My 88 Firebird was running a straight pipe as the exhaust had been cut off by previous owner and it's been fine just loud, but no fumes. Last week it started getting fumes inside the cabin. finally had some money and just took it in and got exhaust done with flowmaster 80 series out back. Sounds great no cat. Thought that would get rid of my fume problem but it seemed to get worse. The fumes are only stronger. First thing I did was check exhaust manifold bolts as it has new manifold gaskets. 2 bolts were loose on driver side, I tightened them down. Next made sure bolts on manifold flange were not loose, they weren't. I've been spending the last 2 days covering all holes in the firewall. Today I remembered the fuel pump access hole was open so I sealed that . At idle with the windows up, I don't get any fumes. Slow driving I don't really get any fumes. Once I get going 40+ the fumes start coming. I usually start smelling the fumes on deceleration. Soon as I let off The gas it's like the exhaust is getting piped in to the cabin. Also fumes are harshest on turns and deceleration, it doesn't matter if windows are up or down. The fume strength is the same. Going crazy with this. My next step is rear hatch weatherstripping. But that wouldn't make sense, bc windows up or down gives me fumes. Where else should I look?

 

More info, my charcoal canister has the tank vent line running to it, that's it, one other port is plugged, and the other line going to nowhere, there's really no gas fume smell while running or not.

 

So thanks for the replies, but I have been filling every hole in the. cabin trying to stop the exhaust smell from entering. There are no other holes where I could imagine exhaust coming in. Yet with the windows up or down I get exhaust when moving only, not at idle. Makes me wonder about the rear hatch seal. Looks fine to me but IDK where else the exhaust could enter. How would I know if the hatch weatherstripping was bad? How can I test if it's sealing? Also would it make sense to get fumes while windows down bc rear hatch seal?

 

Is the blower motor normally on or do you have the switch on vent? Wondering if the fumes are coming off the engine into the blower motor vents in front of the windshield. Only other place I can think of. If your interior is weather proof, i.e. no rain rain gets inside, you should not be getting fumes through the weather stripping.

 

No heater or ac box installed, PO removed, so all the holes from ac box on firewall have been sealed, I've triple checked firewall inside and out and there are no holes for fumes that I can see. Been extensively checking exhaust and it doesn't sound or look like there's any exhaust leaks. I just temporarily taped the back hatch up, tried to make as airtight as possible, so I will know tomorrow morning on my commute home if it was the weatherstripping.

 

Was messing with charcoal canister And pulled vent line from tank on top of charcoal canister and blew into the line. I heard gas bubble then I couldn't put any more air into line, I opened gas cap then gas started coming out of vent line for charcoal canister. Gas wouldn't stop coming out of the line so got a bolt and plugged it, why would this happen

 

So after taping up rear hatch best I could, fumes are noticeably less, still a little there. With windows up, hardly any fumes penetrate inside, but there's still a little smell. Is weird on turns I get strong whiffs of exhaust. Does anyone have a pic with all the 4 fuel lines labeled on top of gas tank? Bc I'm thinking vent line might be swapped with return or something by I was getting gas out of vent line until I plugged it.

 

I replaced all the lines going to the canister including the "canister purge valve". This little valve has small springs and rubber valves that corrode over time. They are calling it a "tank pressure control valve" in the diagram.= http://www.oreillyauto.com/site/c/de...21s%21solenoid


It looks like this.

 

hood to cowl seal missing?

 

Bottom inboard face of the evaporator housing crumbled to dust and created a gaping hole, right above the exhaust manifold?

 

OP says heater box deleted and firewall sealed.

 

Where exactly is the gas smell coming from? Is it only when or after it's been running? It could be you're running really rich.