94 Previa Overheating Only @ Higher Speeds

[PREVIA DRIVER]

HI,

I HAVE A 94 PREVIA SUPERCHARGED LE W/ 185,000 MI ON IT... IT RUNS GREAT, BUT AT HIGHER SPEEDS, AND ONLY @ HIGHER SPEEDS IT STARTS TO GET HOT... THE HEATER TRICK BRINGS IT BACK DOWN, BUT ONCE IT GETS HOT THE HEATER WILL HAVE TO STAY ON INTILL YOU GO BACK TO LOWER SPEEDS FOR A SMALL DURATION. I HAD FLUSHED THE SYSTEM AND REPLACED THE THERMOSTAT ABOUT 100 MI AGO, AND IT DIDN'T HELP. I HAD CHECKED THE WATER PUMP, NO PROBLEM THERE; I HAD CHECKED THE FAN CLUTCH, NO PROBLEM THERE; CHECKED THE RADIATOR, SEEMS TO FLOW FINE; PRESSURE WASHED AND BLEW OUT RADIATOR FINS AND STILL NO SUCCESS... IT WENT IN FOR SMOG ABOUT 2 MONTHS AGO AND FAILED DUE TO A BAD CAT. REPLACED THE CAT AND PASSED WITH FLYING COLORS. PLEASE HELP, I AM VERY MECHANICALLY INCLINED, BUT THIS IS SOMETHING THAT I JUST DON'T GET... I HAD CHECKED ALL OF THE MOST COMMON PROBLEMS WITH A COOLING SYSTEM, AND I JUST CAN'T GET THIS THING FROM GETTING HOT AT HIGHER SPEEDS. I HAD LET IT IDLE FOR ABOUT 45 MINS TO SEE IF IT WOULD GET HOT AND IT DIDN'T. DRIVE IT SURFACE STREETS ALL DAY AND STILL NORMAL; GET ON THE FREEWAY, IT GETS HOT. IT IS A COOL 70 DEGREES YEAR ROUND HERE, AND HAVE NEVER HAD A PROBLEM; INTILL RECENTLY. DOES ANYONE HAVE ANY IDEAS???

THANKS

[propnut]

Ok by virtue of the fact that you have a supercharger it is safe to assume you are in America as we never received that option here in Europe. Nevertheless the engines are the same.

Two things here, flushing the rad is not sufficient you need to either strip it or change it. They suffer extreme sedimentation in the upper section that flushing does not tackle. A reduced flow at speed especially in light of the fact that you are running boost will give rise to your symptoms.

The other and I fear more likely problem is a head gasket which is on its way out. Go down to your local garage and get your water jacket tested for the presence of combustion gases, otherwise known as a block test. Takes a few minutes and will tell you for sure.

Even if the gasket is still ok the water ways in the block and the gasket start to block and at your mileage I can guarantee this has happened. As a result cooling efficiency is impaired.

Either way it means taking the head off.