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93 4runner running cold and idling high

Hey everyone! I have a 94 4runner 5 speed that was running great one day then the next morning I’m driving down the highway and I notice the engine temperature gauge barely moved. When I got off the highway and came to a stop I noticed it was idling very high, around 1800rpm. My first thought was maybe my coolant was low so after letting it cool I opened the radiator and it was full and the bottle was at the proper cold level. So, now I’m guessing the thermostat is stuck open but thought I’d just check here first and see if I’m going down the right path. The other thing is I’m not sure why it started idling so high, though. Perhaps because the engine is running cold it’s just idling high as it would when first starting until it gets up to temperature? Even then, I feel it’s still idling much higher than it did, even in that case, but I could be mistaken. Any help is appreciated, thanks!

I would assume engine is hot after driving and radiator is hot too at this point, is it?

in this case you might have a temperature sensor failing and giving a low reading (“barely moved”?), whcih ECU in turn gets as a justification to keep idle high until temperature gets more reasonable

I would not touch thermostat until you are sure it is at fault, as it is much messier work to compare to sensor

The problem is probably with a stuck open thermostat.

There’s two sensors for the coolant temp. One for the computer and one for gauge.

If the coolant doesn’t come up to the proper temperature, the computer will stay in the open loop mode causing the high idle.

The temp gauge is just confirming that the coolant isn’t getting up to temperature.


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Does the heater blow hot air?
Are the upper and lower rad hoses hot or cool?
If you have hot hoses and it’s blowing hot from the heater, look at the sender. Otherwise, look at the thermostat.

You can’t go by if heat comes out the vents or by feeling the hoses.

Engine management computers don’t go into the closed loop mode until the coolant reaches 140-150 degrees.

So at 130 degrees, there’s heat coming out the vents and the hoses will feel hot.

But computer is still in the open loop mode and the engine stays at high idle


I meant to add this to the original post but must have spaced it. At highway speeds it blows warm air, but not hot as it should. Slower speeds, blows cold.

Tester, this is a key detail that it’s not a matter of 130 versus 150.

Thermostat would be what I was replacing…

The gauge offers me no insight of even an approximate temperature number, but by barely moves I mean the gauge moves, at most, a quarter inch.

But the problem is, you don’t know at what temperature gauge the starts to move.

Is it 100 degrees? 110 degrees? etc…


I appreciate the help, I think thermostat will be my next step given the heater blowing cold too. Now I just need a somewhat warm day so I’m not getting frostbite on top of it all. Luckily it’s not my DD so no biggie waiting.

I have a Corolla of similar vintage that had similar symptoms a few years ago. Inspection of the thermostat showed it was deformed by corrosion, unable to close completely. Replacing the thermostat returned the coolant temperature to the correct position on the gauge and returned the idling speed to normal. fyi, It’s always a good idea to bench test a replacement thermostat in a pot of hot water to make sure it opens correctly and at the right temperature before installing it.