Moyer

Please help.

The car runs fine until it gets up to running temp, then it goes a little higher and pukes out a large amount of coolant(close to a gallon) onto the pavement through the overflow tube on the overflow tank. After this, the temp will just keep rising and overheat.

The first time this happened was at the end of a 2hr drive down the interstate. I pulled into a parking lot and it just dumped out everywhere. I refilled it and was able to drive it the 2hrs back home before it happened again. Now it seems to do it every time as soon as it gets hot enough.

From searching around, I think there's a good chance it's either the overflow cap or a sticking thermostat, but I'm not sure which. The overflow cap doesn't appear to be damaged. Both upper and lower radiator hoses get too hot to touch. Is there anyway to tell without replacing things?

One thing though, I tried just starting it and letting it idle. The temp gauge got up to running temp, then went down slightly, then went back up after a bit when it finally puked coolant again. The fans never came on during this time, but I assume they would have if I let it keep running. That dip in the temp gauge when it got to running temp makes me think that was the thermostat switching?

Also, if anyone can point me to directions on replacing the thermostat, I would appreciate it. It's hard for me to tell what else needs to be removed to get to it. Thank you.

rkuelbs

There are a few issues here.

The radiator cap is designed to hold a certain amount of pressure in the cooling system, and a car will puke coolant for basically three reasons...

1) Radiator cap failure. This is the least likely cause in your situation, and we'll get to that. But basically, if the cap can't hold enough pressure, excess coolant will leave past the cap. The lower pressure creates a lower boiling point, and the coolant will in turn boil over.

2) Overheating. If the engine overheats, and the coolant starts to boil, the pressure in the system will quickly vent past the radiator cap. When the coolant passes the radiator cap, the pressure drop will cause it to instantly boil.

3) Overpressure. The cooling system naturally builds pressure from the expansion of the coolant when it heats up. However, external sources of pressure can overpressurize the system and allow coolant to escape past the radiator cap. The most common cause would be a head gasket failure. It's very common for a failed head gasket to allow combustion gasses to overpressurize the cooling system. Also, on many OHC engines, head gasket or oil cooler failures can allow engine oil pressure to overpressurize a cooling system.

So how do we diagnose yours? At first, the sporadic overheating made me think head gasket for sure. But then you mentioned that the cooling fans never came on before the engine overheated. The cooling fans definitely should have switched on first, so I would first recommend checking the fans and their relays. With the fans not working, the vehicle should be able to stay cool at highway speeds, but should quickly overheat if you were to exit the highway and slow down. Is this what initially happened?

The temperature dip around operating temperature makes me believe that the thermostat is functioning, however any overheating can damage a thermostat, so I would recommend it's replacement at this time simply as a precaution.

After checking and repairing the fans, hopefully your problem will be fixed.

If you still have a problem, I would first recommend checking for combustion gasses from a leaking head gasket. For a simple check, you can fill the cooling system and let the vehicle run until the thermostat opens (with the heater turned on). Then shut the engine off and let it cool. Do this a few times to bleed all trapped air from the cooling system. Then make sure the system is full and start the engine. If you see excessive bubbles that keep rising in the coolant (not boiling, just air bubbles while it is still in the 180-200 degree range), that is likely combustion gas leaking past the head gasket.

Parts store sell a fairly cheap "Block Test Kit" that will actually test the air coming from your cooling system for the presence of combustion gasses. Basically, you install a special tube in place of the radiator cap, and fill it with a liquid that changes colors when it detects combustion gas. One of these kits can check for a blown head gasket quickly and easily.