Hey all. I've been browsing the forums for a long time now, and browsing some more, and some more, and I've found plenty of issues+solutions to various overheating problems, but nothing that seems to be able to help me out. I have a 2002 3.5L KJ Verada 4spd auto, and if the car is left to idle after about 30mins of driving it overheats. Temp gauge reads at roughly 3/4's full before it drops back down to normal operating temperature.
I changed the thermostat and coolant, but had no success with the problem. Watching the engine while it was idling at home, when it does begin to overheat, the thermo fan will kick into high speed, skipping low speed altogether. I disconnected the fan completely and energized the motor manually and I can run it at both low and high speeds, so the fan itself should be fine. I've checked all fuses and relays, they're good too. So by the looks of things something further up the chain is not calling for the low speed fan to kick in, in the first place. I did suspect the two plugs near the thermostat housing, the temp gauge and coolant flow sensor, I unplug the flow sensor with the car running, the engine warning light comes on and both fans kick in, which should be correct? When I unplug the temp gauge sensor I simply get no temperature reading, as expected. That tells me those two items are fine too? I'm at my wits end here, and cant for the life of me work this out.
My only other idea is that the car is due for a timing belt and water pump change, but I wouldn't of thought a slightly overdue water pump would cause overheating at idle?

Any other ideas or advice would be MASSIVELY appreciated, Cheers!

Not sure if this helps, but according to the workshop manual I have (free downloadable), the cooling fan has LO - MED - HI speeds and you can test if they work individually by shorting out terminals on the fan connector. There also appear to be 3 relays operating the fan - one for each speed. This is illustrated in section 14 (cooling). I'd guess that if a multispeed fan was going to fail, it would be initially on the speed that is used most, which will be the LO speed, so maybe it is the fan that is your problem.
Good Luck.

you guys really have two speed fans? they don't just have on or off?
either way the fan should cut in long before the car actually overheats(even to high mode before getting that hot) what actually controls the high/low speeds? is it a relay of some sort? you may have a faulty temp sensor is your other thing, I know in the GTO it has one sensor that sends info to the ECU as to how hot the car is and a second one that triggers the fans...

water pump generally either works or is ceased....when it goes faulty it either starts leaking or ceases and shreds your timing belt (wonderful) I'd be swapping the thermostat, replacing your radiator cap (it may not be holding pressure) and giving the whole coolant system a good flush then see how it goes.

Hey dude.
I've got exactly the same issue with my car. Have you found any resolution yet?

you guys really have two speed fans? they don't just have on or off?
either way the fan should cut in long before the car actually overheats(even to high mode before getting that hot) what actually controls the high/low speeds? is it a relay of some sort? you may have a faulty temp sensor is your other thing, I know in the GTO it has one sensor that sends info to the ECU as to how hot the car is and a second one that triggers the fans...

water pump generally either works or is ceased....when it goes faulty it either starts leaking or ceases and shreds your timing belt (wonderful) I'd be swapping the thermostat, replacing your radiator cap (it may not be holding pressure) and giving the whole coolant system a good flush then see how it goes.

I tend to agree on all points here. The only thing I can add is that you should also check that the engine is in fact as hot as it thinks it is. If the car really is hot, then there could be an engine issue that needs checking rather than a cooling issue. Is your servicing up to date? Do you change your oil often. Change it again. This could be one of a hundred things so sort out the easy ones first.

Car is definitely hot. Ive had to replace my front rocker cover because it had heat warped. Ive had thermo switch and temp sensor replaced, now Im wondering if its the whole fan or an actual ECU fault. If its a faulty ECU I might cry a little! :)

Although unlikely it could be your thermatic switch.

Easiest thing is to just go to your Mitsubishi dealer and talk to the service department, that's what i usually do and they don't seem to mind giving me free advice :happy:

Just bouble check your thermostat and change your coolant. As said above, your radiator cap may not be holding pressure and that can cause this fault. There are also fuses for the fans so check these.

Alrighty, thanks heaps for all the help and input guys, really appreciated. Found the problem so I thought I best put it up here (Better late than never?) This is a fault in the ECU and it not calling for the fan to come on at the right temperature. I kept getting told to check under the radiator for a second thermo switch no matter how much I swore it didn't have one. I checked and checked convinced this little switch was hiding somewhere, but these model magna's dont actually have thermo fans as such, they are computer controlled (So anyone else being told to check under the radiator, dont waste your time :P) So eventually I just had to bite the bullet and take it into an auto elec. Luckily for me I had a mate look at it for free, but it wasn't fixed :'( That will happen when I have the cash to spare :s for now driving with the air con on to make use of the second fan is doing a fine job, even if it is a little thirstier. Hope this helps you Smith75, I will let you know how much it actually costs to fix the problem in a couple of weeks hopefully. Definitely an ECU fault.

Oh, before you take this as a definitive answer though Smith, have you checked your fan relays & fuses too? You mentioned you replaced the switches, just throwing out another idea! :)

Thanks JimmyRada, I have had this problem for ages and have never been able to find a fix!

Have you replaced the ECU yet? and if so what was the cost??

Cheers mate. :-)

To both of the above posters, ECU replacement can be done reasonably easily depending on what model vehicle you have without the need to worry about replacing locks, BEM and Keys. If yo have a H or J series I can flash the contents of your exisiting ECU into a new one. Most of the time these sorts of failures are actually electronic and not programming issue.

Cost would depend on what model you have, ie Manual TL would be far more expensive than an Exec auto TJ for instance. Reason is that some ECU's are very hard to come by where others are very common

Hi - did you get to replace your ECU and fix this issue? I have a 2003 TL Magna which has the same symtoms - and am just about to start testing the 3 relays, as I thought this might be the issue. Would appreciate knowing if the ECU was the problem and what the cost was. Thanks Stu

Bumping this thread for some more input as i seem to have the same problem in my TJ. Secondary thermo fan does not seem to engage automatically so temp guage will keep rising if I am in heavy stop start traffic. Have to force the secondary fan on by turing on the aircon, temp guage will then drop back to just under halfway almost instantly.

Seems strange that this would be an ECU issue that you have to replace a whole ECU to fix. How can one small part of an ECU's programming suddenly stop working?

It won't be the programming that has stopped working (if any part of that stops working, engine operation would be haphazard at best), but rather some "driver" electronics has been damaged if the ECU is the problem. It would be more likely that the fan relay or wiring is faulty than the ECU damaged. It is possible to check the ECU outputs that control the fan relays with a multimeter to confirm that the ECU is commanding the relays to operate.