I searched and found some good info that I will apply to my issue I am having. I just wanted to bring this to anyones attention to see if there are anything else that could be the issue.

92 XJ 4.0L

When I start it up it warms up to proper operating temp. When I turn the heater on it only blows air out of the top dash vents closest to the windshield. And it barely even blows air out of them even on high. I switch to the lower dash vents and get nothing and the floor vents nothing as well.

So I did some searching and found the common issue is a clogged heater core. My question is: Could the heater core get so clogged it would block practically all air from entering the cabin? From what I read, and I could be taking this wrong, but the clogged heater core is only attributed to those who have properly working air vents, just no actual heat.

Any help is greatly appreciated.

Its the vacuum exchanger for the heater. The heater uses vacuum to switch vent locations. Usually its quits due to a vacuum leak somewhere from the vacuum canister to the vent door changer. Behind the front bumper on the passenger side theres a footballish shaped canister, its a vacuum canister. If no vacuum or low vacuum is present the heater will default to defrost and more than likely your cruis control wont work. Check for vacuum leaks on the lines to and from the canister behind the bumper. If the core was plugged it would blow cold or cooler air but still change vent locations.

Alright. I will start with the canister and see if that does the trick.

Thanks

Make sure no one has crushed the tube to the vacuum canister with the battery.

Happened on my latest XJ.

Yeah i broke my brittle lines to the canister when i put in a bigger battery. Then relocated it to the firewall.

Now that you two mention it. I will check that too cause the previous owner put a Optima battery in it and those suckers are pretty damn heavy.. That could very possibly be the issue. Thanks for the pointer

Just a little update. Today I topped off the coolant and the amount of air coming from the top dash vent got a little better but still not enough to even think about keeping the cab remotely warm. So I left the jeep warm up to running temp and checked the hoses..

The two hoses that run right next to the valve cover, the top one was very hot to the touch and the bottom one was slightly less hotter to the touch.

I followed the hoses back to the valve looking thing, off of that valve ran two hoses into the firewall (heater core box)... Both of those hoses were a night and day different cooler than the two next to the valve cover. On a 1 too 10 scale......

Valvecover Top Hose - 9
Valvecover Bottom Hose - 7

Heater Core Top & Bottom Hose - 2 maybe a 3 at best

Here are the two hoses illustrated in red: