lokokkee

I noticed that my MY97 WRX (stock standard with 28K on the clock)has a rather sooty (i.e. black, as against greyish) exhaust tail pipe. Is this normal or is the air/fuel mixture too rich? I thought the ECU is supposed to take care of this. I have been doing mainly urban commuting lately and has not taken it on a long drive since buying it second hand less than a month back, but the journeys are at least 20 km a piece, allowing the engine to warm up sufficiently.

Stuart Knight

iTrader: (2)

Don't worry, they all do this. Something to do with Unleaded fuel, and the standard ECU tends to run rich (so I'm told).

John H-S

Yeah - mine does this too - 99my 7500 miles.
I just clean the sooty stuff off when she gets a wash and the tailpipes come up as new.

The car does look a bit of a mess from the back tho'

IainY

My MY98 15000 miles does it too!

lokokkee

Checked with the local dealer and was told too it has something to do with unleaded fuel. But the car was never designed nor intended to run on leaded fuel anyway with cat. fitted as standard, so the excuse is rather lame. Can anyone tell me if things improve after a long hard drive, where supposedly the mixture runs leaner?

John H-S

Mine never changes - after a hard thrash it's still sooty.

S.Mather

I picked up my RB5 yesterday and with 156 miles on the clock i too noticed a sooty pipe, reading all the replies now puts my mind at rest.

Thanks!

Pete Croney

Turbo engines overfuel.

The action of the fuel misting into the intake air helps cool the intake air and reduce the temperature of the charge, inside the cylinder. This helps reduce the incidence of detonation, as the fuel is compressed.

Putting too much fuel in adds to the cooling process, with negligable effect to burn efficiency.

For car manufacturers, this practise will be outlawed in years to come, as emission regs get tighter and tighter. Water injection will probably become more common on turbo charged cars.

lokokkee

Pete, thanks for the comment on turbo overfueling. I'm under the impression that multi-point fuel injection directs fuel straight into individual cylinders and not spraying it into the intake manifold, hence there is no cooling effect on the charge. Are you suggesting that's what Subaru is doing on the Impreza that leads to sooty exhaust?

Nick

WRT Turbo engines overfuelling.....

My BMW 333i Turbo runs on leaded fuel & has a Zytek management system. The tailpipe after a run, is a nice light grey colour which puts my Scoob to shame. As John mentioned, the soot cleans off easy enough though.

RaymondH

My first post............
A friend of mine who runs his own garage business tells me that the sootiness in the exhaust of a car running on unleaded is nothing to do with the mixture being rich. If a car is run on leaded fuel it is the lead itself which colours the exhaust but it will be darker if that car is running too rich, if you see what I mean.

Raymond.

lokokkee

Raymond, if the color of the exhaust is due to the difference in leaded and unleaded fuel, then the sootiness should apply to most cars running on unleaded. My other car (fuel injected, non-turbo)has always produced a nice greyish exhaust on the same fuel without the latest ECU gadgetry.

Darren Soothill

Try looking at the exhausts of some of the Turbos after a trackday though and you will find that the exhausts are a light grey/brown colour as you would expect the biggest thing with the Impreza compared to other cars is that it is so difficult to push it hard for a reasonable amount of time and then to stop quickly and look at the exhaust.

If you listen to cars that have been decated or some with backboxes like Scoobysports you will very often hear them bang when they are coasting down as this extra fuel is ignited in the exhaust system.

Darren

fubar

I get similar black soot on my MY99 turbo.
I've only used SUL from day 1, so I guess it's not just NUL that causes the problem.

lokokkee

If Subaru is overfuelling the Impreza Turbo, this may be one reason why that model is not sold in US as it supposedly failed to meet the tough emission standards there. If this is so, then it is a great commercial sacrifice on their part to give up what can be potentially its biggest market in order to extract the last few bhp from the car. Can the idling period while on the turbo timer be the culprit if as Darren claims the exhaust is normal after a hard day at the track?