Strange coolant loss, random overheat.

[black75dog]

Hello everyone, I am new here so please be patient with me. I recently purchased a 2005 LS hatchback cause my other car is a Suburban and I needed to spend less on gas. I researched the heck out of Aveo's before I bought it, and surprisingly, I still bought it. Having said that, before i bought it I was aware that the plastic t-housing had been replaced, and the guy said, with receipts to back him up, that he had the head milled, compression check done, all belts/hoses replaced. The car was well priced, so I bought it. So, here's the problem, when I cold start first thing in the morning and turn the climate control wheel to hot, the engine overheats. I stop, pour some antifreeze in, and I am good to go. This happened 2 days in a row. 3rd day, I wait for the engine to warm up, then turn the heat wheel, no problem, everything works fine. 4th day, I again wait for the engine to warm up, turn the heat wheel and it overheats. I stop, pour in some coolant and good to go. I have searched high and low in the engine for a leak, I can not find anything. I do notice some coolant pooling on top of the transmission under the overflow tank, but I removed that, inspected all the hoses and connections, everything seems ok. Oh yeah, when it is over heating, the return hose off the radiator is totally cool, obviously no coolant flow. Could the t-stat be sticking only when the heater is turned on? I am a fairly smart car guy and I am stumped. There is absolutely no coolant under the car when it sits, nothing inside the car either. The oil looks like oil and is not frothy. Any help would be appreciated. Thanks. I am loosing about 1 gallon of coolant every 3-4 days.

[gclark8]

Difficult without seeing the car, but my guess is the engine compression is forcing the coolant out of the overflow. Any bubbles visible when hot? If so, head gasket may be suspect. How long since the thermostat was replaced? Check warmth of both radiator hoses, may be a blockage there.

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[avguy]

I would say +1 on the coolant being pushed out the overflow and it's likely burning some also, even though you didn't say anything about seeing white smoke.
So what's was the deal with the guy milling the head? Not something normally done, unless there was a blown HG due to an overheat, or else there was a timing belt-related problem which required valve replacement. Do you know which one it was? In any case, I think you need to do both a compression and leak-down test now, and try to figure out exactly what the current problem is.

[black75dog]

What the seller told me about the head being milled was that when the plastic thermostat housing exploded, the engine overheated. He felt like it was not running well so he did a cylinder compression check and after that decided he needed to get the head milled. I have driven behind it while my wife drove it and did not see white smoke. I have not, however, done that when it starts to run hot. I am also not sure about when the thermostat was replaced. If he did it when he did all the other work on the engine it has only been 3 months. I know for certain that there is a blockage at times, which is why I asked in my original post if the thermostat could be sticking when the heat control is turned on. That doesn't seem likely to me, if I understand thermostats correctly, they open at a designed temperature regardless of anything else. Thanks for the replies.

[black75dog]

Also, drove it this morning, waited for the engine to heat up before turning the heater on and it was fine. I didn't even smell antifreeze like I have been. I tried last night to get it to overheat so I could see if it was coming out of the overflow, but of course, I couldn't get it to do it.

[gclark8]

Check the heater hoses' connections, are they the right way round? It could be the heater/control is by-passing the water from the thermostat and radiator to just the heater.

[Thymeclock]

You say you are losing a gallon of coolant every 3-4 days. That's a LOT of coolant. If it were a leak it would be obvious, because that coolant is going somewhere. Coolant from a leak would leave traces or a puddle. The most likely cause is a bad head gasket. Do you see anything coming out of the tailpipe, like white smoke?

[black75dog]

So, thanks for all your replies. I have not seen white smoke or any bubbles coming from my overflow tank. I decided to change the thermostat, I know this is most likely not the problem, but it is cheap and easy. When I took the housing apart, there was no o-ring on the housing between it and the block. I thought that was a bit odd. I have a weekend of driving ahead, i am hoping I can get it to heat up again so I might be able to get some more clues.

[black75dog]

I checked the hoses, they are installed correctly.

[avguy]

.... When I took the housing apart, there was no o-ring on the housing between it and the block. ...

Are you certain that the o-ring wasn't tucked into the t-stat housing? If it is truly missing, that would most definitely be a problem, and the likely cause of your coolant loss. But you didn't mention what you did next. If the o-ring wasn't there, did you install one? Or maybe you haven't reassembled it yet?