I have a 98 with 4.0 that simply will not blow hot air from the vents.

I have flushed the core and found that water flows freely through the core so plugging should not be an issue. My jeep is an SE model and has a mechanical wire controlled blend door, so vacuum on the door should not be an issue. I have made sure that the blend door is free and operating normally.

I have checked everywhere for vac leaks and cannot find any. The system blows out of the vents at all RPM ranges and does not blow from the defroster vents unless defrost is on.

Sometimes it will blow decently warm for a time, then cold again. sometimes the air will get warmer as the RPMs go up while driving.

My jeep will heat up to only 203 and takes a while to get there. The thermostat was replaced with the higher end staunt unit and same with the cap. I have not had issues with overheating at all.

In addition the pump is not leaking and I have not observed any leaking coolant at all.

Is there a possibility that the pump has deteriorated to the point that coolant is not flowing through the system enough? Thats the last thing I can think of at this point. I am thinking it would be a good idea to change the pump anyway as the history of the pump is unknown. My coolant is nice and green and doesnt have any rust in it that I can see.

I need to get this licked as its starting to get a bit nippy out!

 

FSM says water pump could cause weak/poor heater performance.

 

Could be the pump, but before I start ripping it out as the vehicle doesn't overheat, I'd look at the heater control valve. That is the culprit in many cases with the symptoms you state. Also, just because the thermostat is new doesn't mean it's not defective. Also, I hope you put in a 190 degree stat. Putting in a lower temp one can cause the heater not to function correctly as the coolant isn't allowed to get hot enough.

 

ditto on the heater control valve, make sure it is opening, I bypassed mine because it's a trail truck, and it was leaking.

 

'97 up do not have heater control valve.

 

Is the return line to the waterneck of the pump even warm?
Check that next time you park.

If you can't tell just from feeling it, pull the return coolant hose off the waterneck of the pump.
Aim it into a bucket, coolant will come out if your pump is working.
Start the Jeep once everything is clear of the fan.
Put your thumb on the waterneck, and see if you got suction.
Dont get your arm cut off by the fan...

 

Thanks guys, I'll check to see it the pump and housing is warm.
The hoses all get hot that's another thing that is driving me nuts. Seems like I should have heat.

 

if the hoses are all getting hot and your not over heating ,heater cores been flushed it hs to be with the controls see if this link helps.....http://www.jeep4x4center.com/jeep-se....swf#toolbar=0

 

I learned this by accident. If you disconnect the plug behind the temperature control ****, the Jeep thinks the heat is on the hottest setting. If you dissconnect it, and then you get hot air, then it may be the temp control **** that is bad. This is a quick thing to check

 

No way to get to the blend door without dropping the dash?

Yikes.

PITA.

Im convinced thats not my issue but dang if disconnecting the switch for the temp control **** doesnt work as mentioned above, I guess I have no choice

Am I wrong in thinking my SE model had a cable/rod driven blend door and not a motor? I know there is a rod with a clevis on the heater box, and when I turn the heat control **** it moves left and right. I assumed this was the blend door ?

 

Here's one. Check your coolant level. Make sure your system has all the coolant it can handle.
I've seen systems that do NOT over heat but were low enough on coolant to not keep the heater core supplied.
Your's sounds like this because the RPM.
Last I'd check the water pump.
The blades of the pump may have deterioted enought to not supply the pressure needed to supply the heater core but will still be enough to flow through the rad.
It's much cooler now so we won't be seeing the overheat sysmptom we would in the summer with a weak pump.