Well sounds like the problem isn't THE exhaust ,but what your EXHAUSTING..,Running rich. Have you tuned the engine using a vacuum gauge? And if so what were your readings? A vacuum gauge is like ah machanical computer It'll tell you every little secret your engine is hiding.

M1Mustang69 wrote:

Hi Everyone, it has been a while since I've been here.  Much has been done to the car over the past 1.5yrs.

I am writting cause I can no longer tolerate this Problem.  I get headache's, Migraines and often avoid driving the car cause of the FUMES ENTERING the cabin while idling or driving with THE WINDOWS DOWN.  it is a 69 Mustang Fastback, REBUILT and modified 351W (done less than a year ago), Long tube headers, 2 1/2" Magnaflow exhaust, T5.  I have gone over as many holes and plugged them, filled them with expansion foam ect ect.  I have extended the tailpipes past twice (maybe still not far enough) but my concern is they'll look stupid too Far Out.......

CAN ANYONE HELP????  Only happens when windows are down!  even just a Crack!

Many Thanks,
Jeff

Not that this is your problem, but I recently had a exhaust leak, and although i couldnt see any holes in the exhaust, when I took it apart,  the mufflers had holes on top, the intermediate pipes had huge holes too.   These peices are right under the driver seat. 

is it exhaust or is it fuel?  Header gaskets?  gaskets between headers and pipes?  Heads have any openings that need plugs?  Firewall openings all sealed? door seals good? trunk seals good? Fuel leak?  fuel line leaking fuel? 

M1Mustang69 wrote:

Thanks Maarkin.

I appreciate your idea's, and some could apply. But this occured with old stock motor with new exhaust (brand new headman headers and magniflow X-pipe system w/mufflers). Engine is now rebuilt blue printed and all. Its essentially ALL new as is everything attached. And this only occurs when the windows are opened. Even if they're opened only a half inch.

I'm lost. Its the smell of combusted (or richly combusted) fumes. No fuel leaks, as far as I can tell all holes are plugged, firewall included. My trunk vallence is damaged (pushed in) and the seal doesn't sit right, but in my opinion, this should be a problem esp or even with the windows closed.

I don't know what the heck the problem is

any holes in the floor or in the trunk, since its a fastback, it will come right in from the trunk. 

what if you are standing outside the car while its idleing with the windows closed
 

I used expansion foam all in the quater extension and have doubled up the trunk seal with that generic tape on foam stuff.  Its getting squashed.

All this doesn't explain why it only occurs when the windows are open, and keep in mind even just a half inch.  Soon as they're shut, no problems.  It vexes me......I'm vexed

Opening the windows can create a natural draft. If you think the smell is comming from rear exhaust go to the extreme. Get 2, 4 feet pipe and place them on the rear. I'm not saying drive it like this. If you still smell something, you must have a leak some where.

M1Mustang69 wrote:

I used expansion foam all in the quater extension and have doubled up the trunk seal with that generic tape on foam stuff. Its getting squashed.

All this doesn't explain why it only occurs when the windows are open, and keep in mind even just a half inch. Soon as they're shut, no problems. It vexes me......I'm vexed

If that leak is directly under your driver seat, then you would notice it when you open the window.  Its an all new exhaust so i'm sure you have it installed correctly with no leaks or missing gaskets at the collectors, etc.

I do think there is some merit to what the others are saying about there not being any  pressure when the windows are closed and then when you open them, a vacuum is created that would suck in air from everywhere it could. KInda like if you have central air in your home, there are vents that supply the cold air into the room, but if the room is already filled with air, it cant force the cold air into the room. so you have return vents at the ceilings in each room or a central return vent in the center of the home where the central air will be able to push in the cold air into the room because the other vents are actually pulling air out of the room into the return vents making room for the cold air to come into the room via the cold air vents at the floor.  Man i'm bad at explaining that.

also check your gas tank, it could be rusty on top and actually have tiny holes in it. not sure if the gas tank is part of trunk floor on your year mustang or not. 

Last edited by MarkinSC (5/17/2013 7:33 PM)