Please shoot holes in this

[Adnan]

Hi,

My starting woes continue but the situation is rather intriguing. I have a theory but I'd like you to poke holes in it.

After a three week vacation, the car started up perfectly, drove normally to the store. Twenty minutes or so must have elapsed and upon my return, she refused to start, only offering the ocassional hiccup or so. Cranking was energetic and all electricals appeared to work fine.

The next morning, she started up as if nothing had happened so I drove her home. Over the weekend, I tried again. Here is what I have found - when cold or restarted within five minutes, everything is good. If I let her sit for fifteen to twenty minutes, things seem wrong (rough running and possibly a no-start if I waited long enough). Let her cool again, and all's well.

Now for the theory. I suspect that there is a vapor lock situation somewhere in the system. It takes a while to develop, hence there isn't a problem with a quick restart or a cold engine. What would cause this? My first thought is that the check valve has failed. Bruno's site has a page on adding an external check valve but I don't know if any of you have used the method.

Electricals are fine, no corrosion or blown fuses. The car is in the SF Bay area where the weather gods are very kind to cars. The one thing I still need to check is whether the fuel pump runs always or not but I can't imagine that it would be sensitive to the temperature in the engine bay.

Love to hear your thoughts. Car is a '91 535i.

Thanks a bunch,
Adnan

[Robin-535im]

Given that the hot-start issue is common enough to have it's own entry on Bruno's site... and that your symptoms seem to match, I would put that high on the list. I haven't (yet) had to fix that but many here have, I think it's just as his site spells it out.

[RichG]

Hi,
After a three week vacation, the car started up perfectly, drove normally to the store. Twenty minutes or so must have elapsed and upon my return, she refused to start, only offering the ocassional hiccup or so. Cranking was energetic and all electricals appeared to work fine.

The next morning, she started up as if nothing had happened so I drove her home. Over the weekend, I tried again. Here is what I have found - when cold or restarted within five minutes, everything is good. If I let her sit for fifteen to twenty minutes, things seem wrong (rough running and possibly a no-start if I waited long enough). Let her cool again, and all's well.

Love to hear your thoughts. Car is a '91 535i.

Thanks a bunch,
Adnan

I have a 90 525 with an M20. I have similar problems once in a while. Cold starts fine and usually starts fine after warm. But sometimes it won't fire and I have to hold the pedal to the floor and then it will start within one or two seconds. First time it did this I didn't hold the pedal down and almost killed the battery. After I figured out to hold the pedal to the floor, it hasn't been too much of a problem.

Anyone know what would cause these similar symptoms? And why holding the pedal down to the floor alleviates it?

Rich

[bmwrp8]

I have a 90 525 with an M20. I have similar problems once in a while. Cold starts fine and usually starts fine after warm. But sometimes it won't fire and I have to hold the pedal to the floor and then it will start within one or two seconds. First time it did this I didn't hold the pedal down and almost killed the battery. After I figured out to hold the pedal to the floor, it hasn't been too much of a problem.

Anyone know what would cause these similar symptoms? And why holding the pedal down to the floor alleviates it?

Rich

cps? fuel pump?

[dukester]

Just got rid of a very similar problem in my 525 M20. Fuel pressure regulator diaphragm had ruptured and the engine was getting raw fuel through the vacuum line into the intake. Changed it out and now it always starts easily, hot or cold. You can check by pulling the line from the reg, mine was wet with gas.

[Jeff N.]

Hey Adnan!

A few things I might check.

- funky heat sensitive coil
- bad engine temp sensor
- already mentioned FPR problems

How does it run after it starts warm? Is it just a brief run crappy or does it run poorly for several minutes or more? A short run problem could be flooding due to leaking injectors.

How about power when running correctly? Does it make normal power? If no, then you could have low fuel pressure causing a lean condition.

How about engine codes? Do you have any? What are they?

I don't think it's your fuel check valve, that will just cause problems with building correct FP upon restart. Shouldn't make any difference once running.

[Tiger]

Yep, vapor lock. Get the E39 Fuel checkvalve kit.

[whiskychaser]

Not had this problem on my BM but have had it on other cars. Problem was dry joints on fuel pump relays. Cold start is great but leave them standing warm for ten mins and the buggers wouldnt start. Could never figure why this wasnt a problem when you were driving along. Didnt even buy new relays - just soldered the pins on the old ones