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      10-16-2018, 11:42 AM   #1
drwillb's Avatar

Drives: e30 M3, e30 M3, e46 M3
Join Date: Aug 2008
Location: northern NJ

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Fuel pressure related to ambient temp?

I haven't gotten around to installing any of the new pumps or filter but I did make an interesting observation over the last couple of weeks.

FP_H seems to be linked to environmental temperature. When the temps were in the low 70s, I was running 7 or 8 for pressure as indicated by JB4 phone app. Lately, 70s temp makes the pressure dip to 6. With temps in the high 60s, pressure ran at 8 or 9, somtimes 10. Temps at low 60s gave pressure of 9 or ten. You can see the trend. Two days ago, I was out before 7am and the temp was in the low 40s. This gave me a pressure of 12-13. Same thing this morning.

These readings weren't just for the first couple of minutes during warmup. The numbers hold for at least 20 minutes and then drop. Again, this is temp related. The colder it is outside, the longer the reading is stable and the less the pressure drop when it occurs.

Kinda makes me think this is low pressure pump related since the fuel temp would be a factor vs. engine block temp that would rise to a predictable and steady temp within a few minutes.
Owned so far: 88 M3 x 2, 95 325is, 95 M3 x 2, 06 Mini Cooper S, 08 335 xi, 09 Z4 35i, 01 M3, 12 135i
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