Overheating (changing the thermostat)

• Replaced thermostat and gasket
• Flush coolant
  

Labour day. This generally means barbecues, getting quite drunk, and relaxing in the shade during a pleasant late-summer afternoon. For me, however, it meant hanging over a smoking engine inhaling various gasses while trying to figure out why the engine was overheating. And I loved it.

The manual suggests several causes for the overheating I leave to the enthusiast to check out. I had narrowed down my options to these possibilites:

• Failed thermostat
• Failed pump (either the propeller came loose or the bearing broke)
• Clogged radiator
• Too much anti-freeze in the coolant
  

As the engine oil levels were fine and clean I did not bother doing another oil-change. Another quick check would be the accessory belt tension (slipping causes bad water-flow), which was fine and the belt was new.

I started by testing out the bearings on the water pump. To do this I removed the accessory belt and turned the pump manually. I heard a squeaking sound when I turned it, but it did not shake and so I assumed the bearings were fine. There was also no leakage from the unit itself.

To look at the thermostat and do further testing on the water pump I needed to flush the coolant. Easy, in general, except for some reason my radiator doesn't have a drain plug! I couldn't find any help on the internet on the topic so I ended up removing the bottom hose and draining it like that. The coolant was filthy, and very green indicating a large amount of anti-freeze. This could be the problem. I flushed the engine with clean water for a few minutes before proceeding.

While it was without coolant I decided to take a look at the other parts at the same time. My reasoning was that the water-flow was somehow restricted, which was causing the overheating.Turning the water-pump sent water out the bottom hose, so it wasn't that (although the propeller has been known to come loose at high temperatures, and the engine was cold when I was testing it, but this is very rare). In the end I decided the thermostat needed to be replaced.

31 years can take a toll on some parts, so after removing the upper hose and the three bolts holding it in place (see right picture, circled in red) I needed to use large amounts of a solving fluid (like WD-40) and a small crow-bar to get the thermostat housing off (see left picture, thermostat indicated with arrow). It was difficult, but I got it off. The thermostat looked fine, but I replaced it anyway with a part I bought at Advance. I also bought a new gasket.

Removing the housing I scraped away whatever was left of the old gasket and put the old thermostat aside to test it out later. The new gasket was installed (though it looks symmetrical, orientation matters! By the way this goes on top of the thermostat) and the thermocouple replaced. I also filled up the coolant again with 75% water by pouring it directly into the engine while the thermostat was out. The remaining coolant/anti-freeze was entered after the radiator burped and the engine was running.

They say in old cars that when you fix one problem another ten arise. This car does not defer tradition. I don't know if the engine is overheating anymore because the gauge doesn't move!!! I stays below C all the time, even after driving it around for a while. All the other gauges work normally, and no amount of tapping gets this to even jump. The old thermocouple was submerged in boiling water later on to see if it was defective, which it wasn't (it opened up just before the water started boiling, around 195F/91C ). It is however possible that that the opening was not sufficiently large to let enough water through. I'll be testing the gauge and the sender unit out next.