996 idles perfectly WITHOUT alternator connected, but stalls and low idle otherwise!

Could help with some diagnostics! Having a super weird problem with my 1999 996 tiptronic cab - few weeks ago was driving home when it started misfiring. Codes were for misfire cylinder 6. Have done some maintenance since then including oil change to mobil1 5-50, new filter new spark plug, spark plug tube, coilpack cylinder 6 (as the first code pulled was for misfire on 6) installed 997 oil pressure piston and spring cleaned MAF, IACV, throttle body (ran car with/without MAF connected, no change to bad idle/stalling) new fuel pressure regulator (old one was leaking) new fuel pump Would usually throw codes for misfire on cylinder 6, rarely on 3 and 5. Now the weirdest thing is, yesterday I pulled the serpentine belt and ran the car - it idled PERFECTLY like there was never a problem. With the serpentine belt on, misfires and backfires. You Tube Today I removed the plugs at the back of the alternator and ran it - car runs perfectly (eliminates water pump, power steering pump and aircon compressor and belt tensioners as suspects). Also means the MAF, IACV, fuel system etc are good. Obviously not a longterm solution to have a disconnected alternator. Put a replacement alternator regulator in, reconnected it, and once again bad idles, backfires & misfires. Now that I've narrowed it down to an alternator/electrical issue, where do I go from here? I replaced the Y cable a couple of years back. Not a starter motor or crank position sensor problem (it starts no problem, just idles bad, stalls then dies). Battery tests good and ran the car perfectly fine without alternator connected.

Start following the alt wires. Somewhere there might be a broken or un sheathed wire. Check all your grounds as well. Pull on connector pins to make sure all are seated properly.

Do waterpump, alternator, a/c compressor and idler pullies turn freely with belt off?

Alternator literally means "alternating current device " (A/C volts) there is a rectifier inside with 3 positive and 3 negative diodes that converts the A/C voltage to D/C voltage. If one or more diodes have failed there will be A/C voltage spikes and EMFI all through-out the car that can and will effect all electrical, especially sensitive electronics in many crazy ways. Yes back in the day we re-built them as common practice, so had to know how they work/fail..There are numerous ways they can fail, stator, rectifier, rotor, brushes, regulator ect, but diode/rectifier is the one that can cause your symptoms. Have the alternator tested for all functions..

They can be rebuilt. I had one rebuilt by a good shop 4 years ago and it’s been flawless. Cost 1/3 what a new one does.

I went with a Bosch, can't remember whether it was new or reman, but they wanted the core back. Wasn't cheap, but felt it was as good as the one in a Porsche box.

What are the codes you are getting? Are you only getting P0300 & P030x (x for the cylinder number)? Does your dash lights dim/flicker in unison with the rough idle? Wondering if the alternator is causing similar problems on a car I have been working on. Somewhat similar things and I have replaced some of the same parts you have trying to figure this one out.

Thanks everyone - just fitted in a brand new bosch 0 124 515 042 replacement alternator this evening, no improvement Still low revs at idle and stalls, car is rocking like there's several misfires but no misfire codes generated now. Many people gave me advice it was probably an AC leak from failing diodes in the alternator causing the electronics to go haywire. Here in Australia labour prices are ridiculous so it was cheaper (but not cheap!) to buy a new alternator rather than refurbish. Going to drink some whisky, forget about this car for another week, then start tracing back all the wiring from the alternator and checking all grounds. I was thinking there was no shorts or exposed wiring, since that kind of problem would still cause issues with the alternator disconnected, but looks like I'm going to have to find out the hard way. Anyone got a good wiring diagram for the charging circuit? Already checked the engine ground strap when I change coilpack and spark in cylinder 6.

But I have to say this, this scenario defies all logic. Alternator out of the circuit and all runs well....alternator in the circuit and engine stalls and runs rough. Replaced alternator with a new one...same scenario. The alternator actually has little to do with how the engine is running. It supplies steady voltage to recharge the battery and maintain a steady voltage to the car. But you can run the car for a period of time even without an alternator. So it doesn't make sense that the alternator is causing the car to stumble, misfire, and stall. In my opinion, you've got another problem somewhere. Year and make of car? Cable throttle? Replaced MAF sensor? Cleaned the ICV? Smoked test for vacuum leaks?

Total crap shoot, but I figured I would add my 2 cents. Alternators are pretty simple, assuming it is functioning properly. It should provide around 14 Volts and if the battery and electrical system drop in voltage enough due to demand, the current the alternator provides will grow accordingly. So picture up to 140 amps flowing from the alternator back to the car electronics once the alternator kicks in. Since the car runs fine from just the battery it tells me that when power is flowing from alternator to the vehicle (including the battery to keep it charged) something is seriously messed up. I would agree with Porschetech3 that the alternator would be the easy fix, but since that didn't solve the problem it must be elsewhere. When I look at the diagram, power will flow from alternator through y-cable that is connected to the starter. From there it goes through some connection points and then hits that power distributer. To me it seems that that could be the only place where something odd maybe going on. Not sure what that thing does but for $600 it must be more than just a junction box. I would check ground strap 29 in diagram above and all the connections up to that power distributor. If you have access to an oscilloscope that could be helpful at seeing voltage at the power distributor - not sure if a voltmeter would catch anything bad.

Quotes source post: 17420359

you mentioned in your thread that people have said that a diode may be leaking AC current. This could happen especially if you jump started your vehicle. There is a way to check for AC current. Using a DVOM and the car running set meter to AC and put probes across battery terminals. You should get a reading of no more than 0.1 volts AC.