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Catalytic converter - general information, checking and component renewal
Emissions control systems / Catalytic converter - general information, checking and component renewal


General information
1 The exhaust gases of any petrol engine (however efficient or well-tuned) consist largely (approximately 99 %) of nitrogen (N2), carbon dioxide (CO2), oxygen (O2), other inert gases and water vapour (H2O). The remaining 1 % is made up of the noxious materials which are currently seen (CO2 apart) as the major polluters of the environment: carbon monoxide (CO), unburned hydrocarbons (HC), oxides of nitrogen (NOx) and some solid matter, including a small lead content.

2 Left to themselves, most of these pollutants are thought eventually to break down naturally (CO and NOx, for example, break down in the upper atmosphere to release CO2) having first caused ground-level environmental problems.

The massive increase world-wide in the use of motor vehicles, and the current popular concern for the environment has caused the introduction in most countries of legislation, in varying degrees of severity, to combat the problem.

3 The device most commonly used to clean up vehicle exhausts is the catalytic converter.

It is fitted into the vehicles exhaust system, and uses precious metals (platinum and palladium or rhodium) as catalysts to speed up the reaction between the pollutants and the oxygen in the vehicles exhaust gases, CO and HC being oxidised to form H2O and CO2 and (in the three-way type of catalytic converter) NOx being reduced to N2. Note: The catalytic converter is not a filter in the physical sense; its function is to promote a chemical reaction, but it is not itself affected by that reaction.

4 The converter consists of an element (or substrate) of ceramic honeycomb, coated with a combination of precious metals in such a way as to produce a vast surface area over which the exhaust gases must flow; the whole being mounted in a stainless-steel box. A simple oxidation (or two-way) catalytic converter can deal with CO and HC only, while a reduction (or three-way) catalytic converter can deal with CO, HC and NOx.

Three-way catalytic converters are further sub-divided into open-loop (or uncontrolled) converters which can remove 50 to 70 % of pollutants and closed-loop (also known as controlled or regulated) converters which can remove over 90 % of pollutants.

5 The catalytic converter fitted to the Mondeo models covered in this manual is of the threeway closed-loop type.

6 The catalytic converter is a reliable and simple device, which needs no maintenance in itself, but there are some facts of which an owner should be aware if the converter is to function properly for its full service life.

(a) DO NOT use leaded petrol in a vehicle equipped with a catalytic converter - the lead will coat the precious metals, reducing their converting efficiency, and will eventually destroy the converter; it will also affect the operation of the oxygen sensor, requiring its renewal if leadfouled.

Opinions vary as to how much leaded fuel is necessary to affect the converters performance, and whether it can recover even if only unleaded petrol is used afterwards; the best course of action is, therefore, to assume the worst, and to ensure that NO leaded petrol is used at any time.

(b) Always keep the ignition and fuel systems well-maintained in accordance with the manufacturers schedule (Chapter 1) - particularly, ensure that the air filter element, the fuel filter and the spark plugs are renewed at the correct intervals. If the intake air/fuel mixture is allowed to become too rich due to neglect, the unburned surplus will enter and burn in the catalytic converter, overheating the element and eventually destroying the converter.

(c) If the engine develops a misfire, do not drive the vehicle at all (or at least as little as possible) until the fault is cured - the misfire will allow unburned fuel to enter the converter, which will result in its overheating, as noted above. For the same reason, do not persist if the engine refuses to start - either trace the problem and cure it yourself, or have the vehicle checked immediately by a qualified mechanic.

(d) Avoid allowing the vehicle to run out of petrol.

(e) DO NOT push- or tow-start the vehicle unless no other alternative exists, especially if the engine and exhaust are at normal operating temperature. Starting the engine in this way may soak the catalytic converter in unburned fuel, causing it to overheat when the engine does start - see (b) above.

(f) DO NOT switch off the ignition at high engine speeds, in particular, do not blip the throttle immediately before switching off. If the ignition is switched off at anything above idle speed, unburned fuel will enter the (very hot) catalytic converter, with the possible risk of its igniting on the element and damaging the converter.

(g) Avoid repeated successive cold starts followed by short journeys. If the converter is never allowed to reach its proper working temperature, it will gather unburned fuel, allowing some to pass into the atmosphere and the rest to soak in the element, causing it to overheat when a long journey is made - see (b) above.

(h) DO NOT use fuel or engine oil additives - these may contain substances harmful to the catalytic converter. Similarly, DO NOT use silicone-based sealants on any part of the engine or fuel system, and do not use exhaust sealants on any part of the exhaust system upstream of the catalytic converter. Even if the sealant itself does not contain additives harmful to the converter, pieces of it may break off and foul the element, causing local overheating.

(i) DO NOT continue to use the vehicle if the engine burns oil to the extent of leaving a visible trail of blue smoke. Unburned carbon deposits will clog the converter passages and reduce its efficiency; in severe cases, the element will overheat.

(j) Remember that the catalytic converter operates at very high temperatures - hence the heat shields on the vehicle underbody - and the casing will become hot enough to ignite combustible materials which brush against it. DO NOT, therefore, park the vehicle in dry undergrowth, over long grass or piles of dead leaves.

(k) Remember that the catalytic converter is FRAGILE. Do not strike it with tools during servicing work, and take great care when working on the exhaust system (see Chapter 4). Ensure that the converter is well clear of any jacks or other lifting gear used to raise the vehicle. Do not drive the vehicle over rough ground, road humps, etc, in such a way as to ground the exhaust system.

(l) In some cases, particularly when the vehicle is new and/or is used for stop/start driving, a sulphurous smell (like that of rotten eggs) may be noticed from the exhaust. This is common to many catalytic converter-equipped vehicles, and seems to be due to the small amount of sulphur found in some petrols reacting with hydrogen in the exhaust, to produce hydrogen sulphide (H2S) gas; while this gas is toxic, it is not produced in sufficient amounts to be a problem. Once the vehicle has covered a few thousand miles, the problem should disappear - in the meanwhile, a change of driving style, or of the brand of petrol used, may effect a solution.

(m) The catalytic converter on a wellmaintained and well-driven vehicle should last for between 50 000 and 100 000 miles. From this point on, careful checks should be made at regular intervals to ensure that the converter is still operating efficiently. If the converter is no longer effective, it must be renewed.

Checking
7 Checking the operation of a catalytic converter requires expensive and sophisticated diagnostic equipment, starting with a high-quality exhaust gas analyser. If the level of CO in the exhaust gases is too high, a full check of the engine management system must be carried out (see Section 3 of this Chapter) to eliminate all other possibilities before the converter is suspected of being faulty.

8 The vehicle should be taken to a Ford dealer for this work to be carried out using the correct diagnostic equipment; do not waste time trying to test the system without such facilities.

Component renewal
9 The catalytic converter is part of the exhaust system front downpipe - see Chapter 4 for details of removal and refitting.


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