1987 944S misfiring

eddymcclements · 06-12-2003, 06:30 AM

I recently had an alternator failure on my 1987 944S. I started the car using jump leads so that I could drive home, and when I arrived I needed to re-start the engine after I'd stopped. Shortly (but not immediately) after I disconnected the jump leads the motor started to idle roughly and began running on three cylinders which I initially put down to a weak spark as the voltmeter was by now only showing 10 volts. After charging the battery and having the alternator re-built the car starts, but runs very roughly, like it's only firing on 2 or 3 cylinders. Have tried new dizzy cap, rotor arm, coil and DME ECU unit, all to no avail.

I had a replacement battery six months ago when the original died suddenly due to old age (showed 13.5 volts, and more at idle, but internal resistance meant it died while cranking). Last year the car was fitted with new crank pos sensor and ignitor module, so I'm not inclined to suspect either of them at present. I'm also not sure whether this is a totally new problem or one caused by the alternator failure (or by using jump leads and disconnecting the engine was still running?).

Have replaced plug leads as they were a looking a bit cracked and old - spark now looking a bit healthier, but problem persists. I was wondering if voltage spikes from a bad diode in the regulator might have been confusing the DME and causing the problem, but have isolated the alternator while running and it's still the same.

I'm now well and truly stuck, so if anyone out there has any suggestions I'd be most grateful.

Eddy McClements

Lawrence Coppari · 06-12-2003, 07:48 AM

Is it the same cylinders doing the missing or is it random? Have you changed your spark plugs?

eddymcclements · 06-12-2003, 08:01 AM

Before I changed the plug leads, it appeared to be number 4 that was misfiring, so I replaced that plug. Now, I'm not so sure - the problem seems to move about between all four. What could be causing a weak or intermittent spark other than a faulty coil/plug leads/dizzy cap/rotor arm?

Eddy

Lawrence Coppari · 06-12-2003, 09:06 AM

Check the connections at the rear of the block where the rpm and crank position sensors come together. I have had trouble with mine getting dirty. Each has a union about 1 foot from the sensor that is mounted on top of the clutch housing. You see it from under hood. The sensors bolt to clutch housing.

Check connections where harness connects to flapper area. Flapper valve measures air intake to decide how much fuel to squirt.

I assume missing occurs at all speeds.

mavm944 · 06-12-2003, 11:47 AM

Perhaps you fried your computer chip. It has been known to happen on all kinds of vehicles when they get boosted, and that would probably account for the misfire.

ae1969 · 06-12-2003, 12:35 PM

? how did the plugs look across all 4 ???? any differences? If one is misfiring badly it should slowly become apparent?

I would suspect a bad intake leak as well.....since you mention it occurs over all 4 now