My wonderful daily driver developed a new air conditioner problem this summer. When selecting recirculation mode (see image below, yellow circled button, for those who are not familiar), external air is now drawn inside.



How do I know this? Because on my commute to work every day, I drive within a mile or so of a sewage treatment facility. Unfortunately Miami grew and expanded faster than the city planners 100 years ago predicted. So there is one such facility inconveniently placed not to far from downtown actually. So if you're driving by and the wind is blowing in your direction and your windows are open or your air conditioner is NOT set to recirculation mode, then outdoor air gets sucked in and you know it right away.

So it has always been my habit to select recirculation mode when driving through this area, to avoid contaminating the air inside. However for the past few weeks, when I drive by that area and the wind is blowing my way, despite setting the air conditioner into recirculation mode, outdoor air enters the car anyway and it is awful. The air conditioner blows plenty of cold on both sides, by the way.

I'm hoping for a simpler fix than a dash top disassembly. By the way there are several YouTube vids on this procedure. I viewed this one in its entirety recently. It is well-presented so I recommend that everyone get to know their C5 a little better by watching it. And hopefully there will be simpler fix for mine than this:

 

Here are a few things to check from a 2000 manual. Your 2003 manual may have a different list.

 

I almost hate to ask, but is it completely omitting the recirculation, or just a reduced version of recirculation -- i.e. if driving through the malodorous zone you were to select 'fresh' does the odor get worse? Curious as to whether the duct operator is broken or stuck, or if it might just be age degraded foam seals on the duct door. On the parts manual diagram, item 1 is the air inlet, item 5 is the air inlet valve, and item 4 is the inlet valve actuator. Since the rest of your vacuum controls sound active, it seems unlikely there is a vacuum source issue, wonder if something is just simply stuck.

 

You have either a bad air inlet vacuum actuator or a bad HVAC control head…with a Tech 2 or a bidirectional scan tool you can check the input for the recirculation button…if pushing the recirculation button shows no input on the scan tool the HVAC head is bad…if input is good the actuator is bad…that vacuum actuator is near the right blend door actuator.

 

Thank you ! Dads2k, Redzg & C5 D !

Well, I'll have to ask the maintenance shop for an estimate, next time I bring it in for anything.

And I had no clue that a 2003 vehicle still had a vacuum tank like the 1968 Toronado I used to own !
I sold my beloved Toronado in 2015 and bought my beloved 2003 Corvette. Here's the Generation I Toronado vacuum tank:


I had it removed, re-sealed, and repainted. Large tank, because it was designed to hold sufficient vacuum pressure for the headlight door actuator to function after engine shutdown.



Vac tank is tucked inside the driver's side fender. Photo taken when I had my carb overhauled and a lot of other engine & AC work done.

 

‘Tank’ would be a bit of an overstatement — it’s on the order of 3” X 6”. And that may be optimistic.
It should not be possible for either the vacuum line or reservoir to be the cause of your issue. If the vacuum source is missing, the HVAC will have no control of the air path, and air will be coming out the defroster, dash, and floor vents all at the same time.

 

For ****$ and giggles you can try re calibrating the blend doors. It's been my experience that if the outside air is that contaminated with a foul odor bad enough, recirculate won't prevent some of it from entering the cabin. Even in my Impala if the odor is foul enough it'll still make it's way into the cabin.

 

redzg asked what happens when I change the HVAC selector from recirculation to fresh air, while driving through the malodorous zone. I haven't tried this. Because I enjoy living.

In order to try this, I would have to bring my 3M respirator with me, outfitted with a set of "organic vapor" filters, which fortunately I have. I would have to put it on instantly if the odor gets worse. Like a jet pilot putting on his oxygen mask instantly if there is a cabin depressurization.

I'll let you know . . . if I survive the experiment. When the wind is blowing my way as I drive near the treatment plant, the stench is beyond horrible.

Thank you yellow2007 for the recalibration video. I will definitely try this over the weekend.