1G How long does your rail hold fuel pressure?

[rdeis]

Hey all! I'm chasing a weird low RPM idle stability problem that has a *huge* hot start component. It settles down after its run for a bit.

The fact that I can stall at a light in hot weather and have trouble getting restarted until the fuel pump has been running a while suggests to me that I've got a problem maintaining fuel pressure. I've gone so far as to install a button in my cabin to run the pump via the check connector. That helps, but it's not instantaneous.

I'm showing the proper ~37 plus boost pressure on my FP gauge when its running. I drove the car this morning and now 8 hours later the fuel pressure shows zero.

Am I right to suspect the Fuel Pressure Regulator or the pump check valve are letting it leak down too fast when the pump is off?

 

[Vladimir]

Not really sure what you are asking but both fuel pump check valve and FPR can bleed back fuel pressure. Also fuel pump housing to fuel pump hose can develop leaks and cause loss of pressure. Check the vacuum line that connects to the FPR for fuel.

I don't see a major problem if fuel pressure goes to 0 after 8 hours. After 8 sec/8 minutes yes, you got a problem. If you are maintaining 37psi fuel pressure at idle and the pressure doesn't deep down with you revving the engine up, it's very unlikely you'll have a fuel delivery issue/ clogged filter etc ...

Check your ECT. It can make the car run crappy. No start or hard to start when hot is a very common complaint on a bad ECT sensor or wiring that leads to the sensor.

 

[luv2rallye]

Fuel rail pressure should be maintained for days with engine completely turned off. A leaky FPR is the usual first suspect. Next a leaky fuel injector or fuel pump.

 

[Vladimir]

Fuel rail pressure should be maintained for days with engine completely turned off. A leaky FPR is the usual suspect.

I agree that fuel should be in the rail, but not pressure days after the car ran...at least that's what I've always witnessed.

Fuel pressure always bleeds off. Some GDI systems maintain pressure in the high pressure line a lot longer. All the conventional EFI low pressure system such as our DSM always, bleed off. Some take couple hours some longer but still the pressure will drop to zero. Which is not a problem at all.There is unpressurized fuel in the lines. It builds up in less than a second. As long as the vehicle starts right away without a prolonged cranking time, there is nothing that needs to be done.

Rapid pressure drop and longer cranking time would indicate an issue. Injectors leaking, FPR weak or leaking diaphragm, fuel pump check valve, fuel lines leaking, fuel hanger o-rings and etc... will usually cause a rapid drop and longer cranking time.

Of course in ideal world or by the textbook the pressure should always be maintained in the rail. But I don't know if I ever seen that before. Slow bleed off that take many hours or days do not cause drivability issue. Just regular wear and tear all cars have.

 

[luv2rallye]

I agree minimum fuel pressure (on fuel injected cars) should remain typically for hours for quick starting - the worst case 20 minutes. I guess I should have said all my cars have always kept enough pressure to start within 3 seconds, days or even a week later - even my 1999 Eclipse GST. Of course I keep them up with regular maintenance (change oil, filters, no 6 month old gas, etc).

 

[steve]

If you were seeing some delay on reaching base pressure I'd be looking into the condition of the fuel system but holding pressure for 8 hours is more than enough and I'm guessing you also have almost instantaneous fuel pressure pressure on cranking. Do you still have a fuel pressure solenoid? That should cause the fuel pressure to be hitting base on hot starts where normal fuel pressure while cranking will be lower due to manifold vacuum.

So, I'd start looking for other potential causes

 

[rdeis]

but holding pressure for 8 hours .. I'm guessing you also have almost instantaneous fuel pressure pressure on cranking..

It was zero after 8 hours, so it's not holding. Initial crank for the day always takes a while. Once pressure has built and the car started it seems OK until it's ben stopped for a bit.

On a really, really hot day it doesn't need to stop for more than a few seconds before the restart gets iffy. (Don't stall it at a light on a busy road!!)