Hey guys,

I had issues with the car on the thread "Strange behavior 900 miles from home". The issue then was intermittently running on 4 cylinders. I changed the coils to new (used) ones from 928 Int. i put in an ignition system bypass relay, I put in new ignition amps, the coil wires and ignition wires are Beru and less than a year old. I got it to start running on all 8 after all of that and then I began an Intake Refresh.

After removing and installing the intake 3 times, I finally got the high idle corrected.

The issue now is she stumbles and studders when taking off. After I reach about 50 MPH, it smooths out but you can still tell it has issues. When sitting at idle it also occasionally coughs (as if someone is bumping it). It gets worse as the car gets warmer.

I've done all of the stuff above to correct the running on 4 cylinders issue. Where should I begin to correct/address this issue? Any and all recommendations accepted. Thanks

Ash

 

This sounds like you had a misfire condition which "lit" the overheat relay.
And when you replaced the overheat relay with the bypass relay (a poor idea, BTW) it cured the 4 cylinder mode, but not the true misfire.
And after your intake refresh, you still have that misfire.

My suggestion would to be to diagnosis the overheat system, per the factory literature, and make sure the the two sensors are correctly working.
Plug back in the overheat relay and see if it still kicks out 4 cylinders. (Use the overheat relay system as a diagnostic tool.)
Once you know which 4 cylinders are being kicked out, you only have 4 cylinders to figure out where the problem lies.

 

Thanks for the info GB. I’ll have to find the old or source a new relay. Can you identify the two sensors you are referring to?

 

There's two thermocouples...one on each side of the engine. Their signals end up at the plug for the relay. The relay compares the two signals. If there is a misfire, the exhaust temperature goes up on the side with that misfire, changing one of the signals. The relay then shuts down the injectors to keep the catalytic converter from getting so hot that it lights the undercoating on the car on fire (Which was a way too common thing for the '85-'88 cars and why Porsche added this system.)

BTW....The last time that relay was offered as a spare part from Porsche, it was over $600.
No longer available, now.
....You don't want to throw away good ones.

 

They are located on the underside of the exhaust manifolds in the 4 and 8 positions (for 89 only). They moved them to 3 and 7 on 90 up due to a more stable reading I believe. Should be pretty obvious when looking up from the underside.

 

Thanks guys. I’ve located pics of the sensors. Are these still available?

 

I think the later model ones are. (Simple to convert from early to late.)
Not a very high failure rate part, however.

 

NOTE when the sensor signals fail ,
its usually a corroded connection D100 will restore it.
I would try this route first, before replacing parts

 

Thanks Mr Merlin. I'll get it back in the air and check the sensors this weekend.