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

[GIDDLEPIN]

Have you checked the timing?

Also you say that you have power washed the radiator, this isn't always a good idea as the high pressure can fold the delicate fins over and reduce the air flow, might be worth checking that you can see through the radiator matrix.

Les