Idle jumping up 200,400 rpms, wires or Permatune CDI problem

[heliolps2]

I have a 89 3.3 CSI modified turbo motor in my 76 911S ,Strange thing, for a couple of years now I've been getting this odd idle jump,its not hunting, let me try to explain the best I can, When I start the car it runs fine and idle great, the problem starts after i drive for awhile, when I come to a stop light the idle jumps up from normal idle 200 to 400 rpms with a small violent jump, and lately when the idle jumps it seems to be dieseling as it comes down, it does it, maybe once or twice a drive.

I have checked an rechecked the fuel system and vacuum hoses and intercooler connections, all that stuff looks good,

I have also replaced the spark plugs, The wires are old , from 1999 about 60.000 miles , I have already ordered a set from our host , if that doesn't fix the problem ,I'll have to check my Permatune CDI with built in coil, I may have to replace that also and that not cheap, right now it just a guessing game, it would be good if I had a stock CDI and coil to test with, before i buy a new one.

Can anyone shed some light on the this problem,

thanks
Helio

[James Brown]

nice to know what car it's doing this on, the turbo? getting rid of the permatune is a good start. lots of good parts in there.

[heliolps2]

3.3 CIS turbo motor

[heliolps2]

any one

[Targalid]

I think you are on the right track, suspecting the ignition system. Have you done the night time engine compartment visual test for arcing ignition wires? I had a fluctuating idle which was cured by a new set of ignition cables. In your case, with Permatune, you have another potential area to diagnose if the new ignition wires do not fix the problem. Check all electrical connections for corrosion/tightness. Good luck!

[heliolps2]

Thank you Targalid ,wires arrive on Friday , I hoping that fixes the problem like your did, can you tell what your car was doing ?

[Perma-Tune]

　
There are a few things to check before you throw parts at the car. First thing to check is this (from our web site):
DISTRIBUTOR TRIGGER SIGNAL
The second most common cause of SC engine ignition failure is the ignition trigger circuit. Since the ignition box is controlled by the magnetic pulse generator located inside the distributor, any disruption in the triggering signal will cause ignition problems. Full disassembly and inspection of the distributor is highly recommended due to the intermittent nature of triggering problems. Pay special attention to the distributor trigger shielded cable ( green wire ), magnetic pulse generator magnet and magnetic pulse generator winding connection as these parts are known to fail in combination. An especially common problem is a broken magnet pole.
Evidence of arching between the magnetic pulse generator magnet points indicates a defective rotor. In this case replace the rotor, magnetic pulse winding, green wire and ignition box. The Perma-Tune high performance triggering is so accurate that worn out distributor shaft bearings can be detected by the tachometer. This is indicated by the tachometer needle slightly bouncing at idle and is usually accompanied by excessive end play of the distributor shaft. Trigger signal problems can cause double triggering which results in a weak ignition spark. Due to the lower triggering resolution of the stock ignition box, some symptoms of trigger signal problems can be more evident after the Perma-Tune is installed. Refer to the vehicle maintenance manual for triggering signal waveform specifications and distributor repair instructions.
To quickly test the triggering circuit without the use of an oscilloscope: Unplug the harness connector from the ignition box. Refer to the 911SC902 Installation Instructions diagram; connect an Ohmmeter between pins 3 and 6 of the harness connector. To prevent the possibility of misdiagnosis, tug on the green triggering signal wire located at the base of the distributor while viewing the meter. The published specification for the magnetic pulse generator is 600 Ohms +- 100. Our experience has shown that a variation of +- 25 Ohm indicates a need to repair defective connections or to replace the magnetic pulse generator windings.
　
　
　
This being a turbo, you also need to check the speed relay function if so equipped. The speed relay was fitted to US turbo cars only and is intended to shut down the fuel pumps in an emergency. This relay gets its signal for engine RPM from the ignition system. Sometimes, the speed relay goes bad and interferes with the ignition circuit and can in some cases even burn out the ignition box or tachometer.

An emergency shut down could be required if the engine goes into an over boost condition resulting in compression ignition of the engine (in other words the ignition switch wold not be able to shut down the enigne as the turbo starts screaming while the engine RPM hits the red line resulting in a flaming, loud, and very expensive, bang). Why the European cars do not need this relay is a mystery to me.

The procedure for testing the speed relay is in the shop manual for the car.

You have a C911SC Perma-Tune? Talk about longevity, it has to be about 30 years old.

[Targalid]

Helio,
I had similar fluctuating rpm and bouncing tach. I changed plugs and that helped. New rotor and cap helped some more but the symptoms, though diminished, still persisted. Then I looked into the engine compartment at night and saw quite a light show as the ignition wires sparked and flashed in the darkness. They were original Beru with the braided wire covers. I replaced with Magnecor ignition wires and the engine smoothed out with no problems since. My tach was bouncing and now is steady. That is my story and I'm sticking to it! You are fortunate that Permatune has chimed in on this thread. Between new wires and their diagnostic procedures you should be in good shape.

[heliolps2]

Ok ,I replaced the spark plug wires, it did not change a thing , I also disconnected the vacuum hoses at the Dizzy no change, I check the Dizzy for play and there seems to be alot of play, I would say about 1/8 of an inch, up and down movement .

Could this be the problem ?
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