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Overheating when idle


I have two possibly related things I’ve noticed:

(1) About a week ago I was in a slow moving fast food drive thru line and kept turning off my engine to conserve gas. I had the key in the acc/on position because I was listening to the stereo and noticed what sounded like the fan coming on under the hood. Is the fan supposed to ever come on while the engine is turned off?

(2) Yesterday I noticed that my engine temperatures would start to rise when idling at a light, but then would quickly drop down to 50% when I started moving again.

What’s the diagnosis here and potential cost to fix? It’s a 93 Corolla with 90k miles, btw. And I just had my waterpump replaced 3 weeks ago because it was making unpleasant squeaking noises.

Thanks for any advice!

It looks like a classic case of a cooling system not able to keep up with the demands placed on it. I am not familiar with your model but have seen cooling fans that work independently of the regular ignition on/off. The temp rising at a light but dropping down while driving in my mind are indicative of a low coolant, coolant leak or coolant pressure problem. Given your situation the first thing I would check when the engine is cool is the level in the radiator. That is not the coolant recovery container. I have also seen posts on this most wonderful board about systems needing to be burped. Not good with burps or diapers I am sure someone knowledgeable will provide a better answer if needed.

The overheating could be several different things including radiator hose, thermostats fans etc.

A little off your question however… Don’t turn the engine on and off while in that drive through lane. You put far more wear on the engine and car by doing that than any gas saving.

Hybrids do that, but some time take a look at their starter and charging system. they are designed to do it, your car is not.

On many vehicles, the cooling fan can come on even with the ignition off. Also they sometimes run AFTER the engine is turned off. My guess is that your coolant is low.

Just curious, but why wouldn’t low coolant also cause overheating while moving?

Because it is overheating while you are not moving, my first guess would be the fans not running correctly. On you car, correct me if I am wrong, the fans are controlled electrically and not by a belt. When you are moving, air is being pushed through the radiator, basically doing what the fans do.

If the fans are not running correctly, the next step is to find out why. Is the problem the fan, wiring/connection, a sensor…

Checking to see if you have low coolant is always a good idea. Top off the reservoir if needed. It can also cause this problem. When you are moving you are always creating a better cooling environment for the engine then when you are stopped.

Ok, here’s some additional info:

Coolant as low and I added to the full line, but it didn’t make a difference.
Yesterday when this was occurring (idling in parking lot) I popped the hood and verified that the fan was running.
Today when this was occurring (again, idling in a parking lot) the fan wasn’t running.
It got hot enough this morning that the coolant in the overflow container was making a boiling sound that could be heard inside the car.

I called my mechanic and he said I may need one of the following:

new radiator ($450)
fan temp sensor ($140)
head gasket (???)

I’m getting tired of putting money into this car :stuck_out_tongue: (I just spent $600 on a new waterpump and pulley).

Overheating at idle and the fan doesn’t run points to fan temp sensor, control unit, relay or other wiring issue

Radiator or head gasket would cause overheat while cruising - not your problem

You checked and filled the coolant in the reservoir, but how does it look at the radiator cap?

I think it could be that the system wasn’t properly purged of air when the water pump was replaced. Or it could be a sticky thermostat, which should have been changed with the water pump if over 4 years old, IMHO

When I open the radiator cap should the car be running? Should it be warmed up or cool? BTW, the thermostat was replaced about 1 year ago.

Here’s a more detailed summary of what I’ve observed today with some additional info:

This morning when I noticed that the coolant reservoir was low I filled it to the top (the car was cool at the time). After I completed my drive to work (12 miles) I stopped in the parking lot and let the car idle for 1-2 minutes to see if the temp would raise above normal. At this point I heard the boiling sound and observed the temp gauge approaching the red. I looked under the hood and opened the reservoir cap and noticed that it was boiling/bubbling. Also, it still looked full. At this point I shut the car off and went into work.

About 3-4 hours later I drove 5 miles to lunch. After I reached my destination I turned off the car and again inspected the reservoir container but now it’s back down to below low! I see no coolant leaking anywhere.

What’s going on here?

You do mention the radiator cap,the engine should be cold when you open. I am wondering just how much coolant is in the radiator,you only mention checking the resivoir.

Your engine should be cool. If you squeeze on a hose and feel no resistance that means the pressure is down. Just because the reservoir is full does not mean your coolant system, radiator etc. has a full fluid condition. The radiator will suck coolant from the canister but only to the point of equilibrium which is why you need to pull the cap and add fluid until the radiator is topped off, or go through a few fill up the canister, watch it empty until the radiator is filled to capacity. My guess is you have a leak in the cooling system and are low on fluid in the engine system, not the coolant container. There are good and bad scenarios for cost for coolant loss problems, but a seized engine due to no coolant is a far worse proposal.

I fear you may have a head gasket problem. If you were able to remove the radiator cap when it was bubbling, that tells me the fluid was not actually boiling buy air was entering into the cooling system.

An old mechanic’s trick it to take the radiator cap off, then run the vehicle for a few minutes, if the coolant starts bubbling a bad head gasket is likely.

You should take your car to get the system pressure checked and for them to check if there are hydrocarbons in the coolant.

I never removed the radiator cap, only the cap for the overflow container

So you don’t know how much coolant is in the radiator.

Correct. How do I check the level of coolant in the radiator? I assumed it was indicated by the overflow amount but I guess not.

from previous: Your engine should be cool. If you squeeze on a hose and feel no resistance that means the pressure is down. Just because the reservoir is full does not mean your coolant system, radiator etc. has a full fluid condition. The radiator will suck coolant from the canister but only to the point of equilibrium which is why you need to pull the radiator cap and add fluid until the radiator is topped off if you do not have a radiator cap keep checking the level in the jug when cool.

Well, I’ve filled the overflow container twice and each time it was empty again following a drive and cool down period (ie the radiator is grabbing additional coolant on cool-down I guess).

I also opened the radiator cap on the top of the radiator when the car was cool and it basically looks empty, even after my two fills of the overflow. I’m a little nervous about filling the radiator directly because I don’t want to add too much. I should also mention that with the engine warmed up and car idling the upper radiator pipe is hot but yet still very pliable and I couldn’t feel any indication of coolant running through it while squeezing it.

Yet I have not been able to reproduce the overheat since adding coolant, even though it seems like there’s still not enough coolant in the radiator. I’ve tried idling a number of times over the last 24 hours and it just sits at 50% temp;.

The engine seems to idle fine. I see no white smoke out the tailpipe. The oil looks normal. I see no coolant leaks either while idling or after having left the car overnight.

What’s going on here? Is it possible my mechanic just forgot to refill the coolant (or didn’t add enough) after replacing the water pump three weeks ago? If not, then where is my coolant going? I’m concerned about the possibility of a head gasket issue ($1400?). There’s no way I’m putting that amount into a car worth ~$2000, so it would mean having to get another car =(

Fill that radiator up with the proper 50/50 mix,don’t worry about over filling,worry about where the coolant that was suppose to be in the radiator went too. Keep checking the actual radiator as you have started to do. You are correct to be concerned. Even if the coolant level stays good you still may experience erratic temp gague action due to air in system,with engine hot and running open the upper radiator hose clamp slightly and let some coolant/air bleed out.

I took my hot and running engine bleed procedure to the BMW shop and the techs laughed,we did all bleeds with the engine off and through the bleed fitting,just have it open when pouring in coolant.

This all makes perfect sense to me now.

The coolant level in your engine was so low that at idle your water pump couldn’t overcome the height difference between the coolant and the top radiator hose. Run the engine a little faster and the pump outlet pressure rises enough to lift the coolant. 1 PSI = 28 inches of water rise

Most cars I’ve seen have the temp sensor for the fan on the bottom of the radiator. But in your case no hot coolant was getting pumped to the radiator at idle. So the fan sometimes wouldn’t run even while the stagnant coolant in the block/head came to a boil.


I took it to a shop yesterday and the mechanic pointed at a small leak coming from the drain area (peacock?) while the engine was idling. He tightened it by hand and the small leak appeared to stop.

After leaving I investigated the ground below the leak area a few times while letting the car idle and did continue to see a small leak (maybe 1 drop every minute). So I took the car back. This time they drained the coolant, replaced the radiator peacock (and o-ring?) and refilled and then pressure tested to 20 lbs. They said everything looked fine.

After I got home I did the same idle test and, again, noticed a very infrequent drip. Also, there is now steam coming off the radiator that I never noticed before :stuck_out_tongue:

I guess I’ll monitor the coolant levels for awhile and see how much I loose. If it’s a fair amount I suppose it’s time to get a new radiator. Must it be OEM or am I safe going with an aftermarket one?(they quoted $330 parts and labor with an aftermarket radiator; the oem one apparently retails for ~$450 just for the part!)