Defroster issues

[arveetek]

1995 K1500 Tahoe 6.5L diesel

I have been experiencing issues with my defroster this winter. Turning on the defrost actually causes all the windows and windshield to get fogged up. I dread having to use the defrost setting since I know it will make everything worse before it gets better. After about 10 minutes of driving, it finally clears everything up, but I have to leave it on high the entire time for it to work.

I am noticing a musty smell from the vents as well. There must be excess moisture in the HVAC system.

Here's what I've checked:

A/C Compressor is running
Air is coming out of defrost vents normally
I removed the blower motor to check the evaporator, and it is clean
Evaporator drain is open and flowing normally

What do I need to check next? I suspect an issue with the heater core, but I don't appear to be losing any coolant. Haven't had to add any coolant in months.

Thanks,

Casey

[DmaxMaverick]

Make sure your recirc isn't stuck. Defog mode should force fresh air and disable recirc. If it's stuck, or not cycling properly, it's condensing all the moisture in the cabin onto the glass, until it warms up enough to vaporize it, and condense it again in the evaporator. Other than that, it's likely the heater core. It doesn't take much leakage to fog the glass. A few drops at a time is all it takes, and you may not notice the missing coolant.

[arveetek]

Make sure your recirc isn't stuck. Defog mode should force fresh air and disable recirc. If it's stuck, or not cycling properly, it's condensing all the moisture in the cabin onto the glass, until it warms up enough to vaporize it, and condense it again in the evaporator. Other than that, it's likely the heater core. It doesn't take much leakage to fog the glass. A few drops at a time is all it takes, and you may not notice the missing coolant.

DOH! I didn't think of that! The recirc door has been broken (or rather the actuator) for some time, and I've actually wired it closed into the recirculate position for better summer time A/C performance. I will try that first.

Thank you sir!

Casey

[DmaxMaverick]

Yep. That'll do it.

[trbankii]

Yep, that would do it. Hopefully it isn't the leaking heater core (my first thought) as that's a royal pain to replace.

[arveetek]

I've only driven it 3 or 4 times since moving the recirc door, but crossing my fingers... so far so good.

Casey