Every time I need to release the fuel pressure I pull the fuel pump plug and the car immediately starts idling perfectly at 1500 rpms. For well over a minute. Eventually it starts to go up to 2000 rpms.

It eventually (possibly upwards of two minutes) runs out of fuel and stops, but this takes too long and sometimes I turn the engine off before someone in the neighborhood complains... which is a different issue (racing muffler + super thin walls).

When I actually have the pump connected it has a lumpy idle that is much more rich (with a bit of black smoke).

 

vaccuum from the engine is pulling fuel through the feed line, probably starts leaning out and running smoother causing it raise rpm

I've seen it happen before on other cars, but not my particular FD, it would shut off immediately. I guess it depends on your fuel line size, routing, rails, pump, etc.

 

Why would vacuum pull fuel into the engine? Its normal for an engine to run for a few seconds on residual fuel pressure, but not two minutes.

If anything its too rich when it idles with the pump plugged in and this has it running closer to the right AFR. I suspect my fuel pressure is too high.

Currently the fuel system is all stock except for the pump, although an aftermarket fuel pump shouldn't really be causing this. Before I had a SARD AFR that made the car run like ****. I bypassed it and now it only runs like crap. And for some reason it runs great with the fuel pump unplugged.

 

Only thing I could think of would be is the engine is fulling fuel from a stuck or open injector and run on the residue fuel pressure and return lines. Have you checked the injectors?

 

I swapped them out with some used ones because they were one possible cause of my misfire problem, but I'm going to swap them back since they probably weren't the issue and one of them leaks ever so slightly.

I checked one of the secondaries but not both.

 

Pulled primaries again and both pintle caps were cracked. Which is weird since only one was cracked last time.

 

When an engine is operating, there is a vacuum during the intake portion of combustion. This draws air and fuel into the chamber before being compressed and combusted. The fuel pump pressurizes and supplies fuel to the fuel rail and injectors where it is then metered via the injectors into the intake stream. If you unplug the fuel pump. There will still be pressurized fuel in the fuel lines. If you're running some absurd extreme amount of pressure in your fuel lines, it's not unlikely that it will take a bit for it to bleed off all that pressure once you unplug the fuel pump. Even page F-96 in the FSM covers this under the "fuel pressure hold" test. It should retain more than 21 psi for at least 5 minutes. So again if you're pump is jammin' away blowing your fuel pressure up to like 40psi or something dumb, it'll take longer for it to bleed off.

Long story short is it sounds like you have a fuel pressure issue. Most likely excessive fuel pressure. Soubds like you need to quit messing with it and take it to a professional.

Matt

 

I know all of that. My question why was the issue would be vacuum and not simply excessive residual fuel pressure. Any EFI car should run for at least a few seconds with the pump unplugged.

It actually does the same thing (run at 2000 rpm) when the tank is empty.

Do you mean 40 psi with the pump off? The pump is SARD but currently I only have the stock FPR, which may have been gummed up by old fuel. I had a SARD AFR on there and it was definitely causing excessive fuel pressure (probably gummed up by something), so I bypassed it with a brass fitting, which may be causing a slight restriction on the return line.

That was my theory.

Every time I take it to a professional here I get charged about twice as much as I was really looking to pay... OTOH, swapping out a secondary rail in case it's the stock FPR that's been gummed up by old fuel is not a big deal. I've got a fuel pressure gauge coming in the mail so I'm going to check the pressure.

 

which relay are you pulling? i ask to double check, but there is a fuel pump relay that supplies power and there is a fuel pump relay that switches speed. pulling the speed relay, which is labeled fuel pump, may just be lowering the pump voltage, especially if it runs for more than ~30 seconds, or raises the pump voltage from 9 to 12, i forget which way the FD works.

the other thought is that the Power FC was probably tuned WITH both FPR's on it, or you'd think it would be. i wonder if finding a stock ecu would be a good idea, it'll start and run and spit out codes and other handy stuff.

 

No relay; I pull the actual plug on the pump since I don't have any carpet in the car.

Yeah, it was certainly tuned with BOTH, but I suspect the aftermarket FPR failed and turned up the pressure to infinity, since I burned through 10 gallons of fuel in about 30 minutes of idling and cranking...

I think I'd rather spend the money for a stock ECU I'll never use towards a commander for the Power FC.

 

that is a better idea.

 

Gaige pod still for sale? Willing to part on the gauges?

 

I dropped my injectors off for cleaning and the shop owner gave me a commander. Score.

 

For some reason its showing my boost as 0.36 kg/cm2...with the engine off.

Edit: Never mind, I accidentally changed the MAP sensor settings.

 

I tested the FPR and its giving me 45 PSI with the engine off.

It doesn't hold the pressure, though.

Also, the FPR won't hold vacuum for more than a few seconds. Should I just replace it? Would a slow leak on the MAP sensor's vacuum hose cause the reading to be wrong?

 

ive seen a few bad FPR's and it makes the car run really bad and really weird. plus fuel will actually leak through the FPR through the vacuum line into the engine, which doesn't help.

 

Yeah, I didn't get any fuel coming out of the FPR when I pulled vacuum on it, through... OTOH, it can apparently take a few minutes for fuel to actually come out, and the FPR only holds vacuum for a few seconds.

I think I'll just order a new one next week.

I spoke to the previous owner and apparently the car was tuned for 3.0 bar fuel pressure (probably because its on stock injectors) and gets turned down by one turn for emissions checks... lol

 

i th9nk the idle will go up because fuel pressure is dropping and leaning out the mixture. its weird why it runs for 2 minutes though, mine dies after 10 seconds.. maybe becuase the fuel pump lets more fuel back into the tank when its not running, dropping fuel pressure quicker than yours

 

The fuel system should hold 20 PSI for five minutes, IIRC.

No idea how fast 20 PSI should drop to the point where the car will stall if the engine is running, though... Although I suspect it should only be about 10 seconds, like you mentioned.

 

Why do you need to relieve the pressure so much? All I do in the shop is open the gas cap and wait a few minutes for pressure to release, then pull the fuel lines. Not that much comes out.

 

Because the car sat for about five years and I'm still trying to stamp out possible causes of why it isn't running right yet.

In this case I'm more concerned about why it's running (better than it usually does) for something like two minutes when it should die after about 10 seconds. I'm not overly worried about a little gasoline spilling.

Injectors should be back in a few days so I'll be able to check the stock FPR, which I might just bypass with a proper adapter (from Tomei).