Ok so I have a 98 jeep cherokee xj straight 6. I warm up my jeep and I have heat but when I drive I lose heat. So far I have replaced thermostat with 195 heater core coolant system flush. The 2 hoses to the heater core get hot. the flap inside the heater box is not broke I cut the 2 a/c pipes. Any ideas why I have the in and out of heat would be very helpful thank you.

 

Do you still have the HCV (heater control valve)? If so, you might want to check for a vacuum leak. You lose vacuum when the engine revs higher, that may be why you lose heat when driving.

 

I dont have the valve on the heater core hoses and I have been told the 98 doesnt have that valve but either way theres no valve on the heater core hoses

 

I think 96 was the last year for HCVs.

Bumblebee, look at your vacuum system for leaks. Some of the doors in your heater system are controlled by vacuum.

 

I will check that out as soon as I get home from work today I didnt think that the other flaps would mess up my heat since the heat flap is controlled by a cable.

 

Clarify then. Is it going from hot to cold, or is the heat going out the defroster vents?

 

X2
On this because it sounds like a vac leak.

 

Its going hot to cold then doesnt really warm back up and the heat never gets hot just warm.