I have 1991 350SD. The lower radiator hose failed and was replaced.

I filled block with coolant, and the radiator. Tried to eliminate all air from the system by running, burping heater core hose and top radiator hose.

Now the temperature gauge is acting badly. At first there was no temperature reading, the it jumped up to about 70.

Yesterday it was not reading at all, then jumped to 80C after being parked for a few minutes. Today, no reading at all.

The engine is being cooled. The radiator is hot, there is plenty of heat from the heater core with no gurgling. I think there is air trapped near the sensor, or there is a bad connection.

1. Where is the coolant temperature sensor on the 350SD?
2. How can I make sure there is no air trapped near it?

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1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside

Bad connection most likely. It's almost impossible to trap air in these engines.

The sensor is usually along the left side of the head, behind the IP.

Check your grounds. My instrument cluster didn't settle down and work right until I ran an new ground wire from the back of the cluster to the conglomeration of grounds right behind the cluster. If you do this, be sure to put a spade connector in the line to make removing the cluster easier in the future.

Good luck!

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Baton Rouge, LA
1977 300D Non-Turbo

Thanks, I will check the electrical connection.

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1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside

So I can share some insights on this...

On the 350SD/L the coolant temp sensor is on the cylinder head near the injectors (IIRC it's between #2 and #3). You'll see it has 2 wires going to it, one of them is for the dash gauge and the other is for the glow plug relay. The sensor grounds to the cylinder head.

There is a 2 pin connection near the glow plug relay for that harness (so that you can unplug the wires from the glow plug relay and remove the engine without having to unhook all the glow plugs). Check for corrosion there as well.

-J

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1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

The sensor went out after I lost coolant and the engine heated up rapidly.

This is what I am thinking because it coincides in time: Is it likely that the sensor was damaged by the high heat?

Also I see a video for sale to diagnose the problem on Mercedes Source. Has anyone seen it? Is it worth $7?

__________________
1991 350SD (updated rods)
Biodiesel B100 when I can find it.
Dino when really cold outside