99 Classic 4.0 A/C Auto Trans

I'm experiencing low heat early in the day and better heat as the day warms up. The vent selector seems to work okay. I judge the heat output with the selector is both the high vent position and the floor vent position. Heat output is consistent between the two positions.

Very little heat is available early in the morning - temps recently in the 30s. As the day warms up (50s and 60s) the heat is much better - more than just the change in the outside temp.

Any ideas on what might be going on here? I could see the vacuum system degrading overnight and "improving" with higher daytime temps and also a little heat soak from the engine. Does the heater controls have any vacuum connections? What about the blend door? Maybe it works better or worse, depending on the ambient temps?

Is there any routine I can do with the controls that would help diagnose the problem?

Thanks in advance.

 

Cab heat is a direct byproduct of engine (coolant) temperature.

For any cab heat issues, first thing is to get an infrared point and shoot thermometer (inexpensive and widely available) and point it directly at the thermostat housing of a hot engine. Should be in the 190-200 range, give or take.

When was the last time the thermostat was changed?

Feel the hoses coming and going from the heater core on a hot engine. Are they both hot?

 

Well, maybe if all other parts of the heat control system (not to be confused with the cooling system) is working properly.

Thermostat recently changed - infra red test says it's right at 195* (idle speed). Have not done the hose-to-touch test but, will do soon. That said, what absolute concrete conclusions can we draw from this hose-touch test? If outlet hose cold or significant cooler, we'd guess a clogged heater core. But cooling system clean and always maintained and this is a new problem - no conditions for it to clog quickly. If both hoses hot, then we think the heat isn't being extracted - which seems to be the problem.

Just looking for info - is the vacuum system involved in the heater controls? I know it is on 96 and earlier - don't know about 97-2001. And if it can not be connected to the vacuum system, what would be the best way to attack the blend door possibilities.

 

On yours, the blend door, meaning hot and cold, is driven off an electrical motor and no vacuum. If the blend door does not close all the way, you will have little heat. I would pop the blend door motor, clamp the selector shaft, and move it manually to see if it makes a difference

 

Have you done a heater core flush? You'd be surprised how effective it is. I certainly was. Went from basic ambient temp to 170°.


-T

 

As long as when you change heat settings from defrost to floor to both ((blend door action) and you can hear the doors open and close, thats first step. Then heater motor needs to blow strong. From here, I replaced my tstat, housing, and water pump. Pleased to say my heat will roast chestnuts on any day after about 5 minutes of driving. Really nice. Jeep did this right. Like bathing in a hot tub on a winter day.

 

5-Speed -- Thanks for the info on the vacuum.

97Grand -- Not hearing the door moving (opening and closing). But when I move the heat dial all the way to the left (pegged in the blue) it does go to no heat. So the door must be moving some.

TJ & Thernlund -- Rained hard here yesterday most of the day. Will do the hose-touch test later today and try to rule the core in or out of the problem.

Thanks guys. If no further response from me later today, that means that I'm chasing the blend door. That may take a week or so. But I will come back and update.

 

As 5 speed says above, yours is driven by an electric motor, not vacuum like mine is.
You have to verify that is working, ...must.
Then a heater core reverse flush with the garden hose isn't a bad idea. Careful, tho, the heater core can be thin, use only running garden hose pressure..or you may sprout little leaks.
If you aren't coming up to temperature due to a missing or stuck thermostat, well, obviously you won't get hot either.