280sl Overheating at idle

[mrfatboy]

Hello,

My '69 280sl just started to overheat at idle.  At normal driving the temperature is fine.  When it is overheating at idle I can rev the engine while in neutral and the temperature drops right back down.  All belts seem to be fine and the fan is spinning at idle and spins faster when I accelerate.  I'm assuming fan clutch is fine.

My first guess was a bad thermostat but that doesn't make sense if I can rev the engine and the temp drops back down.  If it was stuck closed I don't think I would be able to do that.

Any suggestions? 

[Benz Dr.]

Could be a bad fan clutch. I use a bottle of water wetter mixed into the coolant which works better than you might expect.

 A lot of times the rad is starting to plug up and may need a re-core.

Did you change the coolant recently or do anything else around the cooling system? You may have trapped air in the system. We drill a small hole in the thermostat to let air bypass yet it seems to have no effect on temps or coolant flow.

[mrfatboy]

I have done nothing to the coolant system.  The problem just started to happen.  How would you tell if it's bad fan clutch?  The fan seems to be operation normally when at idle as well as high rpms.

After doing some more reading it could be as simple as low coolant level which causes an air pocket in the system which causes over heating.  I will try draining and refilling the radiator.

Years ago when I changed out my FIP and refilled my radiator I got an air pocket then.  However, I can't remember how I cleared it.  What is the best way for clearing out air pockets in the coolant system?

[Benz Dr.]

If you have the 100C temp switch on your thermostat cover you can undo it a bit and that might let any trapped air out. As a permanent fix, a small 1/8'' hole drilled through the thermostat works nicely.

[Tyler S]

Running the engine with the heater valve open (set to heat on dash) will help burp the system.
You can check your fan clutch by getting the engine very hot then shutting it off. Try to freewheel the fan by hand. It should have a lot of resistance. If it spins very easily it is most likely bad. The fan should also be very loud when revving the engine when its very hot.
Thermostats dont usually fail in the way your problem presents so as Dan said a plugged radiator is very possible. If there is just enough of a blockage to impede coolant flow at idle.

[mrfatboy]

Sometimes it's just the simplest of things.    The problem was just a leaking radiator cap.  Bought a new one and problem solved.    I should have used Occam's razor to begin with.

[66andBlue]

Well done, that should have left some money in the kitty for a good Margarita!