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

Metrocast schrieb:
> 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

You may have a leak or head crack letting air or exhaust gases gather at
top of the engine head. With closed themorstat the air expands heating
up and the thermostat housing is heated up hot by damp only.

When the thermically isolated thermostat is opening the coolant is
boiling already and in a moment the cold radiator coolant enters the
engine suddenly making the temeperature going down from 120C to 80C.

Check the pressure at the radiator heating up at 3000 rpm.

The easiest check is to look at the open radiator in the process of
heating up. If at high temperature and open thermostat there are bubbles
in the water stream while accelerating you have a head gasket leak or
head crack.

As a first aid against more head cracks by sudden temperature changes
one may drill a bigger hole in the themostate plate for gas bubbles to
pass to the radiator.

--

Roland Franzius