2002 Malibu - Overheating

I have a 2002 Chevrolet Malibu that is having an overheating problem when driving in the mountains. It can sit and idle in the driveway all day and never get hot, but on the way to Black Rock Mountain yesterday it almost overheated (got to about 220-230). I checked the coolant resevoir and it had plenty of coolant, but it looked like it had some kind of (for lack of a better word) goo in it. I had the system flushed recently, and this morning I took the resevoir off and cleaned it really well to help determine if this stuff was still getting in there. Not sure I got all of it since it seems to have coated the inside of the tank but I got a good bit of it out.

I thought it may be the intake manifold gasket (GM 2.8-3.1 V6 engines are famous for having trouble there) but there isn't any coolant in the oil, and it isn't losing any oil that I can see. I've had one blow out the intake manifold gasket before but, in that case, coolant got into the oil instead of the other way around.

So far I've cleaned the resevoir and replaced the serpentine belt, since the Haynes manual says that a bad belt can cause overheating. I was going to replace the thermostat but, due to all the stuff in the way I can't get to it.

Anyone know what this might be, and what I should do about it?

The only thing I can say is when oil gets in the water its the gaskets but i water gets in the oil then it the heads

go ahead and check to see how the oil looks like on the dipstick

[QUOTE=Seneca_Tiger]I have a 2002 Chevrolet Malibu that is having an overheating problem when driving in the mountains. It can sit and idle in the driveway all day and never get hot, but on the way to Black Rock Mountain yesterday it almost overheated (got to about 220-230). I checked the coolant resevoir and it had plenty of coolant, but it looked like it had some kind of (for lack of a better word) goo in it. I had the system flushed recently, and this morning I took the resevoir off and cleaned it really well to help determine if this stuff was still getting in there. Not sure I got all of it since it seems to have coated the inside of the tank but I got a good bit of it out.

I thought it may be the intake manifold gasket (GM 2.8-3.1 V6 engines are famous for having trouble there) but there isn't any coolant in the oil, and it isn't losing any oil that I can see. I've had one blow out the intake manifold gasket before but, in that case, coolant got into the oil instead of the other way around.

So far I've cleaned the resevoir and replaced the serpentine belt, since the Haynes manual says that a bad belt can cause overheating. I was going to replace the thermostat but, due to all the stuff in the way I can't get to it.