Engine overheating

[boeinggirl]

Hi there,
My son and I own 4 Isuzu bighorns between 92 and 96 in age.
They all run great except one of them...and we can't figure out what's wrong with it.
It ran fine and didn't overheat until a couple of months ago.
It starts first time every time and runs very smoothly...but it starts overheating after about 15 mins of driving at the open road speed limit of 100kmph.
it doesn't heat up when idling, stopped at traffic lights or roadworks or going through our local mountain pass at 80kmph.
The temperature gauge sits bang in the middle between hot and cold until we sit at 100 on the open road.
it doesn't misfire or run roughly...we've run out of ideas on how to fix it.
We have put in a new cam belt, water pump, radiator and changed the viscous fan and checked all the hoses to no avail.
Not sure where to go to next. Surely if it was a head gasket blown we'd see oil in the water etc..but we don't and it doesn't leak any oil or water.

Can anyone please help us?
Cheers
Alexa and Jacob

[geeves]

If its auto the trans can create a lot of heat if faulty. Does it have a single or dual core radiator. Previous owner of mine put a single core in and it would run hot. It was only after I put a real radiator in that I realized my thermostat was bad. thermostat jammed part shut. Fast warm up device in exhaust jammed shut. Fins of the radiator blocked with mud. Head gasket although if not losing water this is low on the list

[boeinggirl]

oh yea, it's got a new thermostat in it too.
Same type of radiator as before it started over heating
It never used to overheat, just suddenly started doing it one day
It is auto but doesn't seem to have any problems with the transmission at all..changed when it should etc etc.
I will ask my son for more info.

[geeves]

Add timing to the list

[boeinggirl]

oh..I was wrong.It is using/losing water and when you take the radiator cap off the water bubbles and can spurt out even though it's not boiling

[geeves]

Bad luck. It might be just a head gasket but more likely a cracked head. The 4jg2 will crack heads just from normal running not necessarily from overheating.
If its otherwise a good car the correct fix is a replacement head but thats a $2k repair. Dont mess around with second hand heads
Other option is to get a product from the auto parts store called liquid copper. They probably have several versions of it and I cant advise which is best. Follow the instructions to the letter. Most versions require all traces of antifreeze to be removed first but some versions do not. It has a 75% - 90%chance of a repair that will last a couple of years before redoing but if it fails its only $30 or so. If the cars otherwise tired this is what I would try.