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1994 Corolla Overheats At Idle

by 1995Corolla, May 14, 2015 in Pre-1997 Toyota Corolla and Geo Prizm

Hi all.

So I'm back to the forums. Sold the 95 corolla awhile back but decided I missed it. Recently, I picked up a '94 with 190k on the clock. After picking up the car, I noticed that if I idle for a short period of time (~30sec) the car gets HOT. If I idle for a minute, the car is almost at the red hot symbol on the top of the temperature gauge. The overheating reduces in significance when I start driving, and if I'm cruising the engine stays at normal operating temperature. So what do we think it is guys? I'm guessing the thermostat but I wasn't sure?

Any help would be greatly appreciate!

Thank you.


Could be a number of things - bad thermostat, damaged or clogged radiator, bad water pump, damaged fan, slipping v-belt, bad sending unit, etc.

First thing I'd check - condition of the coolant and verify its level. If it looks good - take a peek in the next of the radiator - see if there is significant scale build up inside (look in filter neck), look for obvious signs of damage (flattened cooling fins, physical blockage of the radiator, signs of repair - solder, etc.) Check the radiator fan, note anything of issue. Check the belts - see if they are heavily worn or slipping. The hoses - look for obvious signs of damage.

If that all checks out - I'd verify with a separate temperature probe the actual temperatures. Unlikely, but it is possible that the cooling system is actually fine, that it is the temperature sending unit or the gauge itself is faulty. This can also be traced down to electrical noise in the system - funky wiring, botched audio install, messed up alarm system install, etc. - can cause all sorts of sensor issues in the cars.

If the engine really is running hot - most likely it is the thermostat, followed by a bad water pump. If the car hasn't been serviced regularly, might be a good idea to flush out the cooling system - note any scale or "mud" that comes out. Run the car and see if that flush helped. Sometimes you will get some debris stuck in the thermostat - preventing it from fully closing. If that happens, the coolant will be constantly circulated, eventually overheating. Happens mostly at low speeds, idle - as the fan can only move so much air past the radiator. A radiator filled with sludge or heavily scaled makes it that much harder to cool off. At speed - you have plenty of air movement to help cool, but even then - eventually, it will overheat.

Hey FIsh,

Thanks for the help. We checked the coolant and the reservoir was at the full mark. Next, we inspected the hoses. We were surprised to find that some of the hoses didn't contain any coolant -- only air. As a result, we performed a coolant service and got the air out of the hoses (so only coolant remained). Next, we replaced the thermostat for good measure (we had already bought the part). Now the car idles like it was new with no overheating whatsoever. Thanks for your help! I didn't suspect the coolant hoses could be full of air at first, but now I know that is indeed possible.

Great to hear! Sounds like they might have done a quick coolant flush before you picked up the car and didn't "burp" the system of air pockets. Since the car is running well and temperatures are in check - sounds like the engine is fine. Since you picked up on the problem so quickly and took measure to avoid any catastrophic engine damage - sounds like problem fixed. Those A-series engines are pretty tough - takes a lot to cause any permanent engine issues with them.

I'd monitor it closely and see if those air pockets come back, ie, the system is not able to purge itself. If this returns, I'd look into the simple things, like checking the radiator cap (wrong pressure or stuck, will not allow air to purge from the system to the recovery tank like it is supposed to). Could also point to a t-stat with the "jiggle valve" in the wrong position or even the t-stat installed backwards. If there is a large amount of gas in the system - then I'll look to even a bad head gasket being a possibility.

But, sounds like things are running well. Enjoy your ride!

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