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Heat Builds Up

My 2001 Mitsu Galant runs fine except when doing highway speeds for more than an hour or two. Then heat builds up somewhere, raising the temp gauge way up near the red zone (I haven’t let it go higher) Also, the center dash area becomes very hot to the touch and if there’s a CD in the player it pops out like it’s been in a pizza oven. The radiator does not appear to be affected (coolant normal, thermostat operating normally) and the two little plastic fans are working, doing their best but apparently not enough.

The only way to deal with this is to turn on the cabin heat full blast and let the fan run top speed. In a couple of minutes, the temp gauge returns to its normal spot. (Not too pleasant in summertime, however.)

My surmise is that something in the air system is blocked somewhere but I have no idea where to look. Any ideas would be appreciated.

By the way in years of listening to Car Talk I have never heard any discussion of Mitsu issues. I’d love to be the first - thanks!

My guess is the radiator is partially plugged. The fans don’t do anything at highway speed. It might also be the thermostat’s not fully opening.

You need to have the car checked by a good mechanic. The water pump needs to be doing it’s job and the radiator needs to flow. The thermostat could be sticking also. Make sure the front of the radiator and AC condenser are free of dirt, leaves and insects. There should be no heat coming in to the dash area if the heat is not on. You could have a blend door problem or a problem with the heat/AC control.

I’ve flushed the radiator twice but wondered about the water pump. I’ve been told it is binary (either works or it doesn’t work) but could the pump be operating less than 100%? There are no signs of leakage. The blend door is mechanical and over time the wire arm that controls it has bent, but I straightened it out and started turning the dial very gingerly.
How would I determine if there’s a problem with the “heat/AC control”?

Flushing won’t always unclog a radiator, sometimes replacement is required (with the modern plastic/aluminum ones, that is). A mechanic should be able to use an IR thermometer to see if uniform flow through the radiator is occurring. Also, some waterpumps have plastic impellers that can partly fail.

Here is a quick check: When the heat is off or the AC is on check the 2 heater hoses running into the firewall under the hood. If both are hot there is a problem. The inlet hose should be hot but the outlet hose from the heater core should be cool to the touch.

Thank you. I tried the test and sure enough, both hoses are hot. Does this indicate that the radiator is blocked or something else?

It indicates that the heater/AC control is not controlling the heater or AC like it should. It needs replaced.

You might consider replacing the thermostat too. It may be sticking partially open and restricting flow.

BTW, this forum is answered by people that have no affiliation with the show. There are plenty of Mitsubishi questions here though…

I am having trouble understanding why how heater hoses temp plays in. I would think if they are both warm there is flow through the heater core, so how does that represent a problem? The fact that he can reduce the temp by turning on the heat indicates a lack of capacity by the system. Have you been checking the coolant level, and also the level in the radiator? It could be low coolant level or a bad radiator cap. Does the system hold pressure?

I have watched the coolant level and it does not seem to have dropped at all since the last flush. I did replace the thermostat early on but not the radiator cap but the radiator always is full whenever I remove it. I’m assuming that it is holding pressure. Also as noted earlier, the problem only appears after more than two hours of sustained driving at highway speed. I guess my next step is to take texases’s suggestion and ask a radiator shop to look at it with an IR thermometer. I really appreciate all the input on this matter.

None of the answers given explain the hot dash and the reason it takes 2 hours to occur. I think his car is running a little rich at highway speed and is overheating the catalytic converter and the added heat in the engine compartment is overwhelming the cooling system.It could all be caused by an egr valve not opening or the egr passages could be plugged.

This sounds like a very promising possibility to me but I’m surprised to think that the EGR valve could fail (it would have to be permanently closed, I’d think) without triggering a trouble code (my 99 Honda gets a code for “Insufficient EGR Lift” every once in a while. I’ll inspect it as soon as I get a chance and report back. Thanks for all suggestions and any other tips you can offer (such as where to look for a plugged passage.) I am very grateful to have discovered this group.