Yet another slant on the 944 rough idling problem.
 

Chaps,

Just browsed through past postings regarding rough idling on the early 85 944 but couldn't see an answer that suits my symptoms.

The car starts first turn of the key regardless of the weather and ticks over nicely from cold. Warm the car up with a brisk ten mile run. Ticks over nicely at around 900 revs. Switch the headlights on and the tickover dips way down to borderline stalling speed - indeed it does stall on occasion. Any town work (daytime) that involves a lot of sitting in jams causes the electric fan to come on and the engine starts to idle poorly and shudders as if the mounts were on the way out - they're not. Obviously the heavy-ish electrical drain is causing the problem - I suspected the charging system but a few revs of the throttle sees the battery gauge pick up nicely.

any ideas?

I've already changed:

All the belts (Porsche main dealer.
Plugs
Rotor
Distributor cap
Plugs
Leads
Ignition sensors
Cleaned the injectors
Ignition coil

One thing worth mentioning - after I had Porsche change the belts there was a noticibly slipping of the alternator belt which meant taking the car back. Apparently the slipping was caused by glazing on the pulley and they cleaned this up and retightened. Is it possibly they overtightened - causing undue strain of the alt?

Cheers in advance

I would suspect the regulator on the alternator and/or corroded wires from the alternator to the battery and starter.

Include the grounds from the engine to the chassis and from the battery to the chassis as this is the other side of the charging circuit.

Well its normal for load on the alternator and hence engine to increase when you turn electrics on, whats not normal is that your flywheel sensor should sense the drop in rpm, and increase the fueling accordingly. It should regulate RPM to 900 or whatever.

So I'm not sure how it works in your 944 but the fuel injections not doing what it should.

Mark

Mark, Your 94S2 has a DME controlled idle possitioner. The 83 through 85.1's don't.

SoCal Driver, I'll certainly give it a try and thanks. I'll stick her on the ramps and see what's what. I'd have thought that poor connections would have been consistantly troublesome throughout the charging system though - battery always seems full of pep. Still it's worth eliminating from the equation.

The grounds on on the top back of the block to the chassis firewall. This is the "other" path from/to the alternator.

I think your right hand drive car has the battery in back. Need to check both ends of the positive cable as well as the ground at the battery. The heavy positive cable on US cars goes to the starter. Need to check yours too (thus the ramps).

Loads on the electrical system are being "felt" by the engine, and since you have an early car, the DME does not automatically compensate for the additional mechanical load.

The first thing to do is check all of your electrical connections, making sure they are clean and tight. Start at the battery, and then work from there. You should find ground points on either frame rail behind and below the headlights, on the centerline of the firewall, and in the cargo area near the tail lights. Also check the connections at the starter and alternator.

Remove each fuse, clean the contacts, and then reinstall. Do the same with the relays. Also check the head lights, side marker lights, and tail lights; remove the lamps, clean the contacts, and reinstall.

Have the battery and alternator load tested.

The major problem with the electrical system in the early cars (other than dirty/loose grounds) is the alternator is actually under-rated (90 amps) for the amount of current that the various loads will draw.

another cheap trick would be to replace the grounds with high quality audio system power wire (4guage or so should do it) and make sure it is not grounded to paint and then metal. if there is paint, then sand it down to bare metal.