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Vehicle air conditioning modes and vacuum leaks
Dash vent airflow seems to be present about half of the time.

I have read many posts about vacuum leaks causing the dash vents to close during acceleration, addressed by bypassing the original buried-deep-in-the-dashboard vacuum reservoir and installing a new vacuum tank in the engine compartment. My local shop just did this, but the problem persists.

What is the expected variation of the vehicle A/C output during various engine power demands? On a downhill grade or level terrain the dash vents provide cool air. On even the slightest uphill grade the airflow cuts off entirely. By "slightest uphill grade," I mean one so slight that I have a hard time realizing that it is uphill at all, other than that the airflow cuts off. For a steep grade, where the engine downshifts, I might expect the vacuum to decrease enough to cause this effect. For minimal to mild uphill grades, I would expect that the modest vacuum decrease would not cause this effect. Somewhere between moderate uphill and steep uphill is where I would expect the transition from normal airflow to no airflow, but I see the transition between level and very slight uphill.

I have two theories:

1) This is not normal, and a vacuum leak remains. More work is needed to find and correct the leak.

2) This is normal operation, and I just never noticed how often the airflow cuts out on slight uphill grades. I need to reset my expectations.

What do other people experience?
2013 27’ Mid-Bath
2005 Honda CR-V
Re: Vehicle air conditioning modes and vacuum leaks
Reply #1 –

Kieth, when we replaced our failed reservoir we had our vent flaps return to proper function immediately.  When I did the repair I added a vacuum gauge with a T in the vacuum system line and monitored if it was holding vacuum.  I used vacuum pump to simulate a running engine.  When we did finally drive our 98~MB some time later the vent flaps worked and the air was distributed where it was set to go.  You could get a Vacuum kit on Amazon to test your system.

       Karen~Liam
         98 ~ MB
           NinA

inexpensive vacuum kit -
Amazon.com: DASBET Hand Held Vacuum Pump Tester Set Vacuum Gauge and Brake...

As an Amazon Associate Lazy Daze Owners' Group earns from qualifying purchases.
1998 ~ MB  WanderDaze
previously a 1984 Winnebago itaska- The Road Warrior, before that several VW Buses and before that a 1965 Chrysler Convertible Newport or our 1969 Chrysler La Barron with an ice box and a couple sleeping bags
Re: Vehicle air conditioning modes and vacuum leaks
Reply #2 –
I suspect theory 1. 

I did the bypass repair last year and everything returned to normal.  You can lift the flap on the dash to expose the vacuum controlled actuators.   You should see them moving in predictable ways when you turn the A/C dials.  If they move in unpredictable ways with acceleration, you still have an issue.

Rich
2003 MB
Re: Vehicle air conditioning modes and vacuum leaks
Reply #3 –
Certainly not normal. You shouldn't notice any difference in operation while driving.

Quote
You can lift the flap on the dash to expose the vacuum controlled actuators.

Pack rats got into mine and chewed through one of those lines. Made a start on the other one too. Caused the same issue you have. I repaired with shrink wrap on both.
jor
09 27' MB
10  Suby Forester
Re: Vehicle air conditioning modes and vacuum leaks
Reply #4 –
Hi Keith; No that isn't normal.  When you had the tank replaced, it's not the tank, it could be the check valve, but more likely it is a compromised vacuum line somewhere.  I guess it could also be a piston, actuator.
   Normally at full throttle there isn't much, if any, vacuum generated by the engine, but the reservoir tank should provide enough for the duration.  Vacuum is only used when the actuators move. 
   I would use a manual pump, usually they come with a gauge, to operate each actuator, and watch it's leak back rate.   https://www.harborfreight.com/brake-bleeder-and-vacuum-pump-kit-63391.html?gQT=1
A tight system will hold vacuum for a long time. I have changed to close the air inlet, a day after shutting the engine off, and heard the damper move to close off outside air.  RonB
RonB (Bostick) living in San Diego
Original owner of "Bluebelle" a '99 TKB
Re: Vehicle air conditioning modes and vacuum leaks
Reply #5 –
What flap on the dash? I don’t see one on my 2013 vehicle.
2013 27’ Mid-Bath
2005 Honda CR-V
Re: Vehicle air conditioning modes and vacuum leaks
Reply #6 –
Look at the photo that Jor posted in Reply #3 above. The cover pops off, exposing the vacuum cans.

Larry
Larry
2003 23.5' Front Lounge, since new.  Previously 1983 22' Front Lounge.
Tow vehicles  2020 Jeep Wrangler Rubicon, 2001 Jeep Cherokee
Photo Collection: Lazy Daze
Re: Vehicle air conditioning modes and vacuum leaks
Reply #7 –
Our 2017 has a small rectangular access plate.  I’m assuming the cans are underneath it but I haven’t popped it off to look. Funny that I never noticed it before now.
Greg & Victoria
2017 Mid-Bath  “Nocona” towing a manual 2015 Forester
Previously a 1985 TK
SKP #61264
Re: Vehicle air conditioning modes and vacuum leaks
Reply #8 –
My 2013 has the same color dash (dark brown instead of beige) as yours, and the same small rectangular panel as yours. Neither of which look like what I see in the photo that Jor posted.
2013 27’ Mid-Bath
2005 Honda CR-V
Re: Vehicle air conditioning modes and vacuum leaks
Reply #9 –
Just out of curiosity, I opened the small panel. I see a steel plate with a hole, which looks like the unused location for a sun sensor or similar optional device. It looks nothing at all like the collection of vacuum actuators in Jor's photo. I assume that Jor's vehicle is simply not the same dash configuration as mine.
2013 27’ Mid-Bath
2005 Honda CR-V
Re: Vehicle air conditioning modes and vacuum leaks
Reply #10 –
Interesting, Keith. So now, one has to wonder where the vacuum cans are located on our newer rigs.
Greg & Victoria
2017 Mid-Bath  “Nocona” towing a manual 2015 Forester
Previously a 1985 TK
SKP #61264
Re: Vehicle air conditioning modes and vacuum leaks
Reply #11 –
The vacuum reservoir is located up under the dashboard, where it is very difficult to get to, hence the solution of bypassing it with a new reservoir installed in the engine compartment. I suspect now that there is a leak in a hose under the dash, or a defect in one of the actuators also under the dash. Rather than taking a deep dive into the dash myself, I am having the shop that installed the bypass reservoir do the dive on Monday. I'll report what is eventually found and fixed.
2013 27’ Mid-Bath
2005 Honda CR-V
Re: Vehicle air conditioning modes and vacuum leaks
Reply #12 –
I have seen all sorts of problems with the vacuum HVAC lines, the most common being the failure of the vacuum check valve. Before tearing into the dash, I try adding a new check valve under the hood, in the vacuum line leadng from the intake manifold to to passenger side of the firewall, as shown int he photos below.

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I believe this is the check valve I used.
[uhttps://www.amazon.com/dp/B000COCU0M?ref=ppx_yo2ov_dt_b_fed_asin_title

Larry
Larry
2003 23.5' Front Lounge, since new.  Previously 1983 22' Front Lounge.
Tow vehicles  2020 Jeep Wrangler Rubicon, 2001 Jeep Cherokee
Photo Collection: Lazy Daze
Re: Vehicle air conditioning modes and vacuum leaks
Reply #13 –

Leaks can be difficult to find. I use a smoke machine to find hidden leaks. A smoke machine injects low-pressure smoke into a system hose and fills the system with smoke. allowing the leak to be found where the smoke leaks out.
It is handy for diagnosing leaks in the emissions system and the HVAC system.
The smoke is made by heating mineral oil until it smokes and using compressed air to inject it.

For use with an air compressor.
Amazon.com: AutoLine Pro EVAP High Volume Smoke Machine Leak Tester – Shop...

Smoke machine with a built-in air supply.
Amazon.com: AutoLine Pro EVAP High Volume Smoke Machine Leak Tester with...

With repair shops charging up to $200 an hour, this machine can pay for itself the first time it is used.

Larry

As an Amazon Associate Lazy Daze Owners' Group earns from qualifying purchases.
Larry
2003 23.5' Front Lounge, since new.  Previously 1983 22' Front Lounge.
Tow vehicles  2020 Jeep Wrangler Rubicon, 2001 Jeep Cherokee
Photo Collection: Lazy Daze
Re: Vehicle air conditioning modes and vacuum leaks
Reply #14 –
The shop determined that the vacuum reservoir was defective and installed a substitute in the engine compartment. On my next outing I realized that the problem remained.

I took it back to the shop and they found a vacuum line pinched where it penetrated the firewall. They fixed this, and now it works pretty well. Pretty well is, like, 95%, but not perfect. I feel a slight decrease in airflow out of the dash outlets under heavy load, which resolves after a few seconds even while the load condition continues.
2013 27’ Mid-Bath
2005 Honda CR-V