Are there any OM636 experts out there?
 

I have a 1958 180D that sat for about 30 years .... Over the past 3 years or so I've been working to get it back on the road. I got it running and stopping fine .... but it quickly developed leaks out of the freeze plugs. In addition to the leaking coolant the coolant temp gauge quit working and the engine over heated when it ran out of coolant. I got the leaks fixed and put everything back together and it runs fine once again. However, now with the external thermostat in the radiator hose in place, the engine heats up and pushes coolant out the overflow pipe until it runs low and overheats. With the thermostat replaced with a straight hose the coolant circulates to the radiator and there is no leakage. The thermostat has been replaced with a new unit, installed a new water pump, and flushed the cooling system twice. I thought that perhaps when it overheated the head gasket was damaged or the head cracked causing higher than normal pressures in the cooling system; however, there is no oil in the coolant and no coolant in the oil so I'm thinking the head gasket is ok. Any thoughts of what is happening?

Thanks.

Just making a guess but it sounds like something is wrong with the thermostat.

debris in the rad from the overheat?

Run the engine with out the radiator cap on and see if it pushes coolant out.

Did you ever flush the engine? I have several of these engines and they run pretty cool in general unless blocked.

What have you done to clear out the block after 30 years of crud?

A good couple of citric acid flushes would probably help a lot. Directions listed in the forum. Use the search function.

Run the thing in place with the rad cap off each time you flush it.

How clean is the radiator?

Pull the external thermostat set-up and take a look at the inside. Is there gunk in there?

Do it all and report back. Good luck!

Thanks for the suggestions. The radiator is clean and free flowing, I flushed the engine with cleaner twice .... the flushing water is clear and clean not a hint of rust or debris. Did a little more testing and removed the thermostat completely and replaced it with a straight hose and the engine doesn't overheat, it stays around 180 degrees, it doesn't loose any coolant, there are no hot spots when testing with one of those infrared digital heat testers, and I can clearly see coolant circulating in the radiator. Without the thermostat in place I drove it for a number of miles on the freeway and lost no coolant and the engine didn't overheat ..... so maybe I do have a bad thermostat .... but that is crazy in that it is the second new one I've had installed with the same overheating result .... could both of them be bad? I'm going to do some testing on the thermostat later ..... but for right now it isn't overheating and my son's prom is this weekend (he really wants to take the little 180D) so I'm not going to mess with it anymore and just let him go with it. My fingers will be crossed that all will go well ..... but I'll be prepared to have to rescue him if need be! He and his date are excited about the adventure of it all ......