Weird HVAC Question, I searched

I have a 98 xj and I think I have read every post about blower motor replacement and heater core flushing. Here is what my Jeep is doing : When I turn the temperature dial all the way to the 'hottest' setting, while the blower motor is on '4', it gets warm, but the amount of air coming out of the floor vents seems to be cut in half. It FEELS like I turned the blower motor down to 2 when I didn't even touch it. If I turn the temp dial back to about halfway the air starts blowing out at full speed again and is obviously cooler. Everything seems to be working like it should, it just doesn't blow as fast as it should when set on high heat. The heat (temperature) seems a little more weak than usual so I'm suspecting I need a heater core flush but would that cause the problem I'm having? Thanks alot in advance guys.

could be the resistor that controls fan speed- but usually on "high" you get nothing with a bad one-

I'm not real familiar with anything above a 91, but on ours- there is always the problem of vacuum opening flaps/diverters to get full blow-

clogged heater core has no bearing on airflow assuming you mean internally clogged now with that being said if the coil is clogged externally with dirt and dust that would cause reduced flow, but if adjusting the dial which i think is vacuum operated makes any differance i would think the problem is going to be located in your blend doors what happens when system is set to defrost or set to come out of the a/c vents? same prob?

Same thing happens on all of the settings, the blower motor is fine, it never changes speed and so are all of the resistors and crap like that because the fan works in all positions. It is only when I set the temp dial to full hot then all of the vents seem to get their amount of airflow cut in half. The fan speed stays the same, their just isn't as much air coming out the vents if that makes sense. When the temp dial is set at about medium or lower the thing blows like crazy. Also if I have it set on floor or whatever it will stay there and wont switch to another position like defrost or something. I checked the temp. dial and cable that closes the door under the passenger side of the dash and it all works fine too. I'm lost.

I think your talking about about the difference between airflow around the coil, and through the coil- when you set full heat, there will be a difference in airflow simply because theres an obstruction- the coil fins the air passes through- although to me, it shouldnt be as noticeable as you state.

Think of it as a giant bypass door in front of your radiator- open, it would go around it- closed it would go through it. There would be a difference.

Reading it as stated, if theres a HUGE difference- I`d suspect something blocking a line in the airbox. A rats nest, a clogged airbox filter, fins with something clogging them like dirt or crap- The key to all this was when you said "the motor speed does not change" but air flow slows- that rules out electrical, and leaves mechanical or obstruction- I`d vote obstruction if its severe, or normal if its not- it simply could be just the air going through the coil is slower.

On my 89, when I have it on fullheat/dash and slowly move it over to halfheat, there is *some* difference- not alot, but its there. There have been cases where a slowly leaking heater core gathers enough crud between the fins to slow airflow through it- and I cant count how many times I cleaned out a nest from the box of my old chevy truck-

Thanks alot, I was kind of thinking something along these lines. I flushed out the heater core and that didn't help at all, not that I was expecting it too, but I still figured I should try it. I also have a 2000 Wrangler and that thing seems to act normal i.e. no changing of airflow when changing the temperature. How would I go about cleaning out the intake box or whatever? I dont even know where to begin with that. Thanks alot guys, I really appreciate it.

At our Dealership, we clean heater cores externally all the time for the road crew/Cal Trans guys. They leave there trucks running all day in the dust and it completely clogs the heater cores and the evap. We use a product called DWD2 which you can do without removing the heater core.

How do you access the heater core to use this product?