380 SL Overheating

Problem getting worse over past 7 years (summers only). Temp gauge would spike out at just before red and then drop rapidly to normal. Happened about 3 times first year, mechanic said, all OK, bad gauge.
Each year, happened more and more, never actually overheating. 2 years ago, flushed radiator, changed thermostat and water pump. Same problem, still happening more often. Last summer, happens each time out when starting from cold, Ok when starting out hot. Changed thermostat again - just in case. No change. Temp rises, and have checked actual temp, it's not the gauge. Gets to near red zone and so long as you are changing speed, temp will drop. Temp drops like a switch was flicked, it's rapid. In past, the sight of it getting hot causes you to take foot off gas, If you stay on it, the temp will not drop until you get off the gas. Turning on heat holds the temp at above normal, but below high. One mechanic says needs new radiator, another says fan clutch. Both seem to make sense, but each is several hundred $. Leaning to fan clutch. Need to make a guess, hope someone else has resolved same problem. Thanks Direct eMail would be appreciated to seway<remove this>@ct.metrocast.net
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The fact that the problem only occurs in the summer and only occurs on cold start, not hot start, indicates a fan clutch defect - in its thermostat. The clutch works but only after being "kicked" by a high engine temperature. I believe the motor wouldn't heat up if the cold started engine were driven at relatively high speed as the high air flow through the radiator would be a substitute for a reluctant fan clutch, IMHO.
A clogged radiator would not suddenly allow the engine's high temperature to dissipate and would create a heat problem at high speed; your's works fine.
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I would assume you got original cat converter... if that is so, then it is the cat converter that is plugged and cause your overhating problem.
I have been in this situation too... I replaced everything... several thermostats, radiator, fan clutch, water pump was replaced long ago so I know that was good... and even instrument cluster.
Instrument cluster is at fault... showing raised temperature... It is about 7 degree difference between the original and replacement cluster. But as usual.. once in a while it raises up.
Make sure your aux fan is working... you can also buy new thermoswitch that will turn on your aux fan earlier.
In any case, I know my cat was getting plugged as I was burning oil... and this is the biggest reason of all. Most people who replaced the cat shows the temperature falls down to normal.
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