air won't stop blowing out of floor vent

[1972BlueC20]

Hi all,

Need some help, I have a 72 C20 with aftermarket AC, so it's technically a NON AC truck originally. I have a busted heater core which I'm going to be replacing this weekend, and I've looped the hose from the intake to water pump to bypass the heater core for now.

I noticed today for the first time on the way to work since it's a little cooler outside that even with the ac and the heater ect completely off, I'm getting a constant flow of air coming through the floor vent and a little I think through the vents in the dash too....I'm assuming this is from air coming into the engine bay from driving down the road.


How can I stop this???

Any help or suggestions are greatly appreciated.


-Brian

[bonnieclyde100]

there is a flap inside your front cowl. make sure it is closed it will cut the air going into the ac system.

[1972BlueC20]

How do I get to the flap? And is it supposed to be open or closed?


also, I put some duck tape temporarily over the holes around the heater core inlets and also over the hole in the heater blower motor and the air flow is cut down about 95% of what it was prior....still need to figure out how to stop it completely.

[bonnieclyde100]

the flap is usually controlled by factory contols but i dont know with your setup. the lever protrudes through the firewall and is opened and closed by a vacuum actuator . they are open when your ac gets air from outside and closed on maximum ac so it gets air from inside the cab. if you have one it should be on passenger side of cab right above where factory air box would be.i will take a picture when i get home and post it.

[1972BlueC20]

my truck is factory non ac and it's a c20 if that helps.

[70shortfleet]

Even though it is non a/c it still has a flap that send air up to your defrost or down below and is controlled by the levers on the dash

[Hutch]

The problem is most likely a flap inside the ducting.
This will become more apparent once you disassemble the unit to replace the heater core.
You can do a search on the forum for flapper.
Remember that the non-a/c units are different from the a/c units
You can also look up a diagram on the LMC site.
You may also want to get the seal kit ordered before you start the rebuild of your heater unit. Another good idea is the seal kit for both the ducts on the inside of the foot wells

[leddzepp]

The flapper inside the cowl is most likely open as stated above.

[FirstOwner69]

Originally Posted by leddzepp

The flapper inside the cowl is most likely open as stated above.

I thought the trucks without A/C such as the OP's don't have the flapper (cold air door) hidden beneath the passenger side cowl inboard of the antenna. Or, aren't we thinking about the same flapper? I think 70shortfleet is referring to the one in the housing under the dash.

[bonnieclyde100]

thats what i was talking about i dont know it they have one or not. here is a pic of mine coming trough the firewall. if yours doesnt have one try to put system to max ac and see if that stops it .max ac usually gets its intake air from inside the cab and may shut off outside air. again not sure of your setup not being stock.

[leddzepp]

The flapper beneath the cowl is the fresh air inlet, allowing outside air into the cab. The flapper inside the ducting directs air flow and has nothing to do with air entering the cab.

[FirstOwner69]

Originally Posted by leddzepp

The flapper beneath the cowl is the fresh air inlet, allowing outside air into the cab. The flapper inside the ducting directs air flow and has nothing to do with air entering the cab.

Exactly. That's why I was looking for clarification. The OP said his truck did not come with factory air and, therefore, no cowl vent. Correct?

In a non A/C cab with the fan off, is it even possible to cut off all airflow that results from forward motion?

[gmc1969custom]

I am having the same problem with my truck. Non factory a/c (now equipped with a Clardy aftermarket under the dash a/c unit). Air coming in through the center floor vent. My defrost lever/door is working properly as no air travels up to upper dash vents. How do I access this fresh air vent under the cowl? Is it easy to replace? Thanks in advance....

[bonnieclyde100]

if your cab has one you can get to it by taking off the cowl cover panel between the hood and the windshield

[1972BlueC20]

I looked and I don't have any flapper(s) or fresh air inlet(s) inside the cowl area.

I only have those 2 ports on the kick panels but they are closed and that shouldn't effect any air coming through the bottom floor vent above the transmission hump.

what should I try to do now?

I too have an aftermarket clardy hang on system, but it just quit working (blower motor for ac is dead, or the resistor blew...it won't turn on now)

[leftybass209]

I understand what you are saying. This system is not sealed, and there are slight gaps and cracks that wind can get through. Is your system new, as in, new gaskets, flapper foam inside the heater box under the dash new, and all that? Getting more into science, when hot and cold air combine, that creates wind as well, which can happen when cool air sits on the inside by the heater core. I personally haven't experienced any air blowing out of my floor vent but I have heard of this and had it happen on newer vehicles.

[1972BlueC20]

my system is original and the clardy system has been in the truck since the 70's I think.

There is no flapper on my system.

[leftybass209]

Not the AC flapper. There is a diverter INSIDE the box underneath the dash. It is wrapped in a foam pad and probably really brittle, shrunk, and useless.

[70shortfleet]

Is this where the air is coming in at? Here is a picture of the flap inside the metal duct work

[Hutch]

Air flow diagram with factory ac

[1972BlueC20]

I don't have a factory ac truck so that diagram only helps a little.

If my flapper is messed up, then how do I fix it? Can I buy a new flapper? Can I buy the whole new box with flapper? If so, where can I buy these things and what is the official name when searching online.


Thanks guys!!

[1972BlueC20]

@ Hutch

Do you have an airflow diagram for the non AC trucks??

[425HP409]

1. Non AC trucks do not have the "Flapper" under the hood in the cowl, that is only AC trucks to switch from outside air and recirculate air.
2. Trucks that only came with a heater form the factory have a cable from the control panel for "air". It controls the air coming in from the outside by opening or closing a door in the heater box that is in the box under the hood. It has a seal on the door. It is available from many vendors, LMC part #36-0564 for 67-69 and #36-0565 for 70-72. The seal you need is the rectangular one that is in the seal kit. LMC shows an exploded view in their catalog. I hope this helps.

[1972BlueC20]

Thanks @ 425HP409

That definitely helps a lot!

[gmc1969custom]

What is the best way to access the old seals and replace them? Can I just remove the heater box from under the hood? Any hidden bolts? Any help would be appreciated. Done lots of work on these trucks but this is my first on heater seals.