car overheats only when speeding or air conditioner switched on

[vishalshah Banned Find all posts View Profile Close vishalshah]

Dear friends,
many thanks for taking the time to read this post.
I have a prado 95 running a 1kz-te engine. we live on the coast and temperatures generally reach 34-36 degrees celcius daytime.

its got a peculiar issue and i was hoping to get some guidance on this group.
The car drives fine within the town and I can push it and it never overheats in tailback traffic or even when speeding around town.
On the highway a different story, the temperature gauge starts creeping up and we usually park on the side idling until it cools down. We have made some observations as follows:
1. if the aircondition is on and we are on the highway, after some distance of driving over 80kph and using the turbo the car tends to overheat. when we get somewhere and check, the top radiator return hose is quite stiff.
2. even with the airconditioning switched off, the car still overheats if driving at over 80kph and rev counter sometimes over 2000 revs.


On the return trip if I keep the speeds at below 60kph and the revs at below 1800-2000 revs, aircon switched off and dont overspeed, the car never overheats.

work done just recently less than 3 months ago:
1. cylinder head gasket replaced and head checked for cracks and bend( they checked with steel rulers and did not do a pressure test as they said there is no bend( the worshop engineers who do refacing etc did that..
1.2 radiator core cleaned with dipsticks to remove gunk.
2. full block reverse cleaned with radiator flush using a machine( glorified water pump, pushing water through in the opposite flow direction). the water was quite clean so no salt or dirt deposits in engine block.
3.water system flushed three times and very good quality coolant put in after that.
4. the gearbox oil was flushed and changed too.
the cooling system was pressure checked to not have leaks.

work done about 2 years ago when we had an overheating issue then as well.
1. radiator cleaned and cooling system flushed.
2. viscous fan coupling changed.
3. radiator plastic top removed and an aluminium top was welded on. radiator cap changed and checked

I am stuck and don't know what to do.

1, should I throw that engine out and replace with another refurbished 1kz-te engine( the current engine has done 286,000 kms(not sure if it was clocked before i bought it) and there is no smoking etc?
2. change the radiator? have heard that radiator channels do get clogged and narrow after about 10 years thus impeding coolant flow.
3. change the torque converter?
4. change/service the turbo?last turbo service was about 2 years ago.
5. am advised to lock the viscous hub so it runs at the engine speed( apparently in Kenya, they open the vicous coupling, cut gasket paper and seal it again. the silicon stays on one side and the fan keeps running at engine speed always)
6. could it be a clogged oil pump?
7. could it be a clogged sieve in the oil sump thus not getting enough oil pumped around when engine under load?
8. remove the aftermarket sump guards ( we are in Kenya and roads sometimes have rocks etc, so we put very heavy gauge fabricated sump guards) and have the engine washed thoroughly from the bottom of the car?


please assist if possible.
thanking you in advance for your help.

[RedAdventureWagon Member Find all posts View Profile Close RedAdventureWagon]

If it's an auto and on the original radiator replacing it wouldn't be a bad idea as you don't want it to leak coolant into the ATF. Did you ever have the water pump replaced?

[404pug Avid PP Poster! Find all posts View Profile Close 404pug]

or the thermostat?

[vishalshah Banned Find all posts View Profile Close vishalshah]

thanks 404pug for writing back.
have replaced the water pump and it was also checked again about 3 months ago to see if the fins were intact. they were fine.

the thermostat was removed and thrown away. we sealed the cover well so there is no leakage

radiator am thinking of buying a brand new radiator anyways. any idea if i can fit a bigger one into the car? if so, what vehicle type would you suggest thats a direct fit?

[LeighW Avid PP Poster! Find all posts View Profile Close LeighW]

If fit is overheating then your not getting rid of the heat, the three main things are one the radiator, it gets rid of the heat, you indicate the radiator has been cleaned several times, was any blockages or or rubbish removed? Air the air passages clear?

Two circulation, you must move enough coolant around the system to shed the heat, have you installed a new water pump?

Three the coolant, when you say the coolant has been replaced, was the correct mix installed, glycol etc is not a very good coolant, adding to much actually makes things worse as you not the water content to shift the heat?

The fact that it overheats under heavier load indicates not enough cool coolant flowing through the engine to cool it, restrictive thermostate is usually the cause if a good coolant has always been used as then there won't have been any dirt or corrosion to block things up.

Has an stop leakage been used at anytime, they can seriuosly block a radiator?

Try placing cheap temperature sensors on the top and bottom of the radiator and go for a drive, if top of the radiator is hot and bottom cool when it overheats then probably not enough coolant being pumped around for some reason. If both top and bottom of radiator are hot then the radiator is not getting rid of the heat.

[vishalshah Banned Find all posts View Profile Close vishalshah]

Dear Leigh
please allow me to answer as follows


If fit is overheating then your not getting rid of the heat, the three main things are one the radiator, it gets rid of the heat, you indicate the radiator has been cleaned several times, was any blockages or or rubbish removed? Air the air passages clear?
yes we did remove blockages. i am assuming even though the channels have had the white deposits removed, maybe the channels themselves have become narrower over time.
air passages are clear, we did pressure blow the radiator to remove all deposits between the fins.
also pressure washed that and the aircon condensor.

Two circulation, you must move enough coolant around the system to shed the heat, have you installed a new water pump?
there was a new water pump installed about two years ago. we checked three months ago and the pump fins were intact, no play and also the pump was turning well.

Three the coolant, when you say the coolant has been replaced, was the correct mix installed, glycol etc is not a very good coolant, adding to much actually makes things worse as you not the water content to shift the heat?
am sending a photo of the coolant we used. it does not say dilute with water. we live on the beach and on the coast, so maybe the salty air causes rust and thats why they say use coolant directly no diluting.

The fact that it overheats under heavier load indicates not enough cool coolant flowing through the engine to cool it, restrictive thermostate is usually the cause if a good coolant has always been used as then there won't have been any dirt or corrosion to block things up.
there is no thermostat at all, just a blank space and we used silicon to screw the mounting back into place. there is no leakage. thus there can be no restriction from thermostat.
am not sure if good coolant was always used. maybe there was a time the previous owners used only plain water and no coolant. there may be deposits. this was the reason, we reverse flushed using the radiator flush to clean deposits from water channels in the engine block.
i am thinking of also opening up the oil sump to check the oil pickup and see if the sieve is not blocked with carbon particles. that may be a reason the oil flow is not good and the engine overheats under load. would i be right in thinking that way?


Has an stop leakage been used at anytime, they can seriuosly block a radiator?
yes, a stop leak was used and also a holts, stop leak and crack filler in cylinder head.
the car was drained thereafter and also the radiator cleaned but maybe the stop leak blocked the channels making them narrow.
is there any chemical to use to remove these deposits from the block. i think a new radiator is affordable so forget cleaning the radiator again but how do i clean the block water channels?

Try placing cheap temperature sensors on the top and bottom of the radiator and go for a drive, if top of the radiator is hot and bottom cool when it overheats then probably not enough coolant being pumped around for some reason. If both top and bottom of radiator are hot then the radiator is not getting rid of the heat.
this is a great idea. i shall look for those and revert.

photos of the chemicals used as follows

[watsea Member Find all posts View Profile Close watsea]

Previously with my Series 90, I replaced the original red coolant with green coolant and unknowingly did not realise those coolants do not go well together. There must have been some residual red coolant in the system even after I drained and flushed. Maybe the heater.
I had overheating problems under load (light towing) and higher speeds. I removed the radiator and a radiator works explained incompatibility of the coolants. Radiator works removed the tanks and rodded the tubes to clear them. Refitted and all good for a couple of hundred thousand km. Traded vehicle at 390,000km