Occasional vacuum leak?

XPLODEE

MEMBER
2010 FORD EXPLORER
4.6L
V8
4WD
AUTOMATIC
150,000 MILES

I seem to have an occasional vacuum leak as when driving in the summer with ac on and I climb a hill, the flow rate from my center dash vents stops. When I throttle back, the ac discharge returns. I suspect a leak drives the mode actuator to switch to defrost vents. I also note that there are occasional downshifting issues under load that causes a rough shift. Mileage is a bit lower than I expected too though on long high-speed trips Mileage seems normal. Engine is in xlnt shape. Virt. No oil consumption and phaser timing done at 120K.

Ideas on likely leak sources? Seems hard to find a vacuum chart for this limited engine. Car has dual HVAC controls and the 6R80 transmission.

Saturday, July 26th, 2025 AT 2:19 PM

7 Replies

KEN L

MASTER CERTIFIED MECHANIC
54,137 POSTS

This can be a low engine vacuum issue due to a clogged catalytic converter. This would be why the transmission downshifts as well. There are two cats so either one could be partially blocked. If you have a check engine light on or a scanner that can read the long and short trims, we can know more. Also, I would check under the hood for small broken vacuum lines. Please go over these to guides to help find the problem:

https://www.2carpros.com/articles/air-vents-stay-in-the-defrost-position

and

https://www.2carpros.com/articles/how-to-test-a-catalytic-converter

Please go over these guides and get back to us.

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Sunday, July 27th, 2025 AT 11:57 AM

XPLODEE

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5 POSTS

Long trims are abt +10% on bank 1, abt +8% on bank 2. Short trims are about 0.

I don't think the cats are partially plugged. Cat Temperatures seem fine.

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Sunday, July 27th, 2025 AT 12:58 PM

XPLODEE

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The harsh downshift seems to only happen when I'm climbing a hill or passing on a freeway, and then only when the transmission is hot from an extended drive.

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Sunday, July 27th, 2025 AT 1:00 PM

XPLODEE

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5 POSTS

The HVAC is not stuck in defrost mode. I can switch to center discharge, but volume is frequently low, but not always.

What I really need is the vacuum map for this car. You have posted 210 Explorer maps, but only for the v6. Mine is the v8 with the dual control HVAC.

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Sunday, July 27th, 2025 AT 1:03 PM

KEN L

MASTER CERTIFIED MECHANIC
54,137 POSTS

Short-Term Fuel Trim (STFT) around 0%, This means the oxygen sensors are currently reading things as balanced, and the PCM isn’t making fast corrections.

Long-Term Fuel Trim (LTFT):

+10% on Bank 1

+8% on Bank 2

This means the PCM is adding extra fuel over time because it sees a persistent lean condition on both banks. So, I would clean the MAF.

On the HVAC problem: Yes, I know but the line could have a crack under the hood, I would inspect the supply line from the engine to the firewall. There is also a vacuum reservoir under the hood that should be checked as well. Please upload a video in your response of the problem. Sorry, for some reason they are not showing the vacuum line diagrams. Check out the images (below). Please let us know what happens.

Images (Click to make bigger)

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Monday, July 28th, 2025 AT 11:09 AM

XPLODEE

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5 POSTS

I tested the black and white check valve that serves to vacuum supply to the HVAC and EGR. The check valve seemed to not reliably close when I sucked on the white side. I believe it should close when the engine vacuum is low, and the reservoir should hold the mode vacuum. Motor. I replaced the check valve this morning.

Not having a vacuum pump tool, I just sucked on the line after the check valve that went into the firewall. It seemed to hold a vacuum.

This tells me I must have a vacuum leak in the engine bay before the check valve? This is why I need the vacuum line plot for this engine?

The vacuum reservoir for the v8 engine is not in the engine bay iirc. I think it is behind the passenger side kickplate. Since my mouth suck test seemed to suggest that line and its various branches are good, not sure I need to dismantle anything.

I had cleaned the MAF sensor last year to no effect.
I also did replace the egg vacuum motor last year.

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Monday, July 28th, 2025 AT 11:30 AM

KEN L

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54,137 POSTS

Yes, a bad check valve can cause the problem, did you drive it after to see if the problem is fixed, all HVAC system leak a little bit which is normal.

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Monday, July 28th, 2025 AT 12:45 PM