82 500sec euro. Suddenly had an overheat problem, went past 110 and still climbing at idle or running at 50 mph so I bought a new thermostat and put it in. Same problem but---- if I turn on the heater until the temp comes down to a little over 80 on the gage and then turn the heater off the temp stays at 80+ at idle, running, or on restart as long as I don't leave the engine off long enough to cool down. If it's cool I have to run the heater or the temp won't stop climbing. Sounds like an air bubble but once it clears why would it come back at cool down? I read on search but no one said they had this happen. TIA

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85 300 SD ~ 115K
82 500 SEC Euro ??K
78 450SL 164K