Engine cuts out

[CoClimber]

I have an '87 325is that cuts out at any speed. It will do that for just an instant most of the time but it is bad enough to make it undrivable. Almost everything has been replaced by me or the previous owner. Last night I noticed that the tach would drop to zero when the engine would cut out. I took an oscilloscope and monitored almost every input and output to the DME. When the engine would die, the signals to the injector, the coil, and the tach would go away, usually for well less than a second. This, to me, would indicate the DME or the crank position sensor. The previous owner replaced the DME and I also bought a second DME, which has the same symptoms. The crank position sensor meters 550 ohms and the signal to the DME does not drop out when the engine dies. I inspected the various grounds that I could find and they look fine. I monitored the input power to the DME and that did not drop when the engined died.

I had also replaced the ICV and checked the intake for vacuum leaks and I think all that is good. Of course, the loss of injector and spark signal would make it seem to not be a vacuum problem.

Any ideas?

Thanks, Doug

[e30:)]

maybe the coil is bad? or fuel filter... both are easy to swap and cheap. im sure you can find someone with a good coil to check.

[jlevie]

That failure can be caused by loss of timing reference data or loss of power to the DME. Be aware that the DME gets unswitched power, a start signal, and power from the main relay. A flaky main relay, a bad connection on any of the power leads, or a flaky ground are possibilities. You could also have a flaky crank position sensor or a problem with the harmonic balancer (the toothed wheel that drives the CPS).

[structured]

I can't really tell by your description what's going on. My car had a really bad misfiring problem up until I replaced the harmonic balancer.

This thread may help: http://www.r3vlimited.com/board/showthread.php?t=147102

I think that at least a few other people with similar problems to me were able to fix their misfire by replacing/repairing the harmonic balancer.

[CoClimber]

E30, both the coil and the filter have been changed, along with most everything else. :(

jlevie, The harmonic balancer seems tight and does not wobble at all when the engine is running. I probed all the power and ground pins at the DME and they didn't fluctuate when the engine cut out. The cps has the correct resistance and its signal seems good at the DME. The DME is out of its case so I am probing right at the circuit board. The cps could have a weak signal, I suppose, as I have no way to compare its signal to a known good one. The thing is, the signal amplitude is bigger at higher revs and it cuts out then too.

I will check the resistance of all the ground leads from the DME connector to the chassis and see that they are all correct. This is frustrating. I've been struggling with this for too long.

[delatlanta1281]

Check the ground to the top of the oil pan under the afm. That's what did it to me.

[CoClimber]

delatlanta1281, is that the flat braided cable? I looked at it and hosed it down with electrical cleaner. I suppose it wouldn't hurt to remove it and sand the contacts. How did you fix yours?

[StereoInstaller1]

Have you replaced the main relay?

It will do EXACTLY as you described.

Check your electrical diagrams: http://www.wedophones.com/BMWManualsLead.htm

I use a normal DPST relay (no 87a, but has 2 87's) in mine: commonly used as a foglight relay. I have also wired 2 SPDT relays in place before, but the stock setup lasts about 20 years, fukkit!

Luke

[structured]

OP: I just realized that you are from Ithaca. Funny...I was born there.

[e30m20beast]

That might be the grounding, check all the cables that connect to the motor, or just go ahead and but a grounding kit,, it might fix it and give you some more power. Good luck

[StereoInstaller1]

An engine ground is not going to have any effect on the tach, etc.

[delatlanta1281]

not on the tach, but on every other gauge. OP, i would def check the main relays and the wiring that leads up to them. Also, I "looked" at the groung when GG told me about it, but I didn't wiggle it. It was really loose. All I had to do was screw it back in.

[CoClimber]

Okay, I removed the ground wires on the right shock tower and braided cable under the AFM and cleaned the contact surfaces. I also removed the main relay and opened it up - I don't have a spare - and checked the contacts. None of this made a difference. I looked at the CPS signal at the DME and I think it is fine.

I was thinking about it and if there is a bad ground, it could cause a voltage spike that could reset the DME. If that happens, the engine would cut out until it could find TDC from the CPS. That's my theory anyway.

[StereoInstaller1]

Shit man, if you have that consistent of an issue, put a test light on it and drive around.

I think the coil is the issue on the main relay.

If you find a "Hella" foglight relay at an autoparts store, they should be the same thing.

[CoClimber]

StereoInstaller, I opened up the main relay so I will definitely be replacing it, hopefully today.

I spent a lot more time last night looking at the signals and saw nothing weird with the input signals. I looked at the power and ground very closely. It could be that the 'scope I'm using isn't the greatest. I'm wondering about the harmonic dampener. I can't move it by hand although I didn't try to really torque on it. It seems to run true when I watch it with the engine spinning. structured, do you think it could still be bad?