Exhaust fumes in cabin of 2000 Honda Civic


About two years ago, I noticed the smell of exhaust fumes in the cabin of my 2000 Honda Civic any time the vents were set to pull air from outside the cabin. Switching the vents to recirculate would prevent the fumes. Looking under the hood, I found the exhaust manifold was cracked, so I replaced it along with the catalyic converter (actually one piece in this model) and two O2 sensors. Since the replacement, the problem with exhaust fumes remains. I have asked my mechanic to look at this a couple of times, and he could not find the cause. I’m looking for suggestions on what to try next to diagnose the problem. We use the car frequently, and just have to leave the vents set on recirculate. This becomes an issue on rainy, humid days when recirculate causes the windows to fog. Any suggestions would be much appreciated.


Do you have a carbon monoxide sensor? You might bring one into the car and test the air with the recirc on and off. That will tell you if you have a CO build-up. You can always buy one if you don’t have one and use it in your house after testing the car. Most run on 120-AC; you may need a converter for the car, too.


Other sources of exhaust fumes under the hood include the EGR and riser pipe. There is also a sensor on the EGR pipe that checks for EGR flow. This sensor generally has two small pipes and flex lines that attach to the EGR riser pipe. A break or crack in any of these can also allow exhaust fumes into the engine compartment.


What about a slow leak around the valve cover seal? I usually see some grim around the seal. If I wipe it clean, it will build up again in a few weeks. Could that leaking oil be burning when the block get hot and causing fumes? I haven’t had to add oil to the car between changes, so it can’t be leaking much oil.


Hi Jonblack1234,
I have the same car and the same smell. I’ve taken it to my local mechanic and he is stumped so I did some research and found your link.

Did you find the problem that was causing this smell?


It could be something other than exhaust fumes being noticed. Checking for CO as mentioned above is a good idea. Oil leaks getting on something hot will cause a stink in the engine compartment which could easily get pulled in through fresh air vents. And might mimic the smell of an exhaust leak. As will transmission fluid leaks.

Are there any fluid drip-stains under where you park the car at night? If so, replacing the suspect valve cover gaskets might be a good idea. I had some oil drops under my Corolla, couldn’t tell exactly where they were coming from but it seemed like the valve covers might be leaking so I replaced those gaskets, fairly simple and inexpensive job, and the under-car drips totally went away. In my case there was no odor problem though. Well, not with the car at least … lol …


Thanks there is a minor oil leak on the front main seal, 200K on the motor and trans but nothing major and below the exhaust system. I’ve read a few issue with PCV, hose, etc…



Ask your shop to use an evap/smoke machine to find any possible exhaust leaks

That’s what I do, and it is very effective