ChrisUlrich

Car runs fine in open loop, dies in closed loop... could that be because of a fuel pump?

OBE1 95Z28

If your fuel pump was bad, it wouldn't run in open loop. You can verify by using a fuel pressure gauge. O2 sensors could be the culprit.

shoebox

The fuel pump does not know the difference between open and closed loop - only on and off.

Injuneer

It more likely it would run worse in open loop with a failing fuel pump, since in open loop the PCM can not use the short term fuel corrections to compensate for low fuel flow. It would run better in closed loop, because now the PCM would be able to use the ST's to compensate.

Its not likely that its the fuel pump, but why guess. Put a fuel pressure gauge on it and verify it is performing correctly.

ChrisUlrich

A gauge was put on, but it was said that

"Just because it's giving ideal pressure, doesn't mean it's giving ideal volume"

I take it, this statement is false?

The Mechanic claims it's fuel related because once he sprays gas down one of the intake lines, the car runs great with no popping or signs of running lean (at idle).

Dave89IROC

it sounds fuel related, but delivery(injectors, ect.) not supply(pump/lines)

have him check the oxygen sensor signals on a scanner, BLMs as well

Z06Z28

Sounds like my car is doing the exact same thing in those conditions, interesting...I'll keep following this thread. Keep us updated so I can try a few things out.

Also...
I have Autotap scanning software for Palm just lying in my garage, anyone familiar with using it? Haven't really used it much.

Injuneer

Not correct. The fuel pressure regulator is in the return line. The only way it can hold the pressure on the system is if the system is flowing enough fuel to supply all the injectors, and have a bit left over to return to the tank. You put the fuel pressure gauge on, tape it to the windshield and take it out on the road and up to max load/RPM. If the system holds 40psi or higher, it is OK. It means all the injectors are getting all the fuel they could ever use, and there is still enough left over to pass through the regulator and control the system pressure.

If the system can't supply enough fuel, the pressure in the system will drop.

The only way the above statement could be true is if you only measured the system pressure at idle, and not at full load. Then, the system may run out of fuel when the engine is under load. But that would mean the fuel pressure was not measured correctly.

Z06Z28

Chris...what's the update on your ride...just curious?

ChrisUlrich

I have a new fuel pump going in today/tomorrow so we'll find out then.

He said the car ran perfect when he sprayed Starter Fluid/Gas into the intake or vacuum line... I forgot. haha So we're trying the fuel pump/fuel regulator first.

hammersz28

I have one word, OPTI-CRAP..

MarcR94v6

What is open and closed loop?

My car stalls at low RPMS sometimes, especially when going in reverse it seems. Is that similar to the problem you're having?

Injuneer

When the engine is started, the PCM is in "open loop". It ignores the feedback from the O2 sensors, because they do not work until they warm up to over 600*F. After the sensors reach operating temperature, the coolant has reached 120-140*F and a timer has expired (approx. 2 minutes), the PCM switches to "closed loop". In closed loop, it looks at the O2 sensors, and adjusts the fuel flow using the short term fuel corrections, to try and maintain an A/F ratio 0f 14.7:1. If it can't control it to that limit using the short term corrections, it starts to alter the long term corrections (aka "BLM"s) to reach the required ratio. The BLM's are stored in the PCM, and reflect the majority of its "learning" capabilities.

Check out the air/fuel management section of this writeup:

http://members.aol.com/InjuneerZZ/ScanMast.htm

Z06Z28

So hows the car running, fixed?