Hi, I am having an issue with my 1985 300D temperature gauge climbing quickly. The car is new to me and has no recent road time.

The gauge starts cool when the engine is cold, once the engine is started, the gauge will start to climb within a minute. The gauge doesn't just peg out, but it takes a minute or so to climb. I've tried restarting it after shutting down a moment and it goes right back to hot.

I haven't run it for long periods yet, but the radiator doesn't seem warm at all and the upper hose is cool to the touch.

Changing the thermostat had absolutely no effect.

Additional info - The car is a California model and it has 325k on it. I am working to bring it back to good health after its been sitting for years. It has no issues starting ie.. Glow it, hit the key, and boom she runs, as soon as you hit the key it starts.
Also I have no exhaust visible at all (weird to me). The car does have potential turbo issues and when trying to move it around it has low power in general.

Thank you in advance for your help.

It sure sounds like the thermostat is installed backwards with the spring end pointing towards the front of the car and not pointing towards the engine block.

I am pretty certain that the spring is inserted well into the block. This is not the first thermostat that's been installed. I'm not sure that I could mount the housing to the block with the thermostat backwards, is that even possible?

Could be a bad temp sender

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Yes the thermostat goes in backwards quite easily and does just what you describe.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660

I would get a temperature reading before doing anything. I can not see the engine idling developing that much temperature in just a minute. Could be just a temperature sender way out of internal calibration.

You changed the thermostat. These coolant systems need refilled through the top radiator hose I assume you know.

The top radiator hose remaining cold to me is another clue the engine is probably not hot. There can be exceptions but in general I would want to read the block temperature near that sender first with a hand held temperature scanner.

Update:

Today I went out and re-installed the thermostat and verified the direction (It was installed with the spring facing into the motor) that the thermostat is facing. It was filled through the upper radiator hose as recommended. There is no sign of the engine running hot.

The problem persists. I am going to order a new temperature sender and see if that fixes the issue.

Get one of those non contact I.R. thermometers (some of the medical ones they point @ your forehead also have a "surface" setting.)


Point it at the head near where the temp sensor is, and if there is a major difference between the thermometer reading and what the temp gauge says, you have found your problem. If both agree, maybe the water pump isn't working correctly, or the radiator is clogged.