rotten eggs...

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Active Member
Jan 13, 2008
Wolves, UK
Mercedes s320 limo business edition
Hi everyone, my car has been on sorn for the last few months.

Anyway its back on the road now. As per my last posts there is black smoke coming from exhaust when hard revved.

Today i took the car for a drive, i put shell v power fuel in it. On my way home i floored the pedal-the thing flies!!

However, after this i could smell rotten eggs & there was a misfire on idle too.

Im having it diagnosed at acms walsall (merc specialist), But what are your thoughts on what it could be?
Last time i had this (albeit on a BMW 328i Sport with the battery in the boot) it was the alternator going kaput and completely overcharging the battery, which was cooking nicely. See if the battery is particularly hot...
Cats smell like rotten eggs as they break down.

If your cats are blocked/blocking then they'll get too hot and melt. That's the sulfur smell you got.

I wouldn't floor it or drive hard again until you've had the cats inspected.

You can try cataclean, which works well, but it's not going to fix a melted cat.
I-conica, thanks for the advice. The car is going in for a diagnostic soon
The rotten egg smell is sulphur compounds, specifically/mostly hydrogen sulphide.
It used to be a more common occurence because petrol contained higher levels of sulphur, those days are long gone as sulphur levels have been reduced several times from something like 3000ppm in the ninties (according to the internet) to whatever todays ultra low sulphur blends contain
Catalytic converters can't produce hydrogen sulphide without a source of sulphur
Misfire at idle and black smoke sounds like massive overfuelling. Modern fuels can go stale relatively quickly which won't help and they're also fussier about contamination hence the fuel retailers association having documents like this in circulation... While the microbe thing is more about environmental blends of diesel i'm thinking any contamination with microbes etc could raise hydrogen sulphide levels? Maybe it's as simple as it's just running sooooo rich the cat isn't getting hot enough to 'light' and is seeing enough sulphur compounds to produce hydrogen sulphide?
I'd be exploring the battery like mattc43 says, particularly the charging circuits. Hot sulphuric acid (H2SO4 or hydrogen sulphate) smells pretty similar to hydrogen sulphide or H2S
Update..i put a bottle of cataclean into the tank & drove around for 15 mins. The black smoke has reduced vastly. Also the smell has reduced too, but the misfire is still there.
You've fixed the symptoms of the problem, and cleaned the cat up a bit, but as said if you've a misfire, that unburnt fuel will destroy your cat.

If you get a cheap £15 ODBII reader, and an app on your phone you can monitor the LTFT (long term fuel trim) which will show a rich condition, until that comes down your cat is going to be damaged.

Once the misfire is fixed I'd put another bottle of Cataclean through it.
Cheers i-conica. My car doesnt have an obd2 port (i have a obd2 scanner which i got off ebay). My cars got the round socket under the bonnet. I see what you mean about clearing the symptoms-good stuff that cataclean is. Will have it seen to at a specialist. This misfire was not there before, it came when i booted it & hasnt gone since although smell & smoke has gone

So im thinking theres something underlying which needs to be fixed

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