Thread: Heater airflow significantly weakens when temp is set to ‘hot’

Hi, I have a problem with the heater on my Grand Prix GT 2003 with dual zone manual temperature control.
The problem is that the air flow is close to nothing when the temp sliders are moved to maximum 'hot'. The air is hot though. When sliders are in 'cold' position - the air flow is nice and strong.

What I've done or checked by now:

- I've flushed and cleaned the cooling system. The heater core output pipe is almost as hot as input one. So I don't think the heater core is clogged. And I had a new one installed about 2 years ago.

- Blower works fine

- I've checked the Air Flop Actuators (both of them). I saw that the metal pin in the middle moves nicely from one max position to another max position (according to the marks on the plastic housing). I did not remove the actuators though and did only a visual inspection.

About 2 years ago my old heater core started leaking coolant and I had it replaced. Also the Dexcool was replaced with the regular coolant.
I think there is something mechanically blocking the airflow through the heater core. I think they could leave something there or did not remove some protection film when installing the new unit.

Do you think that blocking is a plausible reason?

Do you think I can check it without removing the heater core?

Thank you!

My actuators are electrical and look like those 03 2003 Pontiac Grand Prix Air Flap Actuator - Climate Control - Replacement, Dorman - PartsGeek
I could see that they move all the way in both directions - there are marks on the housing and on the turning thing (a metal pin). Were you talking about those ones or something else?
Thank you scottydoggs!

Thanks again!
I've just checked the "Recirculation Door" - it moves freely and according to the spec (opens at MAX and closes when flow is sent to the floor). It has zero effect on the air flow strength. I move it with a hand - nothing. It means there is nothing blocking the air flow before it enters the blower motor.
Checked one more time electrical actuators - they move nicely and the air flow dies gradually as it moves from cold to hot.
To me it looks like the block is behind the flaps/doors operated by the electrical actuators.
Any suggestions?

Also there is no codes, I have a scanner for few days.

I think my only option now is to get to the heater core and see what blocks it. If I"m lucky I see something in front of the heater. A fun weekend project )

Yeah, I realize how much fun it will be. I've seen some step by step instructions with pictures on this forum and I have that special software installed with all procedures and diagnostics.
Thanks for the tips! I'll post picts if I find a squirrel nest there )

The heater core is out. Looking nice and new, nothing blocks air on either side. No alien objects in the system as far as I can see. Temperature air flaps driven by electrical actuators move well - I can see them now.

I'm out of ideas now (

not trying to jack a thread here but its kinda happening, my car never heats up now that it is winter maybe 120 is the top , this sound like a problem?

Guys, could you please check if you can feel a difference on you Grand Prixes in air flow strength when moving sliders from 'cold' to 'warm'?
Thanks in advance!

Here is how the story ended: While the core was out, I back flashed it with tap water and blew throw it. Some dirt came out, not too much, but noticeable amount. Looks like the Preston cleaners have not done a good job here.
I've put everything back together and so far I think my problem is kind of solved. I realize that back flashing the core has nothing to do with the air flow strength, but it seems to be back to normal. Why? I don't know. The air temp though got hotter as well as the core output pipe. May be that gives a feeling of a better air flow.
The good thing is that now I'm a heater core replacement expert! ))