Interesting heater operation...

[Speedwagon]

I realized today that I have an odd operation with my heater on the 87 Brat.

 

While driving down the highway, some air is blowing on my feet, even with the fan turned off. But when I go up a hill(and the engine is high revving, high load, no vacuum), the air slowly decreases, and will eventually stop if the hill is long enough. As soon as I let off the throttle, the air starts pushing again.

 

If I turn on the fan while climbing, it acts normally.

 

What could be causing this?

Edited August 7, 2017 by Speedwagon

[djellum]

I believe loss of vacuum will cause it to switch to dash vents.  My EA82 is like that, idk for sure on the older gen.  Theres a vacuum can that makes up for minor variations but prolonged deep pedal will overdo it.  my guess anyway.

[DaveT]

The can os supposed to hold for those situations. If there is a vacuum leak, or the check valve in the line to the tank is leaking, the low / loss of vacuum condition will allow the blend doors to move toward their resting positions.

[skishop69]

The Brat system is all cable driven except for the recirc door for AC which is vacuum driven and should only work when the mode lever is all the way to the left. Something is not hooked up right, because it sounds like your recirc  is coming on as you're climbing a hill.

[iluvdrt]

With the fan off and moving at a good pace it will circulate air though the vents as it is forced air from the outside. Going up a hill your speed decreases and thus isn't moving as much air across the heater core. All cars, even new ones, do this when the "fresh air" selector mode is selected because the air intake for the heater is directly under the windshield cowl.

 

If you don't want the residual air circulation either move the selector lever from Heat to Max AC (recirculation), or turn your temperature knob to cold (which will still circulate cooler air).

[skishop69]

With the fan off and moving at a good pace it will circulate air though the vents as it is forced air from the outside. Going up a hill your speed decreases and thus isn't moving as much air across the heater core. All cars, even new ones, do this when the "fresh air" selector mode is selected because the air intake for the heater is directly under the windshield cowl.

 

If you don't want the residual air circulation either move the selector lever from Heat to Max AC (recirculation), or turn your temperature knob to cold (which will still circulate cooler air).

He doesn't say he's loosing speed and the effect you're talking about won't show a significant decrease in air flow until you drop 15mph or so at speeds of 45mph (or so) and up. There is also no 'fresh air' or vent mode on a Brat. The system is always open to outside air unless in the max AC position causing the recirc door to close off outside air intake. No, even new cars do not do this. Some may still, but the vast majority employ multiple mode and recirc doors to stop this effect. My Brat moves air on it's own from about 20mph up and after about 40mph, the amount remains steady. All of my Brats have done this so some vacuum line on his Brat  is in the wrong place and the recirc valve is not functioning correctly.

 

To the OP: If you have the engine running and the Brat is just sitting, turn the fan on high, temp on hot. Slide the temp to max AC and listen for the sound of the fan/airflow to change. If the vacuum is not hooked to the correct port, you won't hear the change and this will confirm the system is not operating correctly. You may have to raise the RPMs a bit to supply enough vacuum if the system is operating correctly.

[Subaru Scott]

I'm guessing the vacuum servo that operates the fresh air/recirc door has a rotted diaphragm after all these decades, and is leaking...

[skishop69]

That's a possibility too if the vacuum valve on the temp lever were stuck open. He'd lose enough vacuum at higher engine loads that it wouldn't have enough pull to overcome the spring.

[Speedwagon]

I'll give that a look and see, thanks.

[iluvdrt]

The Brat does have fresh air. 2 actually. My 82 has a fresh air vent by the clock (Unrelated to HVAC though) AND the HVAC is always on fresh air unless you put it to AC recirculate (If equipped) as denoted in my OP. Which you also confirmed by saying "The system is always open to outside air"....Outside air IS fresh air lol.

 

The system being "Open to outside air" is the fresh air option.

 

Yes, even new cars do DO this. I have a 2005 OBS, a 2015 XV, and a 2017 BRZ that all "leak" hot air while driving with the fans turned off but the temperature set to hot. The 1996 Crown Vic that I commute into work with also does this.

 

And while your diagnostic method is OK, revving the engine actually drops vacuum in the manifold unless it is hooked to the distributor advance port under the carb which is a ported vacuum port. Any other port on the carb or intake manifold looses vacuum when revving, and is less during operation when compared to idling or decelerating.

 

 The Vacuum pod located under the dash hooks to the right side of the intake manifold right on top, which is not a ported vacuum port. The pod is there to store a vacuum so that when operating the engine (when there is no vacuum in the manifold) and the selector is moved, there is still enough vacuum to change the air selector flap, as well as told hold the flap there when operating the motor.

 

The temperature setting is not controlled by vacuum. It is cable operated and operates a blend door which resides over the heater core. This blend door directs the air coming from the air source ( which is controlled by the mode lever which is operated by the aforementioned vacuum servo) through or around the heater core or a combination of the two depending on where the temperature setting is. Only the fresh air/recirculation settings are operated by vacuum.

 

Recirculation does not pull outside air in, it recirculates the existing air already in the cabin. If he has his settings to recirculation, the fan off, and is still getting air blowing through the vents, than yes, the vacuum servo or vacuum pod has failed and is no longer retaining vacuum. So once he downshifts and gives it the beans, the engine looses vacuum, and the recirculation door return spring opens the door to outside air. When cruising or idling the engine gains vacuum, which pulls the door shut thus stopping the air.

 

If he is cruising in the heat mode (which is fresh air) ambient air will continue to blow across the heater core as described earlier.

 

OP, the outside air door is on the right side of your HVAC unit behind the passenger foot well tray and radio trim. You can see it kind of operate when the truck is idling by selecting heat or max AC recirc. To test the vacuum pod (Located behind the glove box) simply unplug each end from the vacuum tube. If you can blow air through both sides than it is no longer functional by holding vacuum.

[skishop69]

Yes, they have a vent, but as you noted, it's not related to the HVAC. I also did say the HVAC system on the Brat is all cable controlled. I did say the MAJORITY of newer cars ( meaning roughly 05 & up) don't do this. Some still do. People whined about it so most manufacturers dropped their cowlings down well below hood level and re-positioned the fresh air intake along with changing the mode doors and HVAC box to eliminate this. My 07 Equinox doesn't do it, but my 02 Park Ave does.

 

I want to recap what his 'issue' is, because it seems like you may have misunderstood, and I don't mean this in a snarky way at all. When cruising with the temp lever in hot (I'm guessing because he didn't say what air temp),  he feels the air without the fan. As he pulls a hill, the air flow diminishes and stops. Since the recirc door is the only thing in this system that can cause that, it means that somehow, vacuum is being applied to the recirc servo causing the door to close. The recirc door closes to outside air when vacuum is applied. The only way for this to happen in his scenario is if the supply vacuum is hooked to a ported line and the vacuum valve in the control head is bad or bypassed.

 

Now, if I'm guessing wrong, and he has the heat lever in max AC and this is happening, then the problem is that the vacuum supply has been routed to ported vacuum and nothing else is wrong. As you already pointed out, ported vacuum increases with engine RPM which would cause the door to close.

 

Like I said, I'm not trying to be snarky. His situation can't be caused by a loss of vacuum since the recirc door only closes to cut off the outside air when vacuum is applied.Maybe I'm confused.  Annnnd go! lol

[Subaru Scott]

I thought it worked the other way, and the servo opened the fresh air door.

[skishop69]

Nope. I had to do some work on mine a while back and it's vacuum apply. What I've seen over the years and what was taught to me is all HVAC vacuum servos are vacuum applied to operate so that the default state has no vacuum to extend the life of the diaphragm. You wouldn't want the recirc servo to fail and get stuck in recirc mode or your car would fog like mad when its cold out.

[Subaru Scott]

Well, you would know then. I never had one of them fancy newer Brats. 

 

I was just thinking the door would default to closed to keep the meeces out.