Guys...... It's more likely something relating to your vacuum system. A leak in the lines, a leaking vac. canister, the plastic/rubber manifold behind the HVAC controls not sealing or broken......
If you're feeling hot air to some degree, then you've got flow from the heater core/engine/radiator.
Yankeefan, (and others) ya gotta remember to include your vehicle stats (at least the year of your rig) so we can help diagnose the problem.
Earlier XJ's have a heater control valve in line on the intake (hot) heater core hose. It's vacuum controlled and they tend to go bad, get crusty, break, etc. It's got a small vacuum actuator that opens and closes a valve using engine vacuum and the heater control in the dash allows vacuum flow to that actuator when you move the selector from cold to hot.
If your heat is working at idle/when stopped, but when you accelerate you lose hot air - that's a classic sign that your vac. system is leaking and not storing vacuum in the canister, or the vacuum actuators (there's 3-4 of 'em) are leaking or opening/closing based on increased/decreased vacuum getting to them.
If you're not having an issue with overheating, then it is usually safe to say that you've got enough coolant in the system to send hot coolant to the heater core. It's not unheard of to see a large enough air pocket in the heating system hoses/core....... but that's usually coupled with an overheating issue unless the heater valve is closed.
Again, depending on the year of your rigs, you'll need to start tracing the vac. system components. If the heater valve is in question, note the position of the linkage when the XJ isn't running. The valve should be closed and you should be able to push the linkage/rod into the vacuum actuator. If that's moving free and easy, get a long piece of vacuum hose, attach it to the fitting on the actuator, and provide some suction. See if the actuator holds vacuum or if it leaks down to the closed position. There's a blend door vac. actuator under the pass. side of the dash near the floorboard - usually partially covered by a dome shaped plastic shield. Test that one the same way. See if you may be getting hot air from the heater core that's not flowing through the blend door/heater box properly. Then the third actuator is usually on the driver's side floorboard just above the gas pedal. That should be the vent/floor/defrost blend door actuator. Repeat the same test.
Clear as mud?
