will fuel pump prime with 02 unplugged?

[bmwm42]

You don't have a true m42 fuel system in your car like I do......we don't have dual fuel pumps.....but who cares if its running

[Dj Buttchug]

but here's the wierder part:

since I'm working on the car, i have the battery unhooked often. When I first hook up the battery and go to start.....the pump does NOT prime.

But once I've started the car and then turned it off, then if I turn ignition on again, the fuel pump DOES prime. I just went out and tested this multiple times, and it works every time.

So maybe the DME has some kind of memory to it, IDK. I don't much care, personally. If the car runs, it runs :)

this is what i think mine does... ???

[Ryann]

Fil is correct. There is a single pump in an original m42 car, and it NEVER PRIMES. The DME will not energize the fuel pump relay until it sees a good CPS signal, which it does not see until the engine is cranking.

[irish44j]

You don't have a true m42 fuel system in your car like I do......we don't have dual fuel pumps.....but who cares if its running

I don't see why that would make any difference. The fuel pumps are still running off the M42 DME and M42 wiring harness/relays. My pump is getting power at exactly the same time as it would if it were an in-tank M42 later pump. The DME don't know what kind of pump is hooked up. So whether I have a 2-pump, stock M42, or aftermarket pump is entirely irrelevant to the conversation.

The fact that there's an electrical load on it remains constant, regardless of what is hooked up......

Again, this is all academic, so whatever. I'd go prove what it's doing....but it's really not worth the effort and getting fuel all over the place, lol.

[irish44j]

Here, I just made up a vid. As DJ and me both noticed....it won't prime if the battery has been unhooked, but if the car is started and shut off, and then ignition "on" is turned, the pump will prime for about 1/2 a second. 100% sure it's the pump.

So.....clearly the DME is activating the fuel pump relay (via the main relay, obviously) for a short time. Whether this is some kind of residual power or something else, who knows. Why it only primes for 1/2 a second, who knows.....

maybe it's not actually priming, but can't think of anything else it could be doing. In any case, the DME is definitely sending power to the fuel pump for a short burst....

video is uploading right now. Here's the link. If you wear headphones it's pretty obvious.

http://www.youtube.com/watch?v=35Y7ytLAGAE&feature=youtu.be

[ForcedFirebird]

Interesting. You do know your DME retains power for several seconds after the car is shut off, though? If you listen to it out in the open, it will buzz for about 45sec after the key is off, so it does in fact retain residual power as you said it might. Does it still prime after the car being off for a few minutes? The e30 fuel pump is not supposed to prime - well using any Motronic 1.x, anyways.

[Dj Buttchug]

Here, I just made up a vid. As DJ and me both noticed....it won't prime if the battery has been unhooked, but if the car is started and shut off, and then ignition "on" is turned, the pump will prime for about 1/2 a second. 100% sure it's the pump.

So.....clearly the DME is activating the fuel pump relay (via the main relay, obviously) for a short time. Whether this is some kind of residual power or something else, who knows. Why it only primes for 1/2 a second, who knows.....

maybe it's not actually priming, but can't think of anything else it could be doing. In any case, the DME is definitely sending power to the fuel pump for a short burst....

video is uploading right now. Here's the link. If you wear headphones it's pretty obvious.

http://www.youtube.com/watch?v=35Y7y...ature=youtu.be

thank you. This is what I was trying to get at and this is exactly what all my m42 cars do.

[irish44j]

Interesting. You do know your DME retains power for several seconds after the car is shut off, though? If you listen to it out in the open, it will buzz for about 45sec after the key is off, so it does in fact retain residual power as you said it might. Does it still prime after the car being off for a few minutes? The e30 fuel pump is not supposed to prime - well using any Motronic 1.x, anyways.

Yeah, it will do it no matter how long the car has been off, provided that the battery is not unhooked (in which case it won't). I just went out and checked it again and it did it - and I haven't turned on the car since I made that vid yesterday.

IDK, whatever. Just an interesting piece of trivia either way, since I don't think it has an effect one way or the other.

[ForcedFirebird]

Learn something new every day. I don't work on the m42's often, but had always disconnected the battery when working in the bay or with the fuel system. They wouldn't prime unless the engine was rotating and never gave any more thought to it since the 6cyl cars never do prime.