On my 240D I've been having an odd temp issue. Randomly my temp will rise up to near 100C, so I'll crank the heat on, and after just a minute or so the temp will come down to normal operating temp. Today it didn't even take that long. It usually doesn't have an issue, but every couple of weeks or so it will do this. During the removal of my veg oil system I replace the t-stat as well as the heater hose coming off the back of the head. I immediately had issues of not reaching operating temp which was solved when I cleaned the plug on the sensor in the engine block. Previous to the VO system removal I never had any temp issues. Does this sound like a t-stat problem? Why would it only do it randomly?

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Sounds like air in the system. fill and drive fill and drive to get rid of an air pocket in the cooling system.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

I've been driving the car for a few months since removing the cooling system and the t-stat was replaced. Can air still linger for that long?

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Mine would do that from time to time, was a bad temp sensor. It had a bad spot on the upper end of the range and when it started getting close to it, the needle would jump up a little, then when it cooled a bit, the needle would drop back into it's normal range.