‘91 fuel pressure question possibly related to long cranking when started

I’m chasing down a long crank issue on my ‘91. It was often a little long, but this week it’s gotten worse.

It’s only when it sits overnight or at work for a long time. When it does crank unusually long it then stumbles for a bit before clearing out and after runs and starts like usual all day. Something I read suggested that could point to a leaking injector.

So far I put a gauge on the fuel rail and got the following readings.

Engine off after sitting all day, key on: 32
Engine running, same
After engine off key off 10 min elapsed: 21
After engine off key off 20 min elapsed: 14

My question is, is that a reasonable drop after running and stopped or does that point to a leak somewhere.

I’m also currently a checking the battery, it tested ok in November but when I put the load tester in it now it’s showing at the high end of the weak band for its CCA. I just put the charger on it and I’m going to check it once more, but may also be due for a new battery.

I’ve been driving it daily to work since November 70 miles round trip each day and it’s been running all weekends so it’s not sitting more than 12 hours at a time.


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Same occurred to me (98 Classic) when I replaced the old injectors with some cheap aftermarket no-names. One or more of them was leaking. The problem disappeared when I swapped the old injectors back in.

Could be the injectors, could be one of the check valves, either fuel pump or regulator. You can check the fuel pump check valve by clamping the fuel line near the filter and see if the pressure still drops. Be careful of that line it gets old and hard and may crack when crushed/clamped. If that isn't it clamp the fuel return line and see if the pressure holds this will tell you if the fuel regulator check valve is OK. I put a BMW part # 16 14 9 068 988 in the fuel line on one of mine and it cured the pressure loss. You may be able to find one at a BMW dealer for a few bucks, mine cost like $4 way back when.
I used to have to turn the key to run then off then run again then start to get a little extra prime to the fuel rail. Worked well for years before I installed the check valve to stop the backflow out of the fuel line.