Engine Rough Idle; Vacuum Leak?

Carson93Fox

Member
Dec 10, 2017
18
5
13
Hi everyone, 93 lx 5.0. The problem I'm having is a surging idle. I started going through the checklist posted here when I noticed that if I mash the brake pedal fast on and off the idle smoothes out perfectly, then goes back to the rough idle if I stop doing it or leave my foot on the brake. Super strange to me, wonder if anyone has heard of this happening before? I couldn't find anything by search. Am I correct to assume there's a vacuum leak somewhere?
 
  • Sponsors (?)


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. ;-)
6A7464BE-C10C-4526-9E37-23D299F5B37D.png
 
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
 
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.
 
  • Useful
Reactions: 1 user
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
I have that vacuum pump but no adapter to go to a big enough hose for the booster line. I ended up taking the source vacuum hose off the vacuum tree and plugging it with my thumb while the car was running. With my thumb off of the vacuum hose, the car idled fine, higher than what it should be, but smooth.... shouldn't it almost not idle at all with that big of a vacuum leak? anyway, when I put my thumb over it, it went back to the rough idle immediately and when I took my thumb off it idled high and fine just as fast.
 
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.
I'll have to try that tomorrow thank you. pressing the brake pedal alone doesn't change the idle, it's when I mash it fast on and off that the idle smoothes out haha it's weird. Car idles terribly whether I hold the brake or don't touch it. didn't know about that check valve ill have to look at that tomorrow too!
 
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. ;-)
6A7464BE-C10C-4526-9E37-23D299F5B37D.png
if only they stood out like that...
 
  • Clever
Reactions: 1 user