Pop Off Valve/Vacuum leak SC

[Joeybear]

Porsche 911 SC with 36,000 miles. Just installed a pop off valve (not because of a problem). Now the car idles roughly, and pops and barks through the exhaust like it has a really free flowing exhaust. Especially when you let off the gas. Starts ok, in higher rpm seems to be ok, but rough. I have not spotted a cracked air box, pop off valve seems ok, but I am suspicious of it. Any ideas? Should I replace the seal on the pop off valve? My pop off valve was sitting in my tool chest for several years. Help is appreciated, my SC has always run like a champ. Oh, 30,000 service was performed awhile back.

[boyt911sc]

Please post a picture of your car. This is what we expect for first time posters and help will come. Ignore this protocol and you will be surprised. Welcome to the forum.

Tony

[T77911S]

did you epoxy it in
is it seated properly/closed

did you remove the air box to do it

[Joeybear]

I will get a pic asap. Yes I used two part epoxy, and did it in the car. I plan on taking it to my mechanic for a smoke test to see where the vacuum leak is coming from. Looked at the seal on the pop off seems ok. The spring on the pop off, how tight is that supposed to me, it seemed slightly weak.

[78-911SC]

Has something to do with that installation. Have someone hold down the pop off valve real hard and see if that alleviates the problem. You can also use plumbers putty were u used epoxy. Will come off very easily but can temporarily seal leaks to track problem. Sounds like a vacuum leak somewhere. Good luck. Keep us posted as to what the problem was.

[T77911S]

the spring does not have to be tight since vacuum should pull it closed but the flapper has to be seated properly, they can get cockeyed and not close all the way.

you probably left a hose off or did something you don't realize you did

[DanielDudley]

Previous owner of a car I looked at last year installed the pop off valve backwards, so that it sprang inward.

[Modifier]

Here is Joeybear's '83 SC. He may not be able to upload a pic on here, but he did teach me how to turn a wrench since the age of 5 or so and how to properly jack up a car - remember Mom's Volvo I jacked up while you were at work!

['76 911S 3.0]

Off topic, but what front valence is that on your car Joeybear?

[Plays with cars]

I have not done this installation myself so I'm just spitballing, but... If you did the install with the air box still on the car I'm assuming you were working in tight quarters. Any chance that you knocked a vacuum line loose or flexed and cracked an old vacuum line?

Certainly coincidental that the problem came up after the install so I would examine the install specifically but also look at things peripheral that may have accidentally been effected by the work you did.

[Modifier]

@'76 911S 3.0 - That is a custom valence he had made years ago. He has a front oil cooler and had a valence made RS style but still fit in with the fogs and lines of the factory bumper.

@Plays with cars - He got the pop off removed easily today from what he told me. So, maybe he didn't get a good seal on it. He ordered a new one in case the one he used was sitting too long and was warped or something. We will update the thread if we figure it out.

[john walker's workshop]

You need to check all around the valve with a small mirror and flashlight to see if there are gaps in the epoxy. Is the valve all the way down and sitting on the airbox ribs?

[LM3929]

any Pictures?

Lorne M.

[Modifier]

To update the thread... Dad installed a new pop off valve with new epoxy and the symptoms went away! The car does however need a good tune up and some more mild maintenance from sitting, so car won't be driven and will be delivered to his mechanic mid-Jan for some lovin'. The other valve must have been not sealed very well. I'll update again once car is back from shop.

[Modifier]

@LM3929 - See post #8 that is his car.