Sat Oct 22 14:07:22 SAST 2016

If your car's not smoking, relax

By unknown | Jul 07, 2010 | COMMENTS [ 0 ]

I DRIVE a 1998 model Toyota Tazz with 228000km on the clock. The car has been using about half a litre of oil every 1500km since I bought it in 2008.

Is this to be regarded as normal oil consumption or do I have to start saving up for an engine overhaul? The engine doesn't smoke and the oil consumption has not increased noticeably since I bought the car.

Also, is it true that modern cars with catalytic converters in the exhaust system can tolerate very little oil consumption because the cat will be contaminated?


IF THE engine isn't smoking and the oil consumption isn't increasing, I wouldn't worry too much about it.

Oil consumption varies from car to car even on brand-new cars because no two cars come out of the factory with identical clearances inside their engines. There will always be slight variations due to machining tolerances.

To allow for this some manufacturers regard anything up to a litre for every 1000km as normal.

As you will know, oil that gets past the oil control rings on the pistons or seeps between the valve stems and their guides, ends up in the combustion chambers where it is burnt together with the air-fuel mixture.

What is perhaps not so well known is that movement past the rings is not a one-way traffic from the sump to the combustion chambers.

While the oil migrates upwards un-burnt fuel and combustion products, such as water vapour, migrate downward to the sump.

On an engine that's very close to blueprint specifications the quantities involved in both migrations will be minute, but there will always be some migration. It might happen that the upward migration of oil is matched precisely by the downward migration of fuel and vapour, in which case the oil level on the dipstick will show no drop at all.

If the amount of fuel and vapour going down exceeds the amount of oil going up, the oil level on the dipstick might even rise, as sometimes happens if a car is used only for short trips and the engine never gets hot enough for the contaminants to be vapourised and recycled.

Reassuring as it might be for an owner to see that his car doesn't use any oil, the oil in the sump is steadily diluted and contaminated by the material coming from the combustion chambers.

This is one reason why regular oil changes are so important, especially on an older engine. It's also the reason why, if you want to be kind to a car that is used exclusively for short trips, you should take it out on the freeway once a month for a full-bore blast at 120kmh so that the oil can get hot enough for long enough to drive off the volatile components.

In your case I would watch the situation carefully and make sure the oil you use isn't too thin. And check that there are no oil leaks on the engine. An oil leak as slight as one drop every 100 metres will show up as a consumption of 500ml every 1000km, not counting the drops that fall while you are waiting at traffic lights.

Resist the temptation to change to a thicker-than-specified oil and steer clear of miracle additives. These might provide short-term cures but will do more harm than good in the long run.

The surest way of deciding if the oil consumption is excessive is to look at the spark plugs. If you see the characteristic, wet, oily black deposits indicative of oil-fouling, you will know that the engine can no longer cope with the amount of oil entering the combustion chambers and an overhaul is on the cards.

Your information that catalytic converters are sensitive to unburnt or partially burnt oil vapour in the exhaust gas is correct. Oil vapour will clog up the chemically active surfaces with which the exhaust gas has to come into contact for the cat to do its job and certain anti-wear additives in the oil will inhibit the desired chemical reactions.

The oxygen sensor, an essential component in the feedback system used to regulate the fuel mixture, may also be affected, which will lead to an overly rich mixture, causing heavy fuel consumption and further compounding the problems for the cat.

Sometimes we have reason to be thankful that we drive older cars without all the modern gadgets.


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