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
I know this is an old thread, but I’m having this problem, but it’s never in summer with a/c. The last 2 winters it started when I had to start using heat. Last winter it actually stopped eventually so I forgot about it. Here I am beginning to use heat again and it’s back, for now anyway. When I warm up the car, I’ve stopped turning on the heat and that seems to keep the smell from building inside the car, but I can smell the fumes pretty strong around the outside of the car. Once I’m driving and not idling the smell is faint (I think it just comes in when I open the door to get in) and eventually fades out. Sometimes I crack the window while driving to pull the smell out. I’m looking for answers, but I don’t see where anyone said they found a solution. I’m hoping to eventually find an answer. I have no idea if this will stop like last winter or continue on.
A mechanic should be able to check your exhaust system for any leaks, and check if any oil is dripping onto it, which causes fumes. Do you see any oil leaks?
What is the make model year miles for your car?
OK , once again I don’t understand this situation . Someone has a problem with smelling exhaust fumes that can make a person very ill or even be fatal … Instead of having a shop solve this they ask a bunch of strangers on the web what to do . 
Because I have found tons of solutions to different problems with both of my cars by seeking online where people have shared when the same situation happened with them and they already did the troubleshooting. Why reinvent the wheel if not necessary. If I don’t find a way to fix it then I will gladly take it to my mechanic.
Unless you have the equipment needed to elevate your car so you can safely inspect the entire exhaust system, and have access to an exhaust leak detector, this is better left to a pro. And it is a major safety issue, if it is an exhaust leak.
There’s a good locally-owned muffler shop in many towns. That’s where I take my exhaust problems. They’ve seen it all and know where to look.
If it’s only happening when your car’s heat is running, and not happening when the A/C is on, it’s likely a leaky and breaking heater core. Have you noticed any coolant loss? This would be the telltale sign that it is in fact the heater core causing your issue. Do you notice any smell from the heater on a cold start?
Are you still driving around using the original muffler? The muffler on my 1998 Civic rusted out at a joint several years ago, and I live in Florida, not the rustbelt.
What is important here is to:
- Get the entire exhaust system checked. You could have a leak anywhere.
- Don’t go by the smell you perceive to be exhaust fumes. You could be inhaling exhaust fumes even when you don’t perceive the scent.
I’ve seen someone have to dump a car because she couldn’t find why exhaust carbon (monoxide? dioxide? both? I can’t remember.) was getting into the cabin of her car.
Probably Carbon Monoxide…that can kill you, and it’s completely odorless and tasteless.
Please explain how a heater core that only carries coolant would emit exhaust gasses into the cabin. Unless maybe the head gasket is gone too.
+1
I think we have to consider that a lot of folks are not able to distinguish between the somewhat obnoxious odor of coolant and other automotive odors. But, the reality is that carbon monoxide–which is odorless–is ultimately deadly for anyone who subjects himself to it for longer than a short period of time.
There are a lot of automotive and home-related issues that I will tackle myself, but anything related to exhaust fumes is best left to a professional.