Possible Vacuum Leak

[1997ThunderbirdLXV6]

Here's the observation: recently when I get into the car first thing in the morning, and I turn the A/C dial to "OFF" (as I habitually do before starting the engine), I hear...nothing at all.

I used to hear the recirculation door close. In other words, the vacuum in the system was maintained overnight.

Now I hear the door as the engine starts.

Functionality while the engine is running is normal, so I suspect a small/tiny leak. Where should I look? I imagine there are a few check valves along the way between the intake and the A/C system (?).


Meanwhile, also recently, I have the impression of slightly increased fuel consumption. I'm not entirely sure really; this may be due to my recent move. Just mentioning it as an aside.

 

[1997ThunderbirdLXV6]

Bumping this months-old question. Anyone?

 

[theterminator93]

The HVAC vacuum canister is mounted to the upper frame rail inside the passenger's inner fender area near the firewall. There is a check valve near the firewall inside the engine bay near the A/C accumulator that tees to manifold vacuum to the vacuum canister and the HVAC head in the cabin. You could start from there to isolate where the leak is.

 

[1997ThunderbirdLXV6]

Ok...I'll investigate.

But since you're mentioning a check valve, is it safe to assume that a vacuum leak inside the cabin would  not affect any vacuum-driven stuff in the engine compartment?

 

[dDUBb]

Ok...I'll investigate.

But since you're mentioning a check valve, is it safe to assume that a vacuum leak inside the cabin would  not affect any vacuum-driven stuff in the engine compartment?

No .. the check valve allows the vacuum to remain inside the cabin when engine vacuum decreases. A vacuum leak inside the cabin would certainly affect the engine.

 

[1997ThunderbirdLXV6]

I'm not making much progress on this. As previously mentioned, there isn't really any sort of malfunction with the engine running.

First thing in the morning, before starting the engine, I can tell that there is no vacuum left if the switch was left in A/C or PANEL the night before. If left in DEFROST, there is vacuum left, but it seems weak.

In any case, the leak must be tiny as it takes many hours for the vacuum to disappear.

Strictly speaking, the consequence of this would be that unmetered air is entering the engine, correct? Likely only a tiny amount; I'm wondering what kind of issues that could cause, if any.

 

[theterminator93]

It's small enough that it likely has no real-world impact.

We're talking about something like a .01% enrichment on your closed-loop fuel trims.

 

[1997ThunderbirdLXV6]

Another observation on this subject. Mind you, I'm not sure if this is recent, or if my attention to it is recent.

Say I'm accelerating moderately, then I take my foot off the accelerator, then for a brief moment, I can hear a whoosh/low whistle. It's very faint. I imagine it's a moment when vacuum is very high because the throttle plate has closed, but the engine revs still need a second to drop.

I deleted the intake silencer forever ago; just a side note. It could just be the normal intake sound in that situation.

 

[theterminator93]

What you're describing reminds me a little of something I used to experience with my 94 where the dashpot was a little... sticky. If I was accelerating or cruising and let off the gas, the car would carry on as if I still had my foot on the gas and take a few seconds to start to decelerate. If I lightly tapped the throttle immediately after initially letting off, it would start to decelerate immediately. I never spent time to try and figure out if it was a sticky IAC valve or just a programming quirk.

 

[1997ThunderbirdLXV6]

, the car would carry on.

In my case, it's really just a sound I'm hearing. The car isn't continuing to accelerate or anything like that.