Fuel pump residual pressure??

[bimmer630]

Ive had a hard starting issue for a while now with my M20. its only got about 80K miles on it and the prev. owner babied the heck outta it so its been maintained.. but every now and then i get in to start it and it just cranks and cranks.. finally kicks over after a few seconds of cranking. (normally it fires right over)
I first suspected ignition and checked the cap/rotor. the terminals in the cap looked kinda spent so i got a new cap/rotor. seemed ok for a while but nope its doin it again. I have been trying to turn the key on and wait a few seconds before cranking in order to let the fuel pump run, and that seems to make the engine start fine (but doesnt the fuel pump only run when it gets a crankshaft signal from the DME??)

I put a fuel pressure gauge on it today and it gets a good 40psi. when you shut it down and come back a few minutes later, the fuel press. is down to like 20psi. what should the residual pressure be on this thing?
maybe i have a bad pump? after the pressure drops to 20 i'll crank it and it goes right back to 40psi.

normal???? i can deal with it but its kinda embarassing when the car wont start up sometimes

[SlowGoinE30]

I'm having this EXACT issue I'd like to know as well, if I find anything Ill post it up here, but so far I'm in the same boat as you.

[twright]

If you ask Mike Miller of Roundel Magazine, he will tell you that you need to go buy a brand new BMW fuel pump. My car starts fine with residual fuel pressures similar to yours.

If you have an early car with two fuel pumps, this problem can be caused by a bad in-tank pump.

The problem can also be caused by vacuum leaks. How are your vacuum hoses?

[jlevie]

The important thing is how fast the pressure comes up when you crank the engine, not what the residual pressure is. That said, a worn check valve (in the pump) is frequently associated with a worn and weak pump.

As twright noted, there are other possibilities, with an intake leak being at the top of the list.

[netcsk]

You'll find the pump builds pressure very fast. I once thought this was a problem of mine (a long time ago) when the car wouldn't start right up. BMW sells an external check valve that you can put right after the pump if you suspect this is the problem. I don't know the part number, but maybe a search will bring it up.

[bimmerboy12]

You could have a drippy injector. The residual fuel pressure has to be contained on both ends, right? So the pump check valve on one end and the injectors on the other. An injector(s) may be leaking fuel into the cylinders. I think you will have to pull the injectors and have them tested, or another trick is to pull all the injectors and fuel rail, and suspend the fuel rail over the top of the intake manifold with the injectors attached. Crank the engine to get up to pressure (have a rag or containers to catch the misting fuel), then see if any of them continue to drip.

[SpecM]

you mentioned cap + roto replacement, but did you change the plugs too?

also, try this: turn the key to KOEO (key on, engine off) position. This will turn the pump on. Leave it there for about ten seconds, then cycle the key off, and then start the engine. See if that makes a difference. If it does, you do have a fuel issue, since the fuel pump will pre-prime the fuel rail before you start cranking. If not, you probably have an ignition issue or a vaccum leak.