Car overheating under different conditions.

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2002 Sienna. 204k miles on it. Decent quality water pump put in 42k ago ( not that that necessarily means anything). Coolant level looks fine.

Doesn't matter if traffic is heavy or light or if driving in the city or on the freeway.
The AC will be going along working fine all of a sudden it will stop and the car will start to overheat.

The weird part is that even if I have not been using the AC but I have the vent fan on, with the temp set to cold, the car will start to overheat. The higher I have the fan speed the faster it starts to overheat.

Again does not always do this. Could be driving along just fine and eventually it will do it.

In both cases the overheating is corrected by turning on the heater and the fan on high and the temp quickly goes back to normal. Thoughts?
 
What if the overheating triggers a fail-safe where the computer shuts down the AC?

Next time it overheats, open the hood. Rad fans should be blowing hard and throwing all kinds of heat your way. If they're blowing cold air you have a clogged rad or water pump circulation problem. Also feel the hoses for heat, flow, and firmness.

I'm liking the thermostat idea because it's something that can be flaky, and b/c the heater core is on its side of the system. A stuck closed t-stat would also have the fans blowing cold air.
 
Try the inexpensive stuff first, like the thermostat. Whatever you do, keep the engine from getting too hot even if it means stopping the vehicle. Quickest way to torch an engine other than running it out of oil is to overheat it.
 
Make sure the engine fans are running without the A/C on and with, if it has two fans one my come on with A/C off and both with it on or a noticeable speed change.
 
Typically if you're having overheating problems while at highway speeds it is usually a thermostat problem or clogged or partially clogged radiator. I just replaced the radiator in a friend's 2008 Dodge Charger that was overheating while driving on the interstate and his radiator was probably 90% clogged. Problem solved for his car.
 
As Trav said, check the radiator fans.

Original radiator? Might be getting plugged, I'd just replace it if it's original.
 
The radiator may also be plugged up with bugs and dirt externally. Blowing out the fins with compressed air or a hose can help. It can amaze you what washes out.
 
THANK YOU EVERYONE.

Both fans did come on when Engine got hot and the single AC did come out even before that when I would turn on the AC switch... even if the AC was not working.

Did not do that thing where when it starts to overheat and I open the hood and check the fans as I just wanted to bleed the heat off and get it back to normal ASAP.

The radiator is a used one I got last year from a reliable shop I have used for many years. Then again, who knows. So I will take your advice and do the thermostat and have a radiator flush and cleaning while I'm at it.
 
If you can , use a Toyota OEM thermostat. Aftermarket thermostats can be hit or miss. In my experience its part that should be OE whenever possible.
 
test the thermostat, backflush the radiator, rebuild and pressure test the system.

how do you know the sensor and the gauge in the dash are correct? you might be chasing a coolant system that is perfect, all because a cheapo coolant temp sensor is starting to fail.
 
Originally Posted by MichaelRS
THANK YOU EVERYONE.

Both fans did come on when Engine got hot and the single AC did come out even before that when I would turn on the AC switch... even if the AC was not working.

Did not do that thing where when it starts to overheat and I open the hood and check the fans as I just wanted to bleed the heat off and get it back to normal ASAP.

The radiator is a used one I got last year from a reliable shop I have used for many years. Then again, who knows. So I will take your advice and do the thermostat and have a radiator flush and cleaning while I'm at it.
Why did you install a used radiator? New ones are dirt cheap, under $100. You can't tell if a used one is any good by just looking at it. My guess is that the radiator core is partially clogged.
 
You said you have replaced the coolant pump ? Wonder is it has a problem ? Is it driven by the serpentine belt or timing belt ? Check the belt .

If it overheats at highway speed , I doubt it is a fan issue .

Any evidence of a head gasket leak ?

Once you find the problem , flush the cooling system and fill , as specified in the owner's manual with coolant .
 
Originally Posted by PowerSurge
Typically if you're having overheating problems while at highway speeds it is usually a thermostat problem or clogged or partially clogged radiator. I just replaced the radiator in a friend's 2008 Dodge Charger that was overheating while driving on the interstate and his radiator was probably 90% clogged. Problem solved for his car.


Clogged on fins with bugs/leaves or inside?

Could not clean it?
 
Originally Posted by Olas
test the thermostat, backflush the radiator, rebuild and pressure test the system.

how do you know the sensor and the gauge in the dash are correct? you might be chasing a coolant system that is perfect, all because a cheapo coolant temp sensor is starting to fail.


I'm guessing the gauge is correct because the temp goes down when I bleed off heat through the heating system and I get a generous blast of heat when I open the hood and my coolant reservoir is way above where it usually is for normal operating temperature.
 
Originally Posted by Thax
If you can , use a Toyota OEM thermostat. Aftermarket thermostats can be hit or miss. In my experience its part that should be OE whenever possible.


Yes, I thought of that. And I agree. When I first got this car in 2012 and my guy was doing a full tune-up and service on it I had him flush the radiator and replace the thermostat. Wellb nbeknownst to me he used a off-the-shelf Stant from O'Reilly's Auto Parts. I didn't think to specify anything better beforehand and by the time I found out that's what he had used it was all back together and too late.

Although this time I'm not exactly going Toyota. I'm hoping premium Gates offering from RockAuto is up to snuff.
 
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