably inside the tank (unless you can visibly see or smell fuel leaking from somewhere else)

We did not have a check valve, so we just clamped the rubber hose on the gauge when turning off the car. The gauge+clamp were attached to the high pressure line in the engine bay right after the fuel damper thingy (before the injectors). The pressure stayed at 40+ psi. Release the clamp and the pressure goes back to 0 in 2 seconds.

There is no puddle of gasoline under the car, so it must be going back though the pump into the tank. This is brand new OEM pump assembly from the dealer! Oh well...

[NSFW]

Sounds like the pump may not have been correctly installed into the bucket or there's an issue with the hoses/fittings that run from the pump to the top of the bucket. I'd take it back to the dealer and describe how you just narrowed down the problem to a leak upstream of that clamp. Heck, repeat the test for them. Then point out that if it was leaking outside the tank you'd all smell it. Then point out that they're the ones who did the work inside the tank.

Or maybe just pull your back seat out and look into the tank/bucket/pump assembly on your own.

[DHRDAN]

Originally Posted by NSFW

Sounds like the pump may not have been correctly installed into the bucket or there's an issue with the hoses/fittings that run from the pump to the top of the bucket. I'd take it back to the dealer and describe how you just narrowed down the problem to a leak upstream of that clamp. Heck, repeat the test for them. Then point out that if it was leaking outside the tank you'd all smell it. Then point out that they're the ones who did the work inside the tank.

Or maybe just pull your back seat out and look into the tank/bucket/pump assembly on your own.

dat fuel pump.

word.