Heating Issues

[alberta_chiclet]

88 Cherokee........OK well this one is for the books, as with many Jeeps I have a heating issue. Thermostat has been replaced, coolant levels are up, winter front on and still no heat. Sealed rad.

BUT I do get heat when I put it on MAX A/C setting..........switch it to HEAT or VENT and it cools right off again. I'm thinking a vaccum line is hooked up wrong somewhere?!? Anyone have any clues or ideas where to start looking? Or what would be causing this?

I love my Jeep but it's driving me insane!

[oljeeptek]

Originally Posted by alberta_chiclet

88 Cherokee........OK well this one is for the books, as with many Jeeps I have a heating issue. Thermostat has been replaced, coolant levels are up, winter front on and still no heat. Sealed rad.

BUT I do get heat when I put it on MAX A/C setting..........switch it to HEAT or VENT and it cools right off again. I'm thinking a vaccum line is hooked up wrong somewhere?!? Anyone have any clues or ideas where to start looking? Or what would be causing this?

I love my Jeep but it's driving me insane!

You may have the heater valve vacuum working backwards. If you pull the vac hose and plug it it should default to wide open for heat flow.

You more likely have a limited flow of water going throught the heater core. In MAX ac mode, the door to the interior opens and youre heating previously heated interior cabin air instead of much colder outside air.

If you have a few minutes and a little ambition, try this:
Start with a fully warmed truck. Set the heater on hot, full fan speed, normal heat mode. Open the hood and feel the heater hoses coming from the firewall, and on both sides of the heater valve.
They should all be hot, and relatively equal in temp.

Drive it a couple blocks, pull over to a safe spot and leave it run. Repeat feeling the hoses. The feed to the heater core (the smaller one) should still be HOT unless your thermostat is bad, the outfeed hose should be a little cooler, but still fairly hot. If its not hot (if it's near the temperature of the heater air output inside) you have a flow problem in the core or the valve.

If the hoses are ok, look for air leaks, air door or control problems.