Ventilation blows hot (when set on cool) only when Jeep is hot

[Landon427]

My '99 SE is doing something odd. When I bought it a year ago, the heater core was leaking, so I didn't run the ventilation (don't know if this was a pre-existing problem). I recently replaced the heater core and AC evaporator (AC isn't running yet). The air blend door looked OK. The ventilation blows hot when the Jeep is hot, when the knob is set on cold (it blows cool air until the Jeep is hot). I replaced the blend door actuator, wondering if the old one was getting flaky when hot, but no change. The blend door swings freely by hand.

Does anyone know of anything else I can check? Is it possible that the blend door edge is 'curling' as it gets hot, or getting flimsy and leaking air past the heater core? I know there's an outfit (Heater Treater) that sells replacement doors, and you cut through the bottom of the ventilation plenum(?) and cut out the old one, I'd just rather not do that if there's something else I can check first. The fan works fine and the position selector (defrost, vents, floor) works fine.

Thanks

 

[Chris]

If the A/C isn't on the only thing you should expect to come out of the vents (unless it's a very cold day) is hot air.

Perhaps that's the issue? The only time my vents have ever been blowing anything close to cold air is winter days.

 

[Landon427]

If the A/C isn't on the only thing you should expect to come out of the vents (unless it's a very cold day) is hot air.

Perhaps that's the issue? The only time my vents have ever been blowing anything close to cold air is winter days.

Thank you - It helps to know that. I thought the blend door would move and bring in ambient air, which is cooler than what is coming out (even when it's hot).

 

[Chris]

Thank you - It helps to know that. I thought the blend door would move and bring in ambient air, which is cooler than what is coming out (even when it's hot).

I've found that isn't the case. Unless it's a seriously cold winter day, if I'm sitting there at idle (on any vehicle, including my newer F150) and turn on the vents with the dial set to cold, at best I get lukewarm warm air (usually it's just hot air though).

The only time it's ever been effective, is again, really, really cold winter days.

 

[Landon427]

I've had a lot of vehicles, and this just seems hotter than I would expect. Thanks for the insight.

 

[Steel City 06]

There can be a decent amount of heat leakage from the heater core, often enough to bump up the vent temperature 5° or more from outdoor.

Some will install a heater core bypass valve to keep the heater core cold. Here is one example: https://www.oldairproducts.com/product/25-1555m-electronic-bypass-heater-valve

 

[Landon427]

There can be a decent amount of heat leakage from the heater core, often enough to bump up the vent temperature 5° or more from outdoor.

Some will install a heater core bypass valve to keep the heater core cold. Here is one example: https://www.oldairproducts.com/product/25-1555m-electronic-bypass-heater-valve

Thanks. This just seems considerably hotter than that, but I haven't measured it.