Plugged rad, bad water pump, or head gasket?

'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

It sounds like you aren't getting flow through the rad, for whatever reason.
Either it is blocked by sediment/scale or you have an air pocket impeding flow.

A blown head gasket wouldn't explain a cold core.

Was the core evenly cold all over? or just in one section of the rad.
It is not uncommon, especially in cars that sit for long periods, to get (partially) plugged in the lower portion.
Flushing them often wont bring them back to life either, and just because you had flow doesn't mean it isn't partially plugged.

But don't rule out the stat either, a/m stats are notorious for causing problems, there is good reason why everyone recommends going OEM.
How did the heater(s) work when it was running hot?
How much coolant did you drain out and how much went back in?

BB

For what it's worth - 2wd w/o rear heater is 8.3 US quarts, w/ rear heater is 8.9 US quarts.

Even if you drained the entire system you likely wouldn't get it all out but you should count on getting most of it out.
I have found it best to measure what I take out, then I make sure I get at least that much goes back in.

Straight coolant should be mixed 50/50 with distilled (to avoid scale build-up) water and too achieve maximum thermal efficiency.
Coolant has lubricating properties which is why you should avoid running straight water for any length of time.

And no, water pumps don't fail intermittently. The only time I have seen a water pump, not be able to pump was when a customer had replaced a unit that suffered catastrophic failure and damaged the internal housing but then it's ability to flow was compromised all the time.
When a water pump fails, it will either leak out the vent hole or make horrible bearing noises.