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A Proper Thermostat To Prevent Overheating

By Stephen Kassis

A thermostat seems a pretty insignificant item when it comes to the operation of an engine. However, it can mean the difference from a cool running vehicle to an overheated nightmare.

Recently, my 1989 Caprice Station Wagon was experiencing overheating issues. An overheated engine – especially in a vintage car or truck – can cause severe damage to the cylinder head, valve springs, hydraulic lifters or even the engine block. It is vitally important that a vehicle that is running hot is cured of the problem as soon as possible.

In this case, I replaced the radiator (which was old and leaking anyway), fan belts, hoses, water pump and the thermostat. None of this seemed to help. The symptoms were simple – at high speed, 65-70 mph for long distances, the temperature gauge would rise to 240-260 degrees. If the vehicle slowed down or stopped, the engine would cool right down. I lived with this situation by only driving the car in town for short runs. Finally a friend (retired GM factory tech advisor) suggested that I change out the thermostat. When I explained that I had just done that recently, he asked if I installed a premium grade unit. Never even heard of a premium grade thermostat, I replied. So I called and ordered one the next day.

Several weeks later, after the new thermostat had been sitting on my desk, I had a trip coming up. Out to the shop I went to change out the thermostat to the new premium grade unit. Most of my experience working on cars has been with early 1930’s vehicles. So when I opened the hood, I figured this would be a half hour project – NOT! Wow, I had to remove vacuum lines, fuel line, air cleaner, cruise control, radiator hose and the water outlet that held the thermostat! What a pain. I definitely prefer working on the older engines!

After removing the old (new) thermostat it was immediately evident how it differed from the new premium thermostat. The water flow opening in the old unit was about 35-40% smaller than the new unit. I was encouraged by the huge difference in the two thermostats. Could this really be the reason for the heating problems? I would soon find out as I headed out on my trip the next day. It was cool and rainy as I started the 200 mile trip – not a particularly good test for heating problems. However, it wasn’t long before the temperature started rising and was soon much hotter than I like to see. Still, it was not as hot as it had run before. Maybe the cooler day was preventing overheating? Time would tell. I stopped for coffee for about 15 minutes. After the stop, the car started to run cooler than it had run in months. Could there have been an air bubble that finally escaped?

Now the car was acting more like expected, still not as cool as it should, but closer to normal than before. The temp would rise when climbing a grade but cool down on the downhill. Before it would get hot and stay hot, only cooling down when the car slowed or stopped. I arrived at my friend’s place (retired GM tech guy mentioned earlier) and we popped the hood to take a look. There was coolant on the manifold. After checking it out, found that a bracket had gotten jammed and stopped the water outlet from seating properly. A quick fix and we were back to testing. He brought out a pressure pump and attached it to the radiator neck and started pumping. When it got to 16 pounds he stopped and let it rest for 5 minutes. Checked again, no leaks!

A gauge he loaned to me told us the actual engine temperature and we found that the gauge in the dash was reading about 10-12 degrees hotter than actual temperature.

He had a gauge to test the radiator cap (which was also new) and found that it was not holding pressure. He had a good one and installed it on my radiator. With the fixes in place, it was time to test out the new system parts. Another 200 miles of local and long distance travel – no overheating. However, we had yet to do sustained high speed driving, which was the true test.

The next day we headed back home, anxious to see if the engine would overheat with 65-70 mile an hour speeds. Factoring in the dash gauge which read 10-12 degrees hotter, the engine was operating within a normal range. When we climbed a grade, the temp would rise to 220-225. But it would drop as soon as we crested the hill and started downhill. Perfectly normal operation!

The lesson I learned was that some lesser quality parts can cause huge problems, even though they are designed for the proper application, they are not up to the OEM specifications. Going with a higher quality thermostat would have saved me hundreds of dollars in parts & labor replacing parts that were not the issue at all.