Overheating
 

Well, I never really solved my earlier problem (it pretty much mysteriously fixed itself... maybe the suggestion that it was "a bad tank of gas" was right), but now I have a new, much more concerning problem.

On the way home from work today, I noticed that the temp guage was rising a little bit in slow traffic, then it went back down. Then it went back up, and it got high enough (about 85%, my gauge has no degree readings) that I decided I had better turn the heater on to cool it off. Running the heater it returned to the normal reading (50%) with minor fluctuations for the rest of the ride home.

Upon arriving home, I checked out under the hood (not that I really know what I'm looking at), and pulled out the Haynes manual. The fluid level is good, and there does not seem to be any coolant leak. The fans work (at least some of the time--I suppose they could be intermittent), and, based on the test in the Haynes manual, the thermostat is at least partially working. I'm wondering if something could be clogged inside the radiator or coolant hoses, reducing (but not quite stopping) coolant flow, or if it is possible for a thermostat to get stuck half-way (i.e. not fully open and not fully closed).

Any other suggestions? I'm totally new to this "fix your own car" stuff, and I can't afford to shotgun the problem with parts that may (or may not) fix the problem. I also can't afford to take it to a shop (although if I really have to, it will just have to go on credit, and then I can have them work on my extremely intermittent rear drum lockup).

Details: 1990 Toyota Celica ST, 1.6L 4AFE engine (shared w/Toyota Corolla and Geo/Chevy Prizm), automatic. The radiator is only about 6 mos old (not sure exactly, but it was replaced shortly before I bought the car, and the radiator looked new at the time). The hoses don't look new, but, as I said, there don't seem to be any leaks.

Sorry for the long post. Any and all help is appreciated!

well i had the same problem
dont ignore it and take care of it
i ignored the slight temp raise and blew a head gasket
cost over 500 to fix
run your radiator fan direct and if it stops the its your fan temp sensor they run around 30 bucks
it beats 500 any day
that is the number one cooling issue that cars have

Two possibilities.

A restriction in the cooling flow through the engine somewhere - clogged radiator is the most common cause for an unexpected overheat - many people don't know that you should use distilled water in your cooling system because of minerals in tap water can clog up the radiator after multiple heating/cooling cycles. So, even if your radiator isn't that old - you could have a clogging problem due to hard well water for example. Municipal city water from a lake or reservoir is not as likely to cause this problem.

A leaking head gasket that is allowing exhaust gas to escape into the coolant through the head gasket instead of through the exhaust valve like it's supposed to. You should have a problem with too much coolant in the overflow container if this is happening.

In my experience this is what happens when thermostats go bad. Usually though this results in an engine that won't reach operating temperature (because the thermostat never completely closes.)

-Jay

How do I do this? Wire the fan directly to the battery?

BTW, my car has two radiator fans, a small "push" fan on the front passenger side and a large "pull" fan on the back driver side.

I really worry about the head gasket... The overflow container is the one where you check the coolant level, right? Would I only have too much there when it is hot, or would the excess stay in the container even after it has cooled down?

Check the Haynes manual to see where your temperature gauge sender is. If it's near the radiator then the issue probably can't be a clogged system (or else why would the gauge go up, right?); if it's near the engine then it can.

Check your radiator hoses too it is possible one of them is coming apart on the inside (partially collapsed) restricting coolant flow. If the thermostat was stuck in the closed position the car would overheat within minutes of starting it. Another thing to do is straight wire the fan to the battery and drive the car and see if it gets hot. If it doesn't it is possible the cooling fan isn't cycling on and off properly. Is the coolant/water mixture approximately 50/50? It's also possible the temperature sending unit is bad, giving a false reading.?

I have absolutely no idea what the coolant mixture is. I've only had the car since March (or something like that), and I haven't done anything to the coolant (as I said, there don't seem to be any leaks, and since the radiator had just been replaced, I assumed there was fresh coolant in it).

Is it important what the coolant mixture is (given that the coolant has worked fine for several months, and just now stopped)?

The Haynes manual does mention checking the temp gauge to see if it is working, but doesn't say how to do that. However, since the car overheats when driven (it doesn't appear to if I just let it sit idling, even with the A/C on), but then the temp comes down when I run the heater (and goes back up if I turn the heater off), and also varies with driving conditions, I think the temp gauge is functioning correctly. Does that make sense?

The mixture is supposed to be 50% coolant, 50% distilled water.

The only reason I mentioned the coolant/water mixture is because of warmer weather if it was a high percentage of water and the outdoor temperature had risen several degrees recently from what it had been maybe that was having an effect. Another thing that I just thought of is the radiator cap. If it isn't holding enough pressure it would cause the car to overheat. Another thing you could do is run the car until the thermostat is open and remove the radiator cap with the car running and see if the water is circulating through the radiator good. It seems to me that it should run hot when sitting and idling more so than when being driven, because when it is being driven it is getting road draft through the radiator to help cool it.