Fluctuating fuel pressure - any fixes?

[Jeff N.]

The fuel pressure on my M30 will typically sit at 42 lbs. Just right.

Occasionally after the car has been running a while, the FP will drop to about 35 or 36 lbs and stay there. At least until the car is off and sits for a while.

The lower FP messes up the mixture and the car runs a bit leaner than I would like, especially on idle.

Any ideas short of replacing the fuel pump? The FPR has been swapped with a different unit and the behavior is consistent so I don't think it's the FPR.

Ideas?

Jeff

[Barney Paull-Edwards]

I know it may be teaching your grandmother to suck eggs but that sounds like a blocking filter.

[Robin-535im]

The fuel pressure on my M30 will typically sit at 42 lbs. Just right.

Occasionally after the car has been running a while, the FP will drop to about 35 or 36 lbs and stay there. At least until the car is off and sits for a while.

The lower FP messes up the mixture and the car runs a bit leaner than I would like, especially on idle.

Any ideas short of replacing the fuel pump? The FPR has been swapped with a different unit and the behavior is consistent so I don't think it's the FPR.

Ideas?

Jeff
Hey Jeff...

Did you replace it with another adjustable one or try the stock / 944 variety too?

I would think the FPR's job is to keep it level regardless of the system variances... and if it's changing then the FPR is not doing it's job.

Could your vacuum to the FPR be changing / high-temp vacuum leak?

This is measured with the gauge that lives under the hood? I wonder if the gauge gets off calibration when hot.

So in summary... no answers, just more questions :)

[ryan roopnarine]

a quart of marvel mystery oil in 10 gallons of gas seems to make fuel pumps really happy to be alive. i don't think that anyone makes a specific fuel pump rejuvenator (sp) that works as well as MMO. that, and a fuel filter change.

[Jeff N.]

Did you replace it with another adjustable one or try the stock / 944 variety too?

It's just a stock 3 bar unit. The adjustable one is on the bench as I was having the same problem with it.


I would think the FPR's job is to keep it level regardless of the system variances... and if it's changing then the FPR is not doing it's job.

Right. Except why would it happen on two different units?





Could your vacuum to the FPR be changing / high-temp vacuum leak?

Now that's an idea. As I think about it...as the vacuum goes to zero, the FP rises. If you pull the vacuum line when things are working right, the FP goes to about 47. So, low FP wouldn't be related to a leak I think.



This is measured with the gauge that lives under the hood? I wonder if the gauge gets off calibration when hot.

Yes. I don't think the gauge is off because I can tell I have a problem via the wideband gauge in the cabin. The car starts to idle like crap and the mixture goes lean. If I then open the hood and check the FP, it's low.

I suppose I could put a new filter in it...