Hey all,
I'm new to this forum as you can see by my post number :)

I've got a 1990 Toyota Celica with about 190,000 miles on it. Its had quite a few things repaired but frankly the mechanic's fees are insane and I'd rather learn a bit about my own car than pay him to fix it for me every time! I was wondering why my car seems to overheat. It overheats especially in the summer time with the AC on. Even if the AC is off and I'm on the highway at 65-75 MPH it will still overheat. To prevent this I have to turn the heat on (even in summer! ugh!). What are some things that could be wrong with the car or where could I look to find out more information about what is wrong and how to fix it myself?

-Thanks!

When was the last time you had the radiator flushed? What color is your coolant? It should be a bright green and not look like dirty water and if it looks like the latter then it needs to be flushed and filled. It should be done every 2 years or 30K miles. Maybe the thermostat is stuck closed and maybe that needs to be replaced too.

Yes I would start with a flush on that radiator and a new thermostat but with a 190k miles you probably need a new radiator. DONT ever take the radiator cap off when the water is hot!! I know elemetary stuff but you be surprise how many people still do this.

It's possible the radiator is clogged, but not likely. You said you can keep the temp. down when you turn on the heater all the way. The heater core has smaller coolant passages, which would tend to plug up sooner than a radiator would. If the heater core cooling passages are not plugged, it's likely the radiator core cooling passages are not either. It also sounds like the water pump is fine, as you are pumping coolant through the heater core to get the cooling you need, so that likely is not a problem.

Items to look for:

- low coolant level
- thermostat stuck in closed position
- if original radiator, copper fins may have rotted away from the cooling passages on the outside of the radiator, so the radiator can no longer dissipate heat
- electric cooling fans operating (you can check this by running your A/C - when your compressor is engaged, both the inner & outer cooling fans on the radiator should be running)
- if this test works, your coolant temp. sensor that is used to turn on the cooling fans may have failed, so when your coolant temperature is high, this switch should engage the cooling fans
- bugs/dirt clogging up up heat-dissipating fins on outside of radiator (probably not very likely)

Good luck!