Originally Posted by camaroman7d
I don't think drilling it will help cure your problem. Do you know what temp these events are happening or do you just have a gauge with no numbers?
The thermosat is going to open at the same temp. no matter if you have holes drilled in it or not. Small 1/8 inch holes will not flow enough to keep the car from reaching the magic temp at which the thermostat will open. Drilling the holes do help removed trapped air and if that's your problem it will help. I doubt that is your problem because once the system is purged (the first time the stat opens) all the "trapped" air should be gone.
I am not familiar with that particular engine or cooling system but, do they make or will a fast acting thermostat fit? If so that is the way to go. I have had some of the "typical" low cost stats do that to me (drilling didn't help, I tried it). They just don't open on time and can cause heat cycling. I have never had a problem with a high flow fast acting stat. Check to see if Robert Shaw, Mr Gasket, Milidon, makes one that will fit your application. they are about $10 an it is money well spent.
You are correct as long as the factory bypass is operational. When the thermo is closed, the water is trapped in the engine and is heated up. The factory designs in a small slip stream (For example Old Pontiac V8 engines have an obvious hose that goes from the water pump to the intake manifold - that is their bypass. Most others us a hidden internal passage). This passage directs a small stream of heating water past the expanding element of the thermo so when the temperature reaches thermo rating, it begins to open and full flow occurs. The engine temperature never exceeds the thermo setting if this is operational and the rest of the system is in good shape. However, when that passage is plugged as in 78 Monte's case, the thermo is isolated from the heating water in the block so it will stay closed until it is heated by conduction from the heating engine which takes a lot longer, all the while the water is getting hotter than the thermo setting. That cold upper hose when the engine is very hot is a dead giveaway of this very phenomenon. The thermostat is seeing that cold hose temperature and until heat is conducted to it from the overheating engien, it will stay closed. Once the thermo cracks open, hot water flows past it and it will function properly until the engine cools down, then it must be conductively heated to open again. Has nothing to do with air in the system. Drilling the 1/8" holes in the thermo is a substitute for the passage, circulating the heating water past the thermo and it will work as designed. Another interim solution is to turn on the car heater which circulates the water past the thermo and it will open OK. The best solution is to clean out that bypass passage.