Engine Rough Idle; Vacuum Leak?

manicmechanic007 said:

Check for unmetered air (vacuum leak) past the MAF
Check the air intake hose from the maf to the throttle plate for being cracked or torn
Go ahead and do sho's recommendation to check the booster
Plug it off as a test if you have no vacuum pump and test the booster in the conventional manner
There are instances where boosters rust out and leak
They are the ones that have no power brakes and will fail a booster test (recalled mostly) in states with high humidity
There has been a rash of booster recalls lately no Mustangs to my knowledge

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90sickfox said:

The brake booster runs off of vacuum. It's one of the most overlooked places for a vacuum leak. When they go bad you can usually hear a whssssshh noise when you press the brake lightly with the car running. Sometimes you can't hear it though. If pressing the brake pedal changes the idle I'd start checking the booster.

You can take the hose loose and plug it. Then start the car and see if idle stays steady. There's a round plug the hose goes to on the booster. It's supposed to be a check valve and only let air flow one way. You can take it out and blow through it to check that.

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7991LXnSHO said:

Carson,
I think the brake thing is a distraction from the real problem, a weird coincidence.
To make sure, do you have access to a Vac/pressure pump and gauge similar to this?Amazon product ASIN B07D2B13DFView: https://www.amazon.com/dp/B07D2B13DF/ref=cm_sw_r_cp_api_glt_i_BDSA9Q6363NJ97VZKM5G

I got one with a radiator and coolant system pressure tester.
Anyway, disconnect the line to the booster, pump up a vacuum, then see if it holds.
Checking the rest of the lines and the cap under the upper manifold certainly would not be a bad idea on a Fox 5.0.
I think going through all the steps in order on the list will either find the issue or will help us lead you in the right direction.

Here is what the red herring under the hood should look like. ;-)

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