My heat only blows Cool/Lukewarm, both the lines going to and from the heater core are burning hot when I touch after being fully heated up from a drive.

Blend door works, I can hear the clank when I turn it from cold to hot fast. Blows on all 4 speeds.

Don't think thermostat is stuck open because I run at 195-210 depending on time I am driving and city versus highway.

1997 XJ 4.0 - 217k Miles

What could be the issue? Inner of the heater core clogged up, if that was true wouldn't the return line not be hot because both lines are burning hot.

 

I often wonder when this happens if the flow is enough to heat up the heater-core return line but not enough to actually make the heater core function properly.

 

Flush your core out and run heat on high in between drains. Put some hot water and vinegar in your core and let it soak for 2o mintues. Then flush it out with hot water until it clean

 

Funny. I have an almost identical issue and have wondered the same thing before.

 

Here's what I did on my 92..

Disconnect the heater core lines and flush with water. Do NOT crank up the hose and jam it on there. Just a small stream of water or you run the risk of popping the core..

After the water flush, I blew through it to drain the water then dumped a bottle of CLR in the lines. Be careful.. Wear eye protection and gloves. Try NOT to get any on your skin as it may burn.

Let sit in the core for about 3 minutes and flush with the water hose again. Blow out the core and re-rinse again.

Hook lines back up and enjoy your hot heat.

If you have that stupid heater valve, feel free to remove it and replace the lines. All newer XJs didnt have it anyways,

NOT responsible for heater-core leaks. If this procedure causes leaks, than your core was shot to begin with.

 

Flush the heater core right at the fire wall stick the hose directly against the fittings. Do both several times. I have also seen the fins inside the water pump be so warn that it doesn't circulate water good enough causing issues too. Its usually flushing the heater core a ton of times usually is the case.

 

Solid disclaimer. I've been dragging my feet until I am willing to replace the whole core if it busts during a flush, pending it is just barely held together by scale or crap or whatnot. Still good advice though.

 

My core was completely blocked with crap when I got the jeep. I had to put compressed air to the line to even be able to get anything to circulate. I would say chances of damaging with the hose is slim unless it already is deteriorating

 

I second that I have been using that system on my vehicles for over twenty years I have never had a problem.