Need help, my 86 runs really bad.
 

I need help !! Not sure what to check next.

Car is a stock 86 911 with 230k on the clock. The engine is mechanically sound, compression is 180 all the way around, valves are adjusted correctly, does not burn much oil. The car has run extremely well for the past several years.

The problem started on the way home from work 2 weeks ago. The car started up fine and ran well for the first 3 miles. Then suddenly it started to miss under part throttle. If you floored it, it was fine, if you let it idle it was fine. It just would not run well under part throttle, It coughed, sputtered, and bucked. Then just as suddenly as the problem appeared, it went away, and the car ran fine the rest of the way home. The next day, and for the next week the car ran just fine. Since then it has steadily become a more consistent problem, and now it seems to run bad all the time. Again, it idles fine, runs well under full throttle, but misses really bad under part throttle, it coughs, and sputters making it impossible to drive.

Here is what I have done so far. I installed a new set of Magnecor plug wires that I had purchased last year but never installed. I did not think this would help, and it did not. I installed my new spare DME relay under the front seat. Then this weekend I inspected and tested the AFM and have determined that it is working correctly. I have also checked all the usual stuff, like looked at the cap and rotor, replaced the fuel filter, checked the air filter, inspected the coil, etc..

I am hoping that someone here may be able to provide some guidance on what to check next. I am fearful that this might be a symptom of the DME going bad.

Thanks in advance.

Have you run any Techron thru the injectors yet? I had a bad ground on my 86, the one on the left side intake that made it run really crappy.

cgarr - I have not run any Techron through the system. I will try to find some today. Good advise on the ground as well. I will check that tonight.

These are the kind of problems that drive a person to drink. Your problem does seem to be symptomatic of a fuel problem, but could also be something like a coil going bad. How much gas have you run through the car since the problem started. I ask this because it could also be that you got some gas with a lot of water in it.

DC,

Cylinder head temperature sensor should be replaced. This, a new DME relay and a new coil solved the identical problem on my '86.

I don't think the coil was the culprit, but replaced it "just in case".

Chris

2nd the CHT. I replaced mine in my '84, much better. Relatively easy replacement. I believe I needed to modify a deep socket for that one (cut a deep slot in it), can't remember for sure though.

Kurt - Agreed, gonna drive me to drinking. It does seem symptomatic of a lean fuel condition, and that could be caused by water. I will pull the fuel filter and see what comes out. I am not ruling out the coil, but I have never seen one go bad, I am not saying they dont go bad, I have just never seen a bad one. I have put a couple hundred miles on the car since the issue started. I took a road trip to Solomans Island (150 miles round trip) 2 days after it first acted up, and it ran perfectly the whole trip. I would hate to think that I got water in my gas, but you never know, and I will check for that.

Chris, CHT was replaced last year with the updated 2 wire unit. I think there was a test procedure outlined in the bently for the CHT. It is a good suggestion, and I will test the CHT. DME relay is brand new, so hopefully the nwe one is not bad. I opened the old DME relay and it did not look like it was going bad. I am hoping I don't have to get into the DME itself.

Fred - Thats two votes for the CHT. Perhaps my new CHT came unplugged. Or perhaps it went bad. I will test it tonight.

Dave, you have a PM.

Update: This issue has been resolved.

This problem drove me crazy for weeks. I tested everything DME, and everything DME tested OK according to the bently procedures.

The best clue to this mystery is that the engine idles perfectly, and runs perfectly at full throttle, but runs like crap at all throttle positions in between. The problem turned out to be the microswitch which senses that the throttle is closed. This microswitch tested perfectly according to the bently procedures, however it was indeed a faulty micorswitch. Aparently this micro switch was sending a false "my throttle is closed" signal to the DME when the throttle was partially open, so the DME was sending just enough fuel to allow the engine to idle even though I had my foot into it. If I floored it, the wide open throttle switch took over and allowed the DME to send maximum fuel, and the engine ran fine.

The conclusive test that isolated this microswitch as the issue, was to unplug the microswitch, and run the car. When you unplug the microswitch the engine RPM increases to 1,100 RPM. This increase in RPM is actually an indication that the switch is working correctly. I believe the DME advances the engine timing just off idle, so when you disconnect the switch, the DME thinks you have stepped on the throttle a bit, and increases the ignition timing to give you a little power off of idle. Anyway with the switch disconnected, drive the car. If it runs perfect in the mid throttle ranges, replace the microswitch.

I am once again enjoying driving my old 911.

Thanks for all the advise, and the encouragement. This one had me scratching my head for several weeks.

Good to hear you found the problem. Had a similar problem on my Mitsu Eclipse 2.0L turbo- a faulty TPS (throttle position sensor) that would operate intermittantly cause the same problem. The sensor would sometimes work, sometimes not and you could make it work/not work by bench testing with a multimeter and applying pressure/no pressure to the male elec. pins in the sensor's plug-in location.

All too often people want to replace typical maintenance items like plugs, rotors, wires, etc. in hopes of fixing. Nowadays it seems we need to check the engine management particulars a bit harder!