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We have a 38 120 in our shop with what seemed to be the same overheating problem that's been on the forum for several years. The customer purchased it 3 years ago and has used it very little due to the problem. The engine had recently been rebuilt he was told. An electric fan had been installed and did not cure it. The shutters did not work and were blocked open. A thermostat had been installed in the water outlet and it does not fit well allowing water to pass it by.

We started by removing a rearward freeze plug to see if there was a buildup of rust. It was clean. Then the water outlet was removed to inspect to see if there was a thermostat, there was. With it off, the mechanic noted some "steam" emanating from a water hole in the head. The motor was cranked and vapor puffed out of the water holes, sign of a leaky head gasket.

The head was removed and it was clear the motor had been recently rebuilt including decking the block. All looked fine. When the head was turned over the problem was evident, the head had been surfaced with a rough cut and compression gasses traveled along the grooves until they met the water passages.

The head was surfaced properly, engine reassembled and idled for an hour with the gauge in the center.

We had assumed the rebuild was good, our first step should have been to check the water for combustion gasses. Learned.

Fixed

Nice...Ernie

I had a similar problem with a daily driver about 10 years ago. It would run fine for a week or so, even in heavy traffic, and then start to overheat. When it did this, the radiator coolant level has always found to be low, but the expansion tank was full to very top. So, I would pour the fluid from the expansion tank back into the radiator and it would fill the radiator perfectly, and the problem would go away....for about another week or so.

After a few months, the problem got progressively worse, with the time between having to refill getting shorter. I had changed the cap twice thinking it was that, and even put on a higher PSI cap.

Long story short, it had a bad head gasket that was leaking cylinder pressure gas into the coolant. This extra pressure caused the coolant to blow past the cap, and once enough coolant was expelled, it started to overheat.

Looking back, I believe it was caused by a shard of wood that I found stuck through the radiator about a month before all this started to happened, and had caused the car to severely overheat. The radiator was changed, and I thought all was fine. But the damage to head gasket had happened and it slowly got worse over the next few months.