How to diagnose rough running engine after warmup?

[djnolan]

Is there a diagnostic procedure for a rough running engine? In this case I have an extremely rough running engine after warmup on a late 85 944. It runs fine for the first 5 minutes or so, and then it starts running rough. The exhaust sounds rough on all cylinders as opposed to a single cylinder. I suspect a bad dme sensor although it could be other things.

Is there a trouble shooting process to quickly narrow this down to the bad component?

If not, where should I start?

[John_AZ]

Pull the O2 sensor connection apart after warm up. Does it run any better?

Engine DME temp sensor-clean connections or replace if original.

Just a place to start.

J_AZ

[GreenWater]

It doesn't sound like the DME Temp Sensor. I had one go bad on my 85.5 and it would cause the car to not start once the coolant was hot. But once the car was running it would run great, warm or cold.

[djnolan]

no change when unplugging o2 sensor.
gets worse if temp sensor is unplugged.
what next?

[Techno Duck]

Check that you have no vacuum leaks, in particular the j-boot and its associated connections if they were removed.

[djnolan]

Checked for vacuum leaks with stethoscope and none found.
what next?

[GreenWater]

It could be the DME temp sensor. If the car is running good until the coolant temp heats up then starts to run bad, that could be from the DME temp sensor. Here is an easy way to check the DME temp sensor.

You already said that you removed the plug to the sensor and the car ran worse? The blue plug?

Try this, warm up the car and remove the blue plug for the DME sensor and then take a small wire and loop it across the two connectors in the plug. That should signal that the engine is warm and tell the DME to supply the correct amount of fuel/air for a warm engine. If the car starts running differently when you do that then the DME sensor is probably bad.

[Gawernator]

If it's the original sensor it might not hurt to replace it. Mine was so brittle the head broke off when I went to remove it, lol

[djnolan]

I have a new one someplace so i will try to locate it and replace, though the resistance of the temp sensor checks out per the procedure on clarks.

Are there any other suggestions?

I can remember using a flow chart for diagnosing a gm ecm. does such thing anything exist for the 944?

[John_AZ]

Originally Posted by djnolan

Checked for vacuum leaks with stethoscope and none found.
what next?

Recheck for vacuum leaks with the soapy water tester. The stethoscope is hard to fit under the "J" tube hoses:

vacuum system test.....soooo easy

Are the reference sensors OK? Do you have a spare to check with?

J_AZ

[John_AZ]

Originally Posted by djnolan

I can remember using a flow chart for diagnosing a gm ecm. does such thing anything exist for the 944?

YUP!
The Porsche 944 Motronic DME
J_AZ

[djnolan]

On a 85.5 N/A, is the DME located in the drivers side kick panel and does it have two 8-pin connectors? Clarke's is confusing me on this.

[djnolan]

disregard

[djnolan]

I made a little progress on the diagnosis. Apparently erratic running engine could be from a bad throttle position sensor switch. I found reference to this in several places. A new one is on order and hopefully it clears up the problem.