M50 fuel pressure question

[Dave M]

Recently been having some minor running issues, hunting idle and CEL etc.

The fuel pump is noisy once running for a few minutes, so I checked the fuel pressure. With the FP relay jumped, its bang on 51-52 psi. With the engine running, it drops to ~43-44 psi. Bentley only states that the pressure drops when running, but what is acceptable?

FWIW, I've purchased a new fuel pump and O2 sensor and they are due for replacement and relatively cheap (TRE performance pump - $60).

As always, I greatly appreciate any and all input,

Dave

[whiskychaser]

Fuel pressure drops about half a PSI for every inch of mercury of intake vacuum. So in theory you have an intake vacuum of 16 inches of mercury. IIRC a healthy engine is somewhere between 16 and 22 in Hg. That is good for up to 1000ft above sea level and from your location I think you are under that. If you are not at home you might need this chart:
http://cts.rdhaglund.com/Impco/LargeEngineCatalog/EffectsOfAltOnVac.pdf

FWIW I'm at about 800ft and get 51psi fuel pressure. With the engine running it drops to 42psi which tallies with my intake vacuum of 18 in Hg.

[BennyM]

This discussion http://www.bimmernut.com/forum/5-series-bmw/42007-tell-me-if-these-readings-make-any-sense.html I had a few years ago might add some insight for you:


I don't think my FPR was bad, but aged. I replaced it anyway in the end, which was nither a difficult nor expensive task.

[whiskychaser]

This discussion http://www.bimmernut.com/forum/5-series-bmw/42007-tell-me-if-these-readings-make-any-sense.html I had a few years ago might add some insight for you:

Nice to note I am at least consistent with my answers:) What sorted the problem in the end - O2 sensor, FPR or neither?

[Dave M]

Nice to note I am at least consistent with my answers:) What sorted the problem in the end - O2 sensor, FPR or neither?

Thank you Whiskey and BennyM for the info. With regard to BennyM's old thread, I'm also interested in the culprit.

I have replaced my FPR in the past few years, but likely still have the old one which was not suspect (just replaced as a precaution). I may try swapping it in if the new pump and or O2 don't solve my problems.

Dave

[BennyM]

A new O2 Sensor did the trick for me. In the process I had replaced several suspect vacuum hoses, the fpr, intake gaskets, and engine sensors, but O2 was the big one.

[Dave M]

Problem solved.

After replacing a noisy fuel pump, FPR and much needed sending unit, I still had occasional hiccups and 1222/1212 codes. The morning of leaving on a 3,000km drive, I replaced the O2, which thankfully solved the problem and provided for a smooooooth drive and great mileage.

Thanks again to all who responded with advice or experiences,

Dave