treemonkee

My heater quit blowing warm air last week. I flushed the heater core and still blows cold air. I drove it 100 miles today and engine temps never got over 180 degrees. Even if my thermostat is stuck open shouldn’t it blow something other than cold air. Temps today were low 50’s.
It’s a 99 and all stock.

Jeepwalker

Take an IR temp gun and quadruple-check the coolant temps at the aluminum upper coolant neck where the upper radiator hose connects. Double-check against your rig's gauge just in case. Couple things come to mind:

1) Plugged heater core - you could try back-flushing with a water hose in reverse direction. Sometimes (often) a bunch of crud comes out! And the heat can be restored. Make sure your rig has proper flow. Are the heater hoses both hot?
2) Is there air in the system? Point the rig nose up high on a steep incline ...or jackstands. Run and let air 'burp' out. Do you hear 'Gurgling' in your rig's heater core in the mornings? That could point to a leaking head gasket (allowing air in the heater core).
3) Is the outside air damper open? Or broken from it's hinges? It could be your rig is putting out heat, but there's too much cold air coming in and overcoming the heater core output potential. Or mice chewed the damper door gaskets.

4) OR, ...what happens is after many years the foam on the heater box damper doors inside the heater box degrades, gets brittle from age and just falls apart. That's real common on these Jeeps. Mine did that too eventually. When that happens, the fix is to remove the heater box and adhere new foam to the damper doors so it seals off outside air properly. I went through the same thing (and so have scores of other XJ owners). It's a lot of work ...lot of videos to show how to repair the heater boxes. Once you fix it, it should be good for another 30 years.

97grand4.0

Reverse flush.
I just had the heat quit on my ZJ last weekend. Cold as ice. Took the heater hoses loose on the front of the engine and rigged some longer hoses into them.
Put the top one in a bucket, took the bottom one and puckered up the old internal bellows, blew as hard as I could. About 5 seconds later BLEWSH! A gob of gobby greasy plug of crud proceeded henceforth to the bucket and I knew I was back in biz. A few more similar pulmonary purges removed the remaining residual refuse. Only a hint of antifreeze on my lips.
Remember, its the top one that feeds it and the bottom one is the return line so you want to apply your air or water pressure to the bottom one.

lawsoncl

The FSM shows filling from the top which make sense to avoid trapping air, but all the ones I've seen are backwards with the thermostat outlet going to the top.. The hoses are two different sizes though, making it a little harder to get them swapped.

Go easy on the pressure, don't want to cause any leaks. Back and forth with a hose until it runs clear in both directions has always worked well for me.

Roler

After a decent drive: yes
Ater a drive, put your hands on the heater hoses. Both hot? One? None?

Jim Malcolm

The proper way to fill any enclosed space is from the bottom, allowing the bouyancy of the gas to help it escape. When you fill from the top, you're fighting nature. Early model XJs with the Heater Control Valve filled from the bottom. It was only when they eliminated the valve that they went to filling from the top. One has to assume that bean counters got involved and the Engineers relented, but it's not "best practice" to fill from the top...

97grand4.0

We may never know...

lawsoncl

More than likely they just decided they didn't want the hoses crossing and any air would eventually purge itself anyway. Same confusion about the trans cooler in the radiator being shown backwards too.