My 95 E300d has about 170k miles and for the last year has had a rough idle when cold - starts great (new glow plugs) just has a bad idle. I have removed the inlet manifold this weekend to replace all the diesel lines - the clear ones and the black main supply lines. Also replacing the temperature sender unit.
When I removed the two pin connector to the sender unit (it is in the top of the head at the back) I noticed some cracking of the insulation in the wiring. The other connectors under the manifold looked in fine shape. I am wondering if this is the problem of the cold idle.
Is there any advice on how hard a job the wiring loom replace is - given that the inlet mainfold is already off I have the access to do the job (I think) - or can I just fix that one connector?

Any advice appreciated.

Same car, and same wiring harness problem. I am confused however. The temperature sender unit that you describe has 4 wires not 2. I fixed mine by running new wires, I also combined 2 of the wires as they are both grounds. Make sure you don't reverse the plug once you put it back. It can go in numerous ways and only 2 ways are correct. Take note how it fits before you unplug it. The symptom of bad wiring in this area (or a reversed plug) is usually either a high idle speed, and/or loss of the EGR functionality.

Phil

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'95 E300 Diesel, 264,000 Miles. [Sold it]

I think you are talking about the sensor for the temp gauge? The one I replaced has two wires going to it and the connector can not be reversed. I am now beginning to wonder whether I have it the wrong way around and I have actually replaced the temperature guage sender unit? If this is the case then the broken insulators are having no impact - the guage is working fine anyway.

Does anyone have diagrams of which plug is which please.

I'm am still confused. On your car, the coolant temperature sensor of which you speak (at the location that you described) should have 4 wires connecting to it. 2 of the wires feed the dash temperature gauge, and the other 2 feed the engine management system. So although your temperature gauge portion works correctly, it does not mean that the other 2 wires, (the ones that feed the engine management system) are not defective. That was my case.

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'95 E300 Diesel, 264,000 Miles. [Sold it]

Thanks for your help. there are two sensors - one has 2 wires and that is definiyely for the tempo gauge on the dash board. The other sensor has 4 wires - you are right. I have now checked the woring to that sensor and am not sure if the connector was fitted the right way. There are 4 numbers ( 1 to 4) stamped on the inside of the connector with the wires attached to it. The socket end (on the sensor in the head) has a number 1 stamped next to one pin. Now it would seem to me that the two number 1 pins should be aligned but mine was 180 degrees out. The funny thing is that the wiring is under tension to locate it in the "correct" position. It is impossible to see the state of the wiring to this plug as it is all wrapped up in insulation tape.
Does anyone know the correct connection for sure - and also how to test the sensor itself for correct operation?