Only idles when temp sensor unpluged

[kjesse]

I've been working on my '84 944 for the past couple of months since I purchased it. Other than having the balance belt tensioner break it has been reliable daily driver. As the weather is getting cooler I've been having a cold start issue where I would need to crank the engine over 8-20 times before it would start each morning. After that initial start it would start up on the first crank until the next morning. I've traced that issue from ignition to fuel to air and nothing I replaced seemed to change anything. This past weekend I was working on cleaning grounds to resolve an issue where the blower motor continuously runs with the key out.

The first ground I cleaned was the one on the center backside of the engine next to the reference sensor. After cleaning and re-tightening the cold start issue seem to have gone away as the engine fired up on the first crank. Unfortunately, the idle didn't hold so I fixed one thing and broke another. Unfortunately, I can't seem to figure out what I broke...

I was able to get the engine to start and idle by unplugging the temp sensor under the J-boot. The idle is mostly smooth and the car continues to run and warm up. If I plug the sensor back in it stalls right after starting. Unplug it again and will start and idle. With it unplugged, I can't open the throttle quickly without stalling. If I feather into the throttle very slowly, I can rev and hold at any RPM - stopped at 4000 - and will stop back to idle without stalling. I've replaced the temp sensor with a new one twice.

So just to recap, the car was running and drivable until I cleaned the one ground on the engine and firewall. I can get the car to idle and rev up with the temp sensor unplugged. I'm quite sure this is electrical but don't know what to start with without replacing the MAF, TPS and/or DME.

[Gawernator]

May be a suspect DME not reading the signal right...

[mytrplseven]

Just for grins, check all the pluggable connectors: TPS, AFM, Ref sensors and any vacuum lines that may be easily pulled apart while you were in there.

[Rasta Monsta]

I would test the temp sensor per FSM, then the wiring between the sensor and the DME.

[kjesse]

Checked all connectors with no change and all vacuum lines are in place.

Checked continuity from the temp sensor plug (solid yellow wire) to the DME (pin 13) successfully following this diagram:



And checked continuity from the temp sensor plug (solid brown wire) to multiple ground points successfully.

Tested temp sensor:

Cold (~40 F) 4.3k
Hot (~192 F) 210

Also re-tested the TPS successfully per Throttle Position Switch - Information, Troubleshooting, Replacement, and Adjustment.

An interesting discovery tonight while testing is that when the engine warms up (temp sensor unplugged still) and I turn the key off, I can't restart it. If I plug the temp sensor back in and start the car, it takes a couple of tries but will start and stall right away. Then if I unplug the temp sensor again and start the car it will stay running.

[kjesse]

Continuity on the wires are good all the way to the DME even with moving the wire around.

Further testing, I've unplugged the AFM, TPS and DME temp sensor with the car idling. Here is a video of the engine and from inside the car:

83 Porsche 944 Idle Throttle Issue - YouTube

83 Porsche 944 Idle Throttle Issue - YouTube

[kjesse]

Replacement DME resolved this issue and I'm back on the road. Thanks for all the help!

[AKCJ]

Glad you got it - thanks for posting.

(Hate to admit but looks like Gaw called this one.)