'86 Van, 5 speed, front and rear AC. ~230k miles.


I've had this van for a few months. Drove it only around town for a few weeks at first, all was well. Took it out on the freeway for the first time and made it about 75 trouble-free miles before it overheated. Replaced the thermostat and flushed the cooling system. Removed the radiator and ran water through it backward and forward. Fan clutch is new and I can definitely hear that it's engaged when the van is warm. Test drove it on the freeway for about forty hard miles. It stayed a bit above the half-way mark. That seemed a bit too warm for me so I bought a 170 degree stat (this one also had a jiggle valve) and installed it. Didn't drive the van until today. New radiator cap, too.


Today it was about 85 degrees out. I took the van for a trip on the freeway today. The temp gauge went to about 2/5ths scale and stayed there. Drove about 15 miles including over one large hill at about 70 MPH. Got to my location, shut off the van for about 45 minutes, got back in and drove back the way I came. Again, traveling about 70 mph, over the hill, down the other side, temp rock steady at 2/5ths. But, about five miles past the hill, I see that the temp has climbed to over one-half and is rapidly gaining.


By the time I got back (about 10 minutes later), the temp gauge was dancing in and out of the red (never pegged out, at least). I let the van idle for a bit and the temp gauge sort of settled in at the very extreme end of the normal scale.


I let the van sit for about twenty minutes and restarted it; the . After idling for a bit, the temp gauge started climbing again, well over half. I popped the engine cover and happened to put my hand on top of the radiator- cold! The lower hose and upper hose are both hot.


So what's up here? Radiator clogged? Head gasket sealing "good enough" until it gets put under enough of a load? Can a water pump fail "intermittently"? Is it possible that my AutoZone fan clutch is failing but still sounds and feels like it's working (that is, feels like it's moving enough air)?


Some guidance here would be great. Cooling system is not leaking from anywhere, van has great power, just can't ever tell when it's going to overheat on freeway trips.

Thanks,

Josh