991 gas smell when coming to a stop

[Benjy911]

I'm getting a natural gas smell (like turning the hob on but not lighting it), that appears for a few seconds when I've come to a stop. If I put the recirculation on it's not happening.

I recently replaced the battery and hadn't noticed it before that, so I've put the old one back in but now can't work out if I'm imagining the smell or not! I don't suppose anyone else has had this and worked out where it's coming from?

[hardlylikely]

Probably your catalytic converters are failing, normally described as a smell of rotten eggs. Mine do the same, but still passes the MOT ok, perhaps give it a good thrashing might help

[Benjy911]

I wondered about the cats but I'm not getting the smell when cold and I think I'd notice it when getting it in/out the garage. It's only done 39k miles, 2k of which were done in the last 10 days blasting through France!

I've also only noticed the smell with the roof up, which makes me think it's coming from the front near the air intake.

[JMandPM]

We have exactly the same problem October 25 with 4 month old 911 Carrera GTS, it’s just back from the garage today- they couldn’t smell it so obviously didn’t replicate it as sometimes the smell is so strong it smells like a mains gas leak. We will try to pin down exactly what factors are occurring when it happens. I will let you know when we eventually discover the cause.

[Mezzanine]

Could be the battery - they can really smell if they are venting/leaking?

[Ed.Neumann]

You did plug the vent hose back into the new battery when you fitted it?

[Discombobulate]

Ed.Neumann said:

You did plug the vent hose back into the new battery when you fitted it?

Thread here is about a 997, but gives you an idea (see images).
https://911uk.com/porsche/loose-tube-under-under-f...

[Roast Beef Racing]

Benjy911 said:

I wondered about the cats but I'm not getting the smell when cold and I think I'd notice it when getting it in/out the garage. It's only done 39k miles, 2k of which were done in the last 10 days blasting through France!

I've also only noticed the smell with the roof up, which makes me think it's coming from the front near the air intake.

Have you got a lump of camembert wedged in the front grille?

[Benjy911]

I didn't disconnect any vent hose but I don't believe the 991 has one, it's an AGM battery.

There's a long standing thread here from when they were new https://rennlist.com/forums/991/718086-gassy-smell...

If the fan speed is low it's not noticeable, but with it over 50% and crawling in traffic it's very noticeable once the car has warmed up.

Interesting to hear it being a problem still on a new one.

[7 CUU]

My brother had something similar to this many years ago.
Turned out to be the air con.
Perhaps try one of these .

[Ed.Neumann]

If it is the AGM battery it could be overcharging due to a failed cell.

Next time it does it, lift the hood and see if it smells strong near the battery.

Just to rule out a failing battery.

[Benjy911]

To add a few more notes, I've tried the following:

- recycle air - this stops the smell entirely
- aircon off - still smells
- turning the fan speed down - far less noticeable
- removed the battery cover to aid airflow round the intake - didn't make any noticeable difference
- standing behind the car when it's idling - no smell
- tried 3 different batteries (the original 2013, one from 2020 and the new one that's only a few months old) - still smells
- sport mode on/off as this does affect voltage into the battery - no difference
- stopped and lifted the bonnet when you could smell it in the cabin, no smell under the bonnet/near the battery and the battery isn't hot
- stopped brimming the tank as I started to notice it on the drive through France when I'd been filling it up at every stop

If anyone has any other ideas so I can pop out for a drive, I'm happy to try them but I do find it odd that some seem to have suffered from this from new.

[Ed.Neumann]

Have you had it from new?

Do you ever turn the climate off?

[Benjy911]

Just to add a further update for anyone else with the same issue.

I still thought it odd that this happened with new cars straight from the factory. Having done some research, the 991 is the first 911 that did away with a battery vent pipe as it uses an AGM battery. The issue with that is AGM batteries still vent gasses, and if they are in an enclosed space they are required to have a battery vent pipe. With the battery sitting next to the air intake for the fresh air into the cabin, it seems silly to not have the gasses being taken further away. I had initially thought they were sealed.

To try and resolve, I've plugged the positive side of the battery and run a vent pipe out down by the drivers arch off the negative side, I've only been for a short run but I couldn't smell anything...further testing to be carried out once the roads dry up!

Picture stolen from the internet to show the proximity of the battery and fresh air

[Discombobulate]

Bingo