starting/running problems 3.2 carrera engine

[shoguneagle]

I am in need of advice and insight into probable causes of my 914-6 Carrera engine starting, running for about 3.5 minutes, surging/fluctuating rpm for 1.5 to 2 minutes, and then suddenly stopping. Engine would not start immediately or a reasonable time afterward. Car started the next day and the same thing happened

I had to rewire the car after it was stolen and I think I got everything right in the conversion (using 3.2 harness and stock engine configuration 1987). I am in the process of rechecking everything. The car does start and run as outlined above several different times after waiting a few hours.

Have not the foggiest idea what is happening. The car has sat for about a year during the rewiring process.

The engine was running and on the road. I did not have any problems prior to the theft. Insight into the problem(s) and possible causes/fixes would be appreciated.

Steve Hurt

[kidrock]

my first guess is that the Cylinder Head Temp sensor is going south. Next time the engine begins the symptoms, you could pull the CHT sensor at the plug harness (white connector) and insert a paper clip to "jump" the sensor. If this doesn't solve the problem, I would check to see if the car is experiencing a loss of fuel or spark.

[ischmitz]

I agree with kidrock, this could be something where temperature plays a role due to the time-dependency of hour? to recover. So the engine temperature sensor could be getting poor contact when warming up. Check if it is the older 1-wire version and if so replace it with the 2-wire version.

Further maybe one or both of the sensors at the flywheel are having issues. How old are those?

Ingo

[cabmandone]

I'm not saying it's not the CHT but even if it's bad I think it will still start. I could be wrong. Check battery voltage, speed and reference sensor and put a good DME Relay in if you haven't already just as a safety to eliminate it as a source of trouble. When I was researching my no start when I installed my engine, I found a couple threads talking about intermittent problems with the DME relay.

[mysocal911]

The OP stated this:

"I am in need of advice and insight into probable causes of my 914-6 Carrera engine starting, running for about 3.5 minutes, surging/fluctuating rpm for 1.5 to 2 minutes, and then suddenly stopping."

So:
1. The engine starts. (so both sensors are O.K.)
2. Then runs for 3.5 minutes. (which further indicates no sensors problem)
3. Starts to surge and then dies. Which is indicative of a very very rich mixture, i.e. CHT.
The engine can run with that mixture being cold but not warm/hot.

Both posts, Ingo & Kidrock, right on!

[shoguneagle]

WOW!!! WHAT GREAT FEEDBACK!! I have never received information including possible causes and solutions in such a quick and fact finding/reasoning manner. Everything was right to the point and justified as possibilities. Without this information, I would have been weeks away from finding a solution and very frustrated.

I will be looking at all the suggestions later today after the heat has gone down. It made sense the way the problem was analyzed with "engine can run with that mixture being cold but not warm/hot". Indication of very, very rich fuel mixture could be causing it therefore CHT. Also, the suggestions relating to the reference and speed sensors, DME/Fuel Relay, and related items.

All the suggestions, reasoning, and solutions were very helpful in putting me into the causes and effects of the Motronic injection system. I have a lot to learn therefore what information everyone gives me is priceless.

Thank You,
Steve Hurt

[shoguneagle]

The advice give here placed me right into the problem area. I found the CHT plug had been ruined and was just barely hanging onto the engine side of the wiring. Car started up with authority but did not have a chance to run it for any duration. I lost my instruments including oil pressure; more "theft gremlins" with more coming to my doorstep. take one at a time and solve! This way I will know what is solid and dependable while eliminating problems.

One question: Can the electrical connections on the CHT be repaired or do you have to get a new harness? I suppose changing the CHT will take care of one side but what about the engine side? It looks like it is a "sealed harness".

Thanks again,
Steve Hurt

[ischmitz]

You might need to get a piece of donor harness from an older BMW, Porsche or similar. The connectors are all the same style. Then cut off the correct plug and repair yours. Splice it to your harness and use shrink tubing in correct sizes for insulation. Stay away from electrical tape as it degrades over time and creates a mess.

In general while good isolation of the harness helps if you life in areas of constant "weather" I don't see that to be an issue for this particular connector. And then again AZ is mostly a dry heat, right?

Ingo

[cabmandone]

Personally, I'd remove the engine side from the block, put the two connectors together and then use some silicone tape (you can get it at lowes and it's great stuff) and tape the two together and forget about it. I still think it's strange that you had to wait so long before the engine would start. When I bumped my CHT sensor wire in the engine compartment, Warm or cold it would fire but die when I put it in gear. I could restart the engine almost immediately whether it was warm or cold.

Good luck with the rest of the gremlins. I hate going through electrical.

[shoguneagle]

The only explanation I can think of for the delay in starting relates to the condition of the plugs, how shaky the connection was, and the engine vibration.

This is really an excellent way of learning the electrical and sensors problems; very bad on the old knees. Have to keep everything positive to get the job completed. The the thieves are not going to get me to quit this project.

Thanks for the advice and encouragement.

Steve

[kidrock]

Is it possible that the thieves bungled with the ignition switch and wires underneath the knee pads? I've found a host of problems in that particular area before, it may be worth checking. Pretty crucial stuff under there, particularly the bulkhead connector from the ignition switch.

[cabmandone]

The oil pressure gauge should be an easy test. IIRC you remove the wire from the sending unit and ground it. I don't recall how the gauge reacts but I think it's supposed to peg with the wire off the sender and zero when grounded. Could be the opposite.

I can see if I can send you some pages from my Bentley if that would help. I think I can save pages as a PDF and forward.