Smelly exhaust!

[camille79]

The exhaust fumes in my L-reg fiesta are exceedingly black and smelly when I first start the engine, but fine after a few minutes or if i drive a distance, stop then start again.  The oil is in need of being changed and I was wondering if this could be causing the problem or if it would be a problem with the actual exhaust itself.  Any ideas? 

[Clanad]

Not being familiar with your particular car, I have to rely on the fact that most internal combustion engines are very similar.  Black smoke is usually an indicator of engine oil entering the combustion chamber, which it's not supposed to do.  There are a number of reasons this could occur. Your description leads me to believe that you have one or more worn rings.  These are devices fitted on the piston that slide against the side of the cylinder wall.  There are usually at least two or three per cylinder.  When these wear, oil from the lubricating system, (which is under pressure) is making it's way past the rings and is being burned on the combustion stroke.  When the smoke ceases after a few minutes, it's because the engine has warmed up and the ring(s) are expanding and fitting better, but it starts all over when the engine is shut down and cools off. I have no idea what a ring job would cost in the U.K. on this car, but shouldn't be prohibitivly expensive... at any rate good luck!

[Tim Baxter]

Sorry clanad I don't agree with your assessment.  Clouds of black smoke indicate to me an over-rich mixture.  Clouds of blue/grey smoke indicate burning oil.  See http://www.scuderiaciriani.com/rx7/smoking.html

So it it's posible that the engine is being over-choked on start up but goes to normal mixture when warm.

Yes oil is pumped under pressure but not directly into the cylinders, the nearest oil should come is from the cylinder wall linings which should be scraped clean by the oil control rings.  It is possible that oil can enter the cylinder via the valve stem inlets from the rocker gear.

When and engine warms up parts do not fit better through expansion, all parts move apart when the get hot. (Remember how iron bands (tyres) are fitted to locomotive wheels?)

[Clanad]

The link you've provided, Tim, is for an RX7 Rotary engine.  My assumption was the engine in question was a standard 4 or 6 cylinder piston engine which are entirely different from the rotary. Having worked with aircraft engines for a number of years, the compression of each cylinder is taken when the cylinders are warm, since this provides the highest ratio measurement (therefore more realistic of engine health).  Rings and cylinder walls being of different metals do expand at different rates dependant on temperatures.  The piston rings, as you know, are not solid rings, but are split.  This split is a very controlled measurement for each piston application, to assure sealing once the engine is up to operating temperatures.  A minor point, perhaps, but the oil rings do not scrape the oil from the cylinder sides, but, rather leave a controlled film for the combustion pressure rings to have an oil film to ride on, since they do not actually touch the walls...

At any rate... good discussion...

[camille79]

Thanks for the advice - either way I'm guessing a trip to the garage is due! 

[Tim Baxter]

You're quite right Clanad, I've never heard of a rotary engine in a Fiesta either!  I missed off the quotes from scrape!  And you're also right about the ring/cylinder expansion!

Try this URL,

http://www.carcareclinic.com/motoriststips.asp

about 1/3rd way down page under heading 'petrol models', "Asa rule of thumb....."

[derekpara]

Come on, Camille79, don't keep us all guessing.

Have you been creeping uphill with your foot on the clutch or has a change of socks cured the problem ?

Please put us out of our misery. 

[derekpara]

Sorry, Camille79. Submitted my answer to the wrong person/ question !

 

It's going to be one of those days.