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In my 2003 Opel Vectra, the cabin air smells of Diesel exhaust while stopped at e.g. a red light.
The exhaust smell goes away when driving, and it also doesn't come back if I turn the ventilation off before stopping at a light. It's not temperature-related: it happens whether it's cold or warm outside, and also whether or not the engine has reached its normal operating temperature. It also happens whether or not the A/C function of the ventilation system is turned on. The car's windows are always closed.

It seems to me that there is some problem with the car's ventilation and/or exhaust system. I'm fairly sure it's not a problem with the A/C itself as it's not a chemical odor but clearly an exhaust smell.

Asking the dealer, he told me that the car has a Diesel-powered auxilary heater that engages when it's cold outside and the engine isn't warm yet. While that heater is working, the heated cabin air might have a weak Diesel smell.
- This explanation would make sense to me if it didn't happen in the summer too (when the aux heater is presumably not used), and if it did happen while driving, but neither is the case.

1. What could be wrong with the car?
2. Is it dangerous to drive? (carbon monoxide gases)

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I retagged ac with hvac as ac deals more specifically with air-conditioning itself and hvac deals with the ventilation system as a whole –  Larry May 2 '11 at 14:20
    
Thanks! I tried "ventilation" but it was rejected, so I entered "air-conditioning" which was then converted to "ac". Please consider setting up a tag alias for "ventilation"->"hvac". –  Torben Gundtofte-Bruun May 2 '11 at 14:47
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3 Answers

up vote 11 down vote accepted

My first concern is that you have an exhaust leak. The exhaust is designed to exit at the rear and or side of the car, far away from your fresh air intake of the ventilation system in the cowl area. I would check for an exhaust leak starting at the front as you stated that it's coming in from through the ventilation system.

This can be dangerous and should not be ignored, you could be overcome by the exhaust which can contain Carbon Monoxide.

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Update: The problem was indeed a leaky exhaust system! Alarmingly, the service shop I had tasked with the repair did not find this leak, but another authorized dealer found and fixed it.

I'm horrified that a repair shop can manage to not find a fault like this, but glad to note that I'm not stupid: there really was a problem. Since then, I've sold the car for unrelated reasons.


Original post:

To follow up, here is how the problem was solved:

The A/C was cleaned and disinfected, the coolant was replaced, and the cabin air filter was replaced by a carbon filter. This is something I like to have done every few years, for the sake of the health of the passengers, and it was long overdue on this car.

Still not sure where the smell came from, but apparently the new filter (and the cleaning) takes care of it.

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I would still check for an exhaust leak as Larry suggested. The smell may be gone but carbon dioxide itself is odorless which is one of the things that makes it so dangerous. The filter may be taking care of the unburnt fuel in the exhaust leak but not the carbon monoxide. It previously going away at speed can be explained by the increased air flow moving the exhaust gas away before it is pulled into your HVAC system. –  ManiacZX Jun 11 '11 at 18:46
    
That makes a lot of sense, @ManiacZX. The service shop that did the A/C work said they'd checked the exhaust for leaks and found none, but you make me want to invest money in a second opinion... I certainly don't want to overlook carbon monoxide! –  Torben Gundtofte-Bruun Jun 11 '11 at 18:54
    
Smoke is a good way to detect air flow items such as leaks. You'd start up at the exhaust manifold and work your way down. The smoke lets you visibly see what the air is doing. Smoke machines are pretty expensive though for a one time use, a lot of people try to rig up a home brew solution, just make sure you are doing it safely and keep a fire extinguisher around as the classic saying goes "where there is smoke, there is fire". –  ManiacZX Jun 11 '11 at 19:02
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I am not a mechanic, not even a homebrew one... thanks anyway :-) Is there a cheap way (kind of test strips, or canary birds, or something) to test for CO presence in the cabin, rather than actually checking the piping? –  Torben Gundtofte-Bruun Jun 11 '11 at 19:22
    
I suppose you could try a home smoke/carbon monoxide detector that operates off of a battery, just take it in the car with you. –  ManiacZX Jun 14 '11 at 1:42
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A Diesel Engine will not emit carbon monoxide no matter what. The Diesel engine, even at idle, has a ratio of fuel / air much-much lower than a typical gasoline engine. Therefore there's more than enough Os for the Cs (oxygen for the carbon) so no monoxide will ever be produced.

There could be other harmful gases in the exhaust or at least it's annoying to smell. It simply shouldn't happen so there definitely is a problem, most likely, as you reported, an exhaust leak.

It can also happen while going in reverse or when there is a strong wind from behind.

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