Fuel pressure question - 986 Forum - The Community for Porsche Boxster & Cayman Owners

tada · 03-11-2016, 03:23 PM

Well the weekend is here and it's time to try to get the Boxster running. I did a fuel pressure test last weekend with a loaned Autozone gauge. Static pressure was 2.4 bars (too low). With the engine running, I saw about 3.3 bars. I assumed fuel pump and ordered one. I also picked up a new relay, but I ran the test with a jumper. Now before I open up the tank, what's the chance that's it's the fuel pressure regulator or a leaking vacuum line to it? I guess I should go get the tester again and try to get the limping engine to barely idle and pull the vacuum line from the regulator. Just doubting myself and probably overthinking it.

78F350 · 03-11-2016, 05:11 PM

Quote:

Originally Posted by tada

... Now before I open up the tank, what's the chance that's it's the fuel pressure regulator or a leaking vacuum line to it? I guess I should go get the tester again and try to get the limping engine to barely idle and pull the vacuum line from the regulator.

Sounds like a good idea. Have you really eliminated a major vacuum leak as the problem already? I have not had to work on the fuel system on mine yet, so not much help from me.
I did pull the pump out of my parts car the other day and got a couple pics. This Tech article is pretty straight forward:
Porsche Boxster Fuel Pump Replacement - 986 / 987 (1997-08) - Pelican Parts Technical Article

A few notes: Take your time disconnecting the vents hoses and wires, don't force anything - It should all come apart fairly smoothly. I broke part of the fastener that holds the fuel lines to the pump, Pulling too hard before releasing the fastener all the way. The pump housing came out easily from mine with one hand grip and a twist.

Here's what came out.

I expect that the pump you purchased will go inside the housing. Note the O-ring that fell to the bottom.

tada · 03-11-2016, 05:38 PM

I just reran the pressure test. Maybe I did something wrong before. This time static pressure was about 3.8 bar (good), and when running 3.8 bar (still good). But after the static test, I performed a leak down test and I'm losing about 1 bar in 10 seconds. Then it levels out at around 2.4. So it sounds like gas may be leaking back into the fuel pump through the check valve. I inspected the vacuum line to the fuel regulator. It looked fine with no gas in it. With the engine running, the pressure dropped about 5 psi with the vacuum line to the regulator disconnected. But now when I jumper the fuel pump relay, I now hear a hissing noise from the regulator. That's with no engine on. Kind of a whirring noise. Car still does not run smoothly or rev at all. So, do you think it's the regulator or the pump? I'm now leaning towards the regulator being bad, it sounds like it is leaking gas during static testing. All help is appreciated.

P.S. The new pump looks identical to the one in your picture above.

tada · 03-11-2016, 06:06 PM

Is there a way to pinch off the fuel return line so that could figure out if the leak was coming from the regulator or pump? If so, where?

tada · 03-12-2016, 01:36 PM

Would a leaking injector cause that quick a fuel pressure loss?

luckyed7711 · 03-12-2016, 04:07 PM

Quote:

Originally Posted by tada

Would a leaking injector cause that quick a fuel pressure loss?

My car has the later, non-return fuel delivery system, but I had similar issues and questions testing the system.

I had adequate pressure, like you, non-running/running, with rapid bleeding off when shut off.

If you do a flow/volume test on the pump output and it checks out, it'll go toward your pump being probably fine. I think on the cars with the return system you could probably pull the line on the pump side of the regulator and check the volume from there. Jump the relay, run for 30 seconds, supposed to produce 850 ml minimum. Mine was putting out over 1 liter, so I ruled the fuel pump out.

At the time I could hear the sort of fluttering click from the area of the #2 cylinder that might be a leaky injector, but didn't know for sure or how bad. I later ran a bottle of injector cleaner through a full tank, and it stopped the sound.

I suspected an issue with the pressure regulator, which on my car is routed in to the fuel level sender assembly. I cracked the old plastic housing on the level sender when I pulled off the fuel line, so I had to buy a new one anyway. It fixed my running problem (which was car running fine, then dying suddenly, then starting after awhile and so on.)
Not your same bad running symptoms, but you're fuel supply test results were similar, so maybe it's your regulator.

tada · 03-12-2016, 04:33 PM

Quote:

Originally Posted by luckyed7711

My car has the later, non-return fuel delivery system, but I had similar issues and questions testing the system.

I had adequate pressure, like you, non-running/running, with rapid bleeding off when shut off.

If you do a flow/volume test on the pump output and it checks out, it'll go toward your pump being probably fine. I think on the cars with the return system you could probably pull the line on the pump side of the regulator and check the volume from there. Jump the relay, run for 30 seconds, supposed to produce 850 ml minimum. Mine was putting out over 1 liter, so I ruled the fuel pump out.

At the time I could hear the sort of fluttering click from the area of the #2 cylinder that might be a leaky injector, but didn't know for sure or how bad. I later ran a bottle of injector cleaner through a full tank, and it stopped the sound.

I suspected an issue with the pressure regulator, which on my car is routed in to the fuel level sender assembly. I cracked the old plastic housing on the level sender when I pulled off the fuel line, so I had to buy a new one anyway. It fixed my running problem (which was car running fine, then dying suddenly, then starting after awhile and so on.)
Not your same bad running symptoms, but you're fuel supply test results were similar, so maybe it's your regulator.

Thanks for answering. That's a good idea. The supply line to the regulator looks kind of rigid. But it makes sense. I think the sound I'm hearing in the regulator when the system is under pressure just may be an injector leak into the manifold. Not o-rings. I'll get it back up in the air tomorrow and start pulling the spark plugs to check for fuel on them. If it is injectors, I guess a rebuilt/cleaning service.

tada · 03-18-2016, 06:06 PM

I changed out the fuel pump and fuel sending unit. The fuel pump was leaking back through the check valve. I broke the sending unit removing it. But I am still getting really rough idle, no ability to rev, and some quiet pops of backfire (I guess). But the fuel pressure is holding rock solid. I'm scratching my head for ideas. I've tried to verify the vacuum system. The vacuum to the fuel pressure regulator is good. And I can trace the other side down to the vacuum canister and the 4 way tee. But I can't see much past that. And I don't know visually that I could see a leak unless it was obvious. Could it be injectors?