A few weeks ago I flushed out my heater core to my '98 Grand Cherokee (a lot of crap came out, and I thought that was it). This seemed to help the problem with my heat not blowing through the defrost, or vents. But It didn't fix the problem. I tried it the other day, still cold! When I don't use the blower, the heat seems to build up enough to blow hot for a few seconds when I turn on the blower fan. But goes right back to cold in no time. I checked the two heater core hoses to see if they were hot. They are, so flow is not the problem. I seem to remember something about the heater doors not working correctly or a vacuum not doing it's job. Anyone able to help me out here?
Thanks'
M

kind of confusing reading that.... but I'm going to assume the modes work correctly. Temp on full hot, blower on high there should be a noticable difference in heater hose temps.... think of it as a mini radiator with one hose being the inlet, and the other the outlet. If both hoses are the same hot temp, I'd suspect the heater core is restricted. Another thing you can check is the blend door motor. This motor is located under the heater-A/C housing directly over the floor panel transmission tunnel, and can be removed from the passenger compartment without instrument panel or heater-A/C housing removal. You can leave it plugged in while it's removed and turn the temp knob and see if it moves. It will also allow you to check the door sweep manually.

lose of vacuum usually puts the air flow through the defrost only, when the blend door has a problem, Most of the time it causes the opposite, all heat, no cool. I bet the heater core is still plugged,feel both the heater hoses. If 1 is cooler then the other,it's restricted. Try reversing the hoses on the heater core,and drive it a few days. the heater core doesn't know directions.

I'm a little confused here. Two posts. One telling me that if the heater hoses are both hot, the core is restricted. The other is telling me that if one is hot, and the other cooler, the core is restricted. I don't think it can be both ways.
The hoses are both hot after running for a while (long enough to get the thermostat (195) to open). As I said above, I can get a little heat to come out the defrost vent after the Jeep is warmed up. But as soon as I turn on the fan, it cools down rapidly and stays cold. I think I'm leaning towards the blend door being the problem. Just wondering if anyone has gone through this recently? I'm I on the right track? I watched a few vids on You Tube, put out be "Heater Treater". Looks like I'm in for a little heater box surgery!

I had a similar problem on my 2000 Cherokee. I believe my problem was air trapped in the heater core. I drained and refilled the system, but it wouldn't blow hot.

The small underhood space for the radiator and hoses leads to air being trapped in the system. Factory probably should have installed an air bleed near the thermostat.

The air doors seem to be doing their job still. All the positions on the dial put the flow of air where it's supposed to go. WOW! I'm stumped.

so is one hose hotter than the other on your radiator? It sure is.... that scenario is the same for the heater core. Hot coolant flows in, heat is transfered from the core, and the return flow is cooler..... does this not make sense???? there should be a notable difference in the temp of the hoses. IF they both feel the same you have a flow issue.... At this point I'd try flushing/reverse flushing the core several times. I personally think you are going to put a heater core in. Your fan theory backs that up. Fan on low you get "some" heat. Fan on high you get none. Plugged core has no/very little heat to transfer.

After much diagnoses, time, and frustration. Finally found the problem. Heater core plugged up over half way. R&R the heater core (not for the faint of hart!). Hotter than hells gates now.