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Burning Smell

Got a burning smell after driving?

Modification Details

Burning Rubber Smell

I get a lot of emails about this and I'm surprised it has taken me this long to realise the information
requires a page of its own. I get these emails from people who have usually driven their car for a while,
usually on a motorway and usually in the height of summer or the middle of winter.

Is It Something To Worry About?

99.9% of the time, there is nothing to worry about however there is always that 0.1%.

Check that the rubber belts look OK, especially if you own an MHD 451.
Check that the alternator pulley is turning as the engine runs.
Check that the air conditioning compressor pulley is turning as the engine runs.
Check that the belt tensioner pulley is turning as the engine runs.
Check the tyre pressures all around the car.
Check the oil level is OK.
Check the engine bay insulation hasn't dropped down.

So, What Normally Causes The Burning Smell?

This is a common problem on cars with transverse exhausts (exhausts that run across the
back of the car instead of front to back like front engined cars). Let me talk you through it.

The problem is the position of the exhaust.

It is exposed, has a large surface area, is very hot and is in an area of low pressure.

Imagine the state of a race track after a Formula 1 race. The "dirty" part of the track is littered with rubber
marbles (F1 lingo) which are burnt pieces of rubber that were removed through friction and abrasion.

This happens on normal roads, however, the pieces of rubber are much much smaller. The movement of
the car, the pick up from the tyres and the lower pressure in the engine bay allows these tiny pieces of
rubber to be picked up and blown around. Some of it lands on the exhaust and sticks in place.

Over longer journeys, the exhaust gets hotter than normal and the amount of pick up increases.
The more rubber you pick up on the exhaust, the stronger the burning smell will be.

The Smell Is Different Sometimes

Yep, it depends on the weather. If it's a hot day, tyres are softer and lose more rubber.
This soft rubber is even more likely to stick to the exhaust. Also, on hot days the tarmac
of the road is hotter and the bitumen is also flicked up onto the exhaust.

If it has been raining, the water mixes with dust, road crap, dead animal roadkill etc and that smells.

If it's cold with a chance of snow, the roads are gritted or salted. The salt is often formed from urea, a by-product
of piss (usually cows). The grit abrades the tyres and that mixes with the salt to give you a urine and rubber smell.

What Happens To All Of This Stuff?

As with everything, it can only burn for so long. Eventually, all the shit that got
flicked up onto the exhaust will burn away, turn into a dry powder and will fall off.

It doesn't take long for this to happen. Usually it has gone by the next day.


43,000 fortwo 451s were recalled in America and 7,000 in Canada in 2018 due to the possibility
of the engine insulation matting dropping down and touching the exhaust. This is a fire hazard
that has, at the time of writing, possibly been the cause of 27 cars being destroyed by fire.

There is no difference between the engine matting in European models and the Euro5 cars have exactly
the same exhaust system as the US models. So there is no reason why this can't be an issue in Europe too.

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