This just started happening yesterday, 1997 Rodeo. I searched the forums and didn't find anything.

At idle, my heater blows cold air, and while driving I get hot air. I fiddled with the temp switch and it seems okay - does this directly actuate the heater core valve or is it vacuum controlled? I used 4WD for the first time in a while last night, so I thought maybe I cracked the diaphragm on the 4WD actuator or something, but it doesn't seem like the valve is vacuum actuated.

This temp change occurs very rapidly, so it doesn't seem like a cooling flow or thermostat issue, but anything is possible I suppose. This thing has 151,000 miles on it, I haven't touched the cooling since I got it at 123,000 miles.

surprise, surprise. unless yours is different from my 1992 trooper, you don't have a heater valve. the coolant flows through the heater core all the time. you get the heat by turning the dial which actuates the flap in the heater box and the heat comes out of the vents.

(i think that's weird. it means you heat the underdash area all the time. do other vehicles use this system, i.e. without a valve?)

i thought i had a heater valve too. while trying to diagnose a coolant leak/odor i found out different.

[in fact, i use to make sure i turned the heat to high when i changed the cooland to make sure it drained, too. and i thought for some time a whisp of coolant when i 'turned on' the heater was just the normal kind of drip you often get from heater valves]

so if you have heat at speed, and not at idle, i'd say you *do* have a coolant circulation problem, and i'd guess it's your radiator, but it could be the heater core (a small radiator).

hth

//bc

Have you checked your coolant level?

Not yet. I've spent the day cleaning out the garage and getting propane for my shop heater. Yes, it was too clutterd with stuff to get a car in - 4 month old baby....

I need to replace the windshield fluid pump tonight, so I'll check the level and turn the bleed screw to make sure no air got in. I hope it's not low, since it's not leaking anything. <img src="/forums/images/graemlins/zombie.gif" alt="" />

I had an overheating issue this summer when I let it sit a while running, and I assumed it was the fan clutch. Maybe I do need a new radiator and/or heater core.

I'd rather juggle chainsaws than replace that heater core though.

The "cold at idle, hot at speed" scenario you describe is exactly what I had on my '92 Trooper at about 150K miles. The problem for me was a "tired" water pump. It wasn't leaking and had no bearning noises, but it was not circulating as well as it should have been ... it took >1300 RPM's to circulate "good heat" through the cab of the truck. With the mileage you have on the vehicle, you could be looking at a tired water pump. HTH ... Fred.

Thanks. I have been getting the dreaded timing belt tensior tapping, and plan on changing the belt/tensioner/idler pulley/water pump weekend after next anyway, so maybe I'll push the schedule on that up and do it this weekend. The original owner gave me the receipt for a timing belt change at 70k, but I have seen some nasty worn out pumps in other cars by that time.

Maybe I'll drop a can of cooling system cleaner in and let it circulate this week, and do all the other deferred maintenance this weekend.

Mine is doing the same thing. I had a gm 3.4L that did this as well , it was the head gasket...I am afraid that is my current problem as well, I am keeping an eye on it but the symptoms are not good so far. My overflow is full, it could also be a bad or stuck thermostat. Good luck.

BTW mine is an 88 2.6 with 191K on it.