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Converter care

By unknown | Jan 23, 2008 | COMMENTS [ 0 ]

The catalytic converter (cat), compulsory on all new cars sold in South Africa since 1 January, is found on the exhaust pipe, fairly close to the exhaust manifold.

The catalytic converter (cat), compulsory on all new cars sold in South Africa since 1 January, is found on the exhaust pipe, fairly close to the exhaust manifold.

It often looks very similar to an extra silencer box, but the oxygen sensor, with electrical wires coming out of it, screwed into the exhaust pipe just upstream of the cat, is a reliable give away.

On many late model cars the exhaust manifold is hidden under cowls and plumbing, and therefore, your best chance of seeing the cat is when the car is on a lift and you can look from underneath.

It is placed close to the manifold so that it can warm up quickly from a cold start. One of the shortcomings of the present converter technology is that a cat can only function efficiently once its temperature reaches 400°C. For the first couple of minutes after a cold start, the cat is doing nothing at all.

On the other hand, too much heat is even worse for a converter, as we shall see below.

That explains why it is placed close to, but not inside, the exhaust manifold.

The three worst enemies of a catalytic converter are:

l overheating

l poisoning

l bashing

Let's look at each one more closely.

Overheating occurs when raw, unburnt fuel is allowed to get through to the converter, perhaps because a spark plug or plug wire is defective, or a fuel injector is malfunctioning, or the oxygen sensor is faulty.

Unburnt fuel vapour will ignite when it strikes the hot ceramic honeycomb inside the catalyst, and this will lead to a build-up of heat, which will drive the temperature far above the normal operating temperature of the converter.

It is not uncommon for the cat to glow red hot in such a situation. If the problem is not detected and rectified quickly, the ceramic material will melt down and turn into a solid mass inside the converter.

The melted ceramic may block the exhaust flow, leading to a loss of power at higher engine speeds, difficult starting, and heavy fuel consumption.

Converter poisoning occurs when the exhaust gas contains a substance which coats the surface of the catalyst and in this way prevents gas from coming into direct contact with the catalyst for the conversion to take place.

The most notable contaminant is lead, which explains why vehicles with catalytic converters must never be run on leaded petrol. One tankful is enough to render the converter permanently useless.

Another common catalyst poison is silicon, which can enter the exhaust stream if the engine has a leak allowing coolant into a combustion chamber.

Heavy carbon deposits, resulting from oil being burnt in the combustion chambers because of worn rings and degraded valve seals, can have the same effect. In addition, such deposits may clog the pores in the ceramic honeycomb and block exhaust flow.

Bashing the converter is never a good idea, whether it be from running over road debris, over-enthusiastic off-road excursions, or loose exhaust hangers which allow the converter to bounce around.

The ceramic honeycomb is rather fra- gile, and once it is fractured, the broken pieces come loose, rattle around and break up into smaller pieces.

Not only will this lower the efficiency of the converter, but it will eventually cause a partial blockage and increased back-pressure in the exhaust.

It is clear from the above that the best way to extend the life of your cat is to make sure that all engine systems which influence the composition of the exhaust gas are kept in tip-top condition.

These include the ignition system, the fuel-injection system, seals and gaskets in the engine, and all the sensors feeding information to the electronic control unit.

The oxygen sensor, placed just upstream of the converter, is particularly important for the well-being of the cat.

Remember, a malfunctioning converter is merely a symptom of a problem somewhere else.


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