HVAC Air output reduces after short time?

[cardiac]

2002 Trailblazer LTZ. This just began out of the blue last spring. Heat and AC are always at the appropriate temperature. What I do not get is output. When I first start the TB up, the heat blows (A/C is the same) just fine at any speed setting. After 10-15 minutes, the output slows down so much that it begins to get cold inside, even with the blower on high. Blower speed DOES NOT change when the output drops. You can hear the fan "sound" does not change.
It seems like someone just put a plug in the vents. I have looked in the vents with a small bore camera and can see nothing blocking. The fan motor is 2 years old and functions just fine. I have messed with this for 6 months, and now it is cold here in IN and am turning to the great minds here for assistance. Ideas?

Help!!

Thanks for your input

 

[mrblotto]

For the heat issue, I’d check the radiator fluid. Is it rusty? Maybe some gunk is clogging the heater core (or whatever it’s called). Had an 01 Jeep that happened to.....

For the AC, could be the compressor giving out. Again, our 01 Jeep had a problem where the AC would blow cold, then hardly cold at all after 10 mins or so


Edit: just re-read:
You’re referring to the actual OUTPUT of air, the FORCE of the air blowing thru the vents, not the temp.
Since it took me forever to type this on the phone, I’ll leave it lol




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[cardiac]

Yep, the temps are just fine, and you are correct that it is just the OUTPUT...

 

[BurnItDwn]

This might be a dumb question, but, could a really old cabin air filter get clogged and cause problems like this maybe?

 

[cardiac]

2 years ago when we changed the blower motor, I made sure there wasn't one in there (Or anything else).

 

[mrblotto]

Does the blower motor have some sort of belt or similar that it uses? Maybe it’s slipping?


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[cardiac]

No, it's direct drive. Has the squirrel cage attached right to the motor shaft. And that is tight. I'm beginning to think that as the heater core gets hot, it is expanding too much and blocking air flow. Because when I shut the TB off and let it cool for a while, it blows like crazy again for 10-15 minutes. It simply can't be anything else. I'll sell this thing before I have to change a heater core in it. Dash removal, AC recharge, bla, bla, bla.... Dang it!

 

[pcgeek11]

Depending on how the air flow is in your system a frozen evaporator core could explain the low flow when running the AC or the defroster setting which also runs the compressor. This would normally be caused by the Thermistor that regulates evaporator temperature not working correctly.

 

[cardiac]

Depending on how the air flow is in your system a frozen evaporator core could explain the low flow when running the AC or the defroster setting which also runs the compressor. This would normally be caused by the Thermistor that regulates evaporator temperature not working correctly.

Would that effect the air flow power when it is on the floor vent with the AC switch off and on heat?

 

[pcgeek11]

I would say no if the defrost isn't being used.

I cannot think of anything that would plug up the air flow over time except the evaporator freezing up... So it doesn't apply unless the compressor is running...

You have a strange one.

 

[cardiac]

I cleaned the heater core with a soft toothbrush and then took out a vent and blew air in reverse down the ducts. Didn't make one dang bit of difference. I guess I need to buy one of those crappy cigarette-plug heaters now or sell the SUV....

 

[mrblotto]

What about disconnecting the vent(s) from the blower motor, and seeing if the force/output coming out of the motor itself changes after 15 mins or so? It may help rule something out


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[cardiac]

Mrblotto, I did do something similar. The 2002-2003 Trailblazer has a port to stick a cabin filter in it. I pulled the cover off and things did not change. It is between the blower and the heater core. I just tried this Wednesday. Great idea!.

Thanks

 

[mindless1]

Despite your claim the blower motor "functions just fine", I wouldn't rule it out yet, seems that what should be consistent both with hot and cold air settings, is that as the motor heats up, the bearings expand and it slops around and loses RPM. Either that or you have automatic climate control and as the circuit board in that console unit heats up, it malfunctions.

If the fan really is spinning at the same RPM, what else could change as a function of time? It can't be hot and cold both having the same effect on something else, nor would a blockage clear itself then come back again and again 10-15 minutes later, nor would a broken blend door create a blockage, then mend itself, then break, over and over again. The only thing I can think of that would go in a cycle like this is a component heating up then cooling down, one that is not in the output airstream of the blower motor.

 

[cardiac]

Believe me, it has me befuddled. I have done most of my own repairs since the mid-70's, and never heard/seen anything like this.
I was able to access the heater core and brush the fan side with a toothbrush, knocking off some dust and dirt. I didn't want to get to wild with it as the heater core is pretty old. I blew some compressed air down the vents to free up more dirt. That seemed to help. While I had the cover open I did leave t running for abouut 30 minutes. The air moving there didn't change, but the output did drop at the vents...

 

[frozentundra123456]

You could have two problems. The evaporator core icing up is a good explanation for loss of cooling over time, but does not explain the loss of heat output. As someone else said, low radiator fluid (or clogged tubing) will cause the heater to blow cold air, but that does not sound like your problem. I still would not rule something like this out though. I have had this happen more than once, that low radiator fluid will cause the heater to blow cold air. The engine temp could even read normal, but if the coolant is not sufficient to fill the heater core, or if it is plugged, you will get no heat. It might happen only after the vehicle has been running if crud in the cooling system settles out and then plugs up things when circulated for a while.

 

[cardiac]

Heat/Cold work properly. It is only the velocity of the airflow that changes.

 

[LTC8K6]

I suspect it is the fan motor slowing down a little as it heats up.

Maybe it is a loss of vacuum causing an actuator to move?

 

[cardiac]

I have accessed the blower motor after running for 30 minutes and it is still running as it should. Haven't visited the idea of vacuum loss and partial actuator movement.... I'll have to look at that. Thank you!

 

[mindless1]

Partial actuator could spit the flow between multiple vents. If it's still only going through (coming out) the one vent that shouldn't be it. Usually (more often than not) a vac leak would make it come out the windshield defroster vents to a greater extent if not entirely.

 

[cardiac]

I can switch vents just fine. I'm just going to live with it. I've had enough... Thank you.