Exhaust smell in cabin when vents on
Thread Starter
Pro
Joined: Mar 2025
Posts: 521
Likes: 252
Exhaust smell in cabin when vents on
Hi All,
So this weekend I smoke tested my exhaust, and didn't find any leaks. Doesn't mean there arent any, as I haven't been able to get it lifted just yet and inspect closer. It's just that the exhaust smell bothers me and I know that the C4 did not have a cabin air filter.
What are some things I should look for when I do get it up in the air and what are some other reasons the vents could be smelling like exhaust? I never really drive with the top off because it smells.
Thank you all!
So this weekend I smoke tested my exhaust, and didn't find any leaks. Doesn't mean there arent any, as I haven't been able to get it lifted just yet and inspect closer. It's just that the exhaust smell bothers me and I know that the C4 did not have a cabin air filter.
What are some things I should look for when I do get it up in the air and what are some other reasons the vents could be smelling like exhaust? I never really drive with the top off because it smells.
Thank you all!
Race Director
Joined: Oct 2012
Posts: 17,458
Likes: 6,563
From: Melbourne, FL
What year? Any known modifications? Does it smell from the outside, or through the AC vents?
Blind guess, it's the exhaust manifolds. Good time to invest in headers and a full exhaust. Probably cheaper than paying shops to search for it...
Blind guess, it's the exhaust manifolds. Good time to invest in headers and a full exhaust. Probably cheaper than paying shops to search for it...
Race Director
Joined: Oct 2012
Posts: 17,458
Likes: 6,563
From: Melbourne, FL
Melting Slicks
Joined: May 2019
Posts: 3,292
Likes: 1,431
From: Mid-Atlantic
2025 C4 of the Year Finalist - Unmodified
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Manifold to front y-pipe nuts might not be tight enough or maybe loosened. Although I've never had that happen or seen it happen. There's no donut gasket there on my '90 but not sure on your '88.
I've been getting a rattle noise under my C4 lately. Even when I close the doors.
I got under there and it turned out that the inside of the front Y-pipe is rattling against the outside pipe where it connects to the cat.
I tried tightening the clamp but couldn't get it any tighter to where the inside diameter an outside diameter of the pipes weren't vibrating against each other. Didn't bother pulling out the air tools but at least I know what the problem is for whenever I do feel like messing with it.
That super small space between the two pipes where they connect also throws off an exhaust smell inside the cabin.
Maybe check that kind of thing out if you haven't already.
Of course, FAUEE makes an excellent point, too. Especially about the rust perforation on that front pipe. Could very well be rusty and perforated pipe.
I've been getting a rattle noise under my C4 lately. Even when I close the doors.
I got under there and it turned out that the inside of the front Y-pipe is rattling against the outside pipe where it connects to the cat.
I tried tightening the clamp but couldn't get it any tighter to where the inside diameter an outside diameter of the pipes weren't vibrating against each other. Didn't bother pulling out the air tools but at least I know what the problem is for whenever I do feel like messing with it.
That super small space between the two pipes where they connect also throws off an exhaust smell inside the cabin.
Maybe check that kind of thing out if you haven't already.
Of course, FAUEE makes an excellent point, too. Especially about the rust perforation on that front pipe. Could very well be rusty and perforated pipe.
Last edited by Natty C; Yesterday at 11:14 PM.
Melting Slicks
Joined: May 2019
Posts: 3,292
Likes: 1,431
From: Mid-Atlantic
2025 C4 of the Year Finalist - Unmodified
2023 C4 of the Year Finalist - Unmodified
2022 C4 of the Year Finalist - Unmodified
2021 C4 of the Year - Unmodified Finalist
I'd like to find some headers with the egr/emissions stuff already welded into them but haven't had any luck. Of course I haven't really looked around too deeply. I cold have sworn I'd seen them around here some place but it may have been a case where the owner fabricated it into them, though.
Here in Delaware, they're equally as **** with that stuff. Maybe even more so. And we also must have mufflers installed by law. I've never failed emissions but I dread having to do it every couple of years.
Racer
Joined: Feb 2010
Posts: 376
Likes: 65
From: south wilmington il.
Exhaust leak between the cylinder heads and tailpipe tips. There, I answered it as easily as possible. Air cones into the car via venting on firewall or cowl whichever you prefer, or through the window if it's open. Now you just have to figure out where. I'd start at the exhaust manifolds, next popular place is exhaust pipe connections. If you have the A.I.R. pump and all the associated parts it's another notorious place for exhaust leaks, EGR valve, A I.R. tubes. Find it before you get brain damage from the fumes.
Melting Slicks
Joined: Apr 2001
Posts: 2,125
Likes: 225
From: Ca.
Get it up in the air so you can get under it. Then (with engine cold) get a leaf blower, stick in an exhaust pipe and duct tape the other exhaust exits closed and tape the leaf blower in place. Get a bottle of soapy water and start spraying everything. I think you'll be surprised at where and how much it leaks-I sure was!
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