Steve Conar

I have had the engine replaced in my 1986 944 Turbo and all seemed well until about a week later when i shut off the car on a quick trip to town...when trying to restart it it just spinned with no turnover...replaced the DEM relay a couple of hours later and the car started just to have the same problem a litle later...actually replaced the fuel pump as this seemed to be the problem only to have the problem reoccur a few days later...the car always seems to fix itself after anywhere from 10 minutes to a couple of hours later...replaced the coil and was good for a couple of days until the problem creeped up again so this time i pulled a plug wire and stuck a spare plug in it and laid it on top of the motor and saw i was getting no fire to plugs...again the car started later so the next time it acted up i was lucky to be close to my mechanic who brought a test light and found we were getting good fire from the coil but none being sent to the plugs...car is now at the shop but i ask what different items can we check...of course we will look at distibuter cap and rotor button but if those are good what else should i check and what test can be performed to keep me from replacing possibly good and expensive parts...IE..crank sensor etc...the car runs very well and never shuts off while running and always seems to fix itself in time...its hard to do test when you dont know when the car will act up and it seems testing while it is working would yield no results...SUGGESTIONS PLEASE

fast951

If you are getting spark out of the coil, then everything up to the coil (including coil) is fine. This leaves the wire from coil to distributor, cap & rotor an plug wires.

Steve Conar

wire from coil to distributer cap has fire also....can just about eliminate the plug wires also because all 4 would have to quit working then start working again which seems unlikely (even with my not always good luck lol)...will know soon about the cap and rotor button but for the life of me i cant figure how the heat is messing with them (even though i do know heat can do strange things to electronics)...the car and motor when running is strong and never misses a beat and the non fire doesnt happen each and every time i drive it...making it an adventure not knowing when i may not restart or be stuck temporarilly....the nearest porsche dealer and autoparts stores or just me looking at diagrams cannot find a pickup/stater etc in the distributer...and some people localy are suggesting maybe the crank sensor which goes to the DEM brain may be the problem but thats a $173 dollar guess just to try replacing it to see what happens...Thanks much John and i hope it is just a cap or button but am still curious how to detect weak coil fire or other testing procedures to truly eliminate the guess work...More thoughts of yours or others are greatly appreciated

Bri Bro

If it is really getting spark from the coil, it has to be in the distributor so I would replace the distributor, cap & rotor.

If it isn't getting spark from the coil, look at the signal going to the negative side of the coil. With the ignition on, you get 12VDC to the positive side of the coil. When you crank the engine/start the engine, the negative side of the coil should pulse to ground (signal comes from DME). If it doesn't, I would suspect the reference sensor or the DME itself. There are sensor at half the price, they just have a longer leads that need to get tucked away so that it doesn't touch an exhaust pipe. See link below.
http://www.paragon-products.com/Refe...61.210.002.htm


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Steve Conar

yes brian there really is fire from the coil...when the coil wire is unplugged from the distributer and the test light is inserted into it and the other end of tester is plugged back into the distributer the light shows fire coming from the coil and being sent into the distibuter but not going through to the plugs...( I am curious to know if there is a way to test the coils output to know it is sending enough power through to make the plugs fire)...again this only happens (and doesnt happen every time) after the engine has been running and is warm...when the car sits for a little while the problem corrects itself and fire starts coming all the way through to the plugs and the car restarts...Thank you very much for the info...the distributer cap and rotor button are not very expensive and hopefully will fix the issue but i have to get them ordered tomorrow...if that doesnt work I guess i keep hunting the problem More thoughts on the issue? All the info of tricks for testing and evaluating firing issues on older porsches like mine will aide my mechanic...he is very good and trustworthy but as you guys know these older porsches have sensors and such that arent encountered on everyday cars and I am learning new things about the car each and every day...there is always something to deal with on these cars but I think it is worth it just for the enjoyment I get driving mine

Bri Bro

To test the spark, you need to get a device that has an adjustable gap inside the tube. You connect the spark plug wire to the tester and measure the sparks length and this will determine voltage of the spark. Something like this,
http://www.denlorstools.com/home/dt1...rk_tester.html
http://www.hotrodautosupply.com/esho...od&productId=2
If the spark voltage is low, it will not generate enough heat to cause ignition. If the spark can jump a 3/4 inch gap and the spark has a bright blue color, it should be OK. We have tested High Voltage pulsed power supplies that can jump a 40 foot gap..makes a lot of noise. Rule of thumb was 10,000 volts per inch of gap with dry air.

I would be leaning towards a defective cap that has a crack in it that opens with heat.

Steve Conar

Thank you Brian and John...I am ordering my new cap and button today....When the problem first occured I started thinking FUEL first as I had had the DEM relay go bad a few years back...Instinctively this got my mind on one track...once it was discovered it was a fire issue I just couldnt get my head to think "SIMPLE" (a common habit of a porsche owner lol)...but cap and button will be replaced soon...Will keep you posted