B8 S4 Exhaust Vapour

[Mr Scruff]

My B8 S4 Avant likes a good smoke - to the extent that I need to use the wiper to clear the rear screen!

Obviously all cars do this when it’s cold but I did a 40 mile journey today and it was still gently smoking away when I got home!

It’s white, so not oil, but the car doesn’t use any oil or coolant. Any idea what it might be?

[catso]

Mine does it when it’s cold. Pull up at a junction and the rear screen fogs, needs a wipe to clean. Just water vapour after a cold start.

Also it gets very dirty on the rear so the aerodynamics obviously encourage it to stick to the screen.

[Mr Scruff]

It’s not the fact it smokes a bit that bothers me so much, it’s the length of time it continues for.

Previously I’ve been used to this stopping within a mile or two, but it was a decent length run yesterday and still did it afterwards.

[catso]

Sounds like something's wrong, maybe something to do with cold enrichment or excessive moisture/condensation? Does the car live outside?

I posted re. mine because it's the first car I've ever known that fogs it's own rear screen from the exhausts, but as above mine only does it for the first few minutes/miles until it's warmed up.

[Adrian E]

Could just be environmental/usage? Our S5 is doing a motorway commute regularly at the moment and it joins the motorway with the temperature gauge barely registering. Takes a good couple of junctions (10 miles) before the coolant is fully warmed up and I doubt the oil is up to temperature until it's been sitting in traffic for a bit after that to allow the heat to soak into the oil. If your usual usage is fairly light load it can take an age to generate enough heat from the engine to actually heat the exhaust system through beyond the cats, which is where all the condensation originates from as the gases cool after leaving the engine.

If it's still doing it in April I'd be thinking there's something amiss, but if fuel economy is as bad as it usually is, rather than much worse, I'd not worry.

[anonymous-user]

Adrian E said:

Could just be environmental/usage? Our S5 is doing a motorway commute regularly at the moment and it joins the motorway with the temperature gauge barely registering. Takes a good couple of junctions (10 miles) before the coolant is fully warmed up and I doubt the oil is up to temperature until it's been sitting in traffic for a bit after that to allow the heat to soak into the oil. If your usual usage is fairly light load it can take an age to generate enough heat from the engine to actually heat the exhaust system through beyond the cats, which is where all the condensation originates from as the gases cool after leaving the engine.

If it's still doing it in April I'd be thinking there's something amiss, but if fuel economy is as bad as it usually is, rather than much worse, I'd not worry.

Exhaust temp not much to do with coolant and oil temp. Exhaust gases are very hot from the moment you start up and the exhaust gets to temp quicker than the engine - and much less affected by ambient temperatures. Clouds of condensate after 40 miles means a) exhaust not warm enough to prevent condensation (unlikely) or b) excess water (coolant) in exhaust (eg from head gasket).
I would check coolant level and oil cap for “mayonnaise”.

[Mr Scruff]

It’s definitely a bit odd. In the 1,500 miles ive had the car it’s used absolutely no oil or water, both at the max lines. No mayo under the oil cap (which ties in with no coolant usage)

Can’t figure where it could be coming from, the car drives normally and mpg is as expected around 21-23. Only thing I was wondering was whether the exhaust is past its best.

I have a 200 mile trip coming up start of Feb, be interested to see if it’s doing it at the end of that run too.

[Adrian E]

Discombobulate said:

Exhaust temp not much to do with coolant and oil temp.

true - it was more that stored heat in the exhaust pipework itself is going to take an age due to engine heat not soaking through when it's cold/damp weather. The length of time for fluids to warm up obviously has a small part to play in that.

I suspect on a wet day the spray off the road keeps the exhaust cooler than on a mild dry day and it just allows condensation to reach the tailpipes for longer?

[Adrian E]

Mr Scruff said:

Only thing I was wondering was whether the exhaust is past its best.

I have a 200 mile trip coming up start of Feb, be interested to see if it’s doing it at the end of that run too.

Ours is now getting on for 6.5 years old and the exhaust is in fine fettle - if anything a nice bit noisier than when we bought the car at 2 years old. I doubt you'll ever 'need' to replace the exhaust - we sold our C5 A6 with the OEM pipes still on it at 10+ years old and it still didn't have any sign of corrosion beyond the usual surface layer.

200 miles of dry and relatively loaded driving (as in engine load) and I'd be surprised if you saw any condensation - if it's a typical damp and cold day you may still get a whisp.