I use my 1992 Mitsubishi Expo LRV for an 85-mile high-speed commute three times a week. About six months ago, I had the water pump replaced as part of the timing belt job. Shortly thereafter, it began running warm (temperature gauge at 1/3 full scale instead of normal 1/4 full scale) and losing about three cups of coolant every month. In the past week it started overheating, as evidenced by a very rapid rise in the temperature (gauge to full scale) and from coolant venting into the reserve tank or out the vent. The temperature rapidly drops (about 1/3 to 2/3 full scale) when the engine is turned off.
I replaced the thermostat and the radiator cap, and verified that all the thermosensors, relays, and fans work. It still overheated. Our local radiator shop found only a few degrees difference in temperature between the inlet and outlet, and rodded it out. It still overheated. The dealership mechanics have suggested a hairline crack in the head gasket or the head could let combustion gasses enter and heat the cooling system. In support of this, the engine warms up in just two or three minutes whereas it used to take between five and ten. But, the oil cap and dipstick look as they should with no evidence of water in the oil. I am stumped. What could the problem be and what should I do to pin it down? If it is the head gasket or head, how much should these cost to repair?
Head gaskets will run you a lot for the labor, but I would try bleeding the system. Since you indicated that the problems started right after a water pump change, I would think that there is air in the system and you are getting hot spots. So, before you go and get a new head gasket, try bleeding it first. Also, my mechanic suggest trying to pressurize the system as well. You can do this with a little device that looks like a bycile tire pump that has an adapter to attach it to the radiator. Watch the gauge, if it falls out of the zone for your car (look at the radiator cap, it will have a pressure rating) in more than 5-10 minutes then there might be a head problem. Now that I am thinking about it, the original problem could have been this bleeding problem, but since you drove around for a few months like this, you could have warped the head. You see, with aluminum heads, you have to be very carefull with the temperature. Also, make sure they are cold before you remove the head. Even if it is slightly warm, they can warp when removed.
Bob, The Auto Answer Man
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