1971 Mercedes 116-981 3.5 Engine

My 3.5 motor is freshly rebuilt and has a new wiring harness.  The engine runs very smoothly when I start it, and for the first 5 minutes as it warms up.  However, when it gets to normal running temperature, it tries to die, and will not idle.  I had to have the car towed home because of how badly it was running.  I went out the next morning, and it was running fine again when it was cold.  Any thoughts as to where to start with this problem?  Thank you.  Julian

Hi Juan,
and Welcome to the forum again!
The first thing that comes to my mind is the engine coolant sensor.
His task is to enrich the mixture when the engine is cold and warming up.
If it is defective or missing contact, the engine stays enriched all of the time, gets the multiple of fuel the warm engine should get and the engine dies.

Check the connection and the values of the engine <coolant> temp sensor directly at the plug towards the ECU.
Here is a link for downloading the pinout and the values you have to expect: jetronic.org/media/kunena/attachments/687/DB_M116_350_EN.pdf

Regards
Norbert

Thank you Norbert, should those be the Ohm readings when the engine is cold and when it is hot? Thank you, Julian

Hi Julian,

you find engine temp sensor resistance curve here: jetronic.org/en/d-jetronic/troubleshooting#Temperatursensoren

What you describe is a totally wrong air/fuel mixture or a very bad ignition timing. If engine temp sensor is off, you should smell fuel from exhaust.

It's important to pull the plug out of the ECU when measuring.

You find the readings for some engine temperatures in the linked document <or the sensor's curve in the document in Volker's link>.

Regards
Norbert

Two more questions, 1st - what is the best way to measure the Fuel/Air mix? 2nd - I know that there is a dial that can be turned on the ECU that adjusts Fuel/Air mixture, but is there any other adjustment? Thank you for your help.
Julian