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Discussion Starter #1
The engine condition, mileage and the use to which the vehicle has been put should also be taken into consideration. The most troublesome vehicles tend to be big-engine low mileage, shopping cars, which never get to full operating temperature. These can and will display all types of elusive problems not only idle/emission related. Renewing emission related parts with this type of vehicle might temporarily reduce emissions to below the legal limit, but the fix will only be temporary and the customer should, at least, be made aware of this.

The minimum test equipment required to diagnose emission-related faults is an accurate four-gas tester, the type used for MOT’s is fine. Ensure that it is regularly serviced / calibrated. Right, now we’re up the first rung of the ladder lets look at what the four gasses/readings mean.

Carbon Dioxide (CO2)

Carbon dioxide is produced as a result of the combustion process. The more complete the combustion, the higher the level of CO2 produced.

Too much fuel or too little oxygen will result in incomplete burning and thus a lower than optimum CO2 reading. The, often quoted, figure for complete combustion is 14.7 parts of air to 1 part of fuel, this is by weight not volume i.e. 14.7 Kg of air to 1 Kg of fuel. This is the Stoichiometry (chemically) calculated correct ratio, but it should be noted that this might vary slightly with different fuels. There are running conditions for engines for which this target figure would not be ideal. Acceleration, for example needs a richer than ideal mixture, light throttle cruise mixture is often weaker to improve fuel consumption. These conditions will lower the CO2 levels and will result in an increase of other pollutants. CO2 is not regarded as a harmful gas. The CO2 level at this 14.7:1 ratio is normally around 13.5%

These other pollutants are the ones that are usually measured on today’s exhaust gas testers. Carbon Dioxide readings, although very popular on older machines, rarely feature on modern gas testers.

Carbon Monoxide (CO)

Carbon monoxide is produced as result of incomplete combustion as a result of too much fuel or too little oxygen. Hence the level of CO is strongly dependent upon the air: fuel ratio. Too much fuel results in a high CO reading. Incorrect or poor mixture distribution in the cylinder can also cause a high CO reading. It also interesting to note that an excessively lean mixture can cause an increased CO reading, this is due to low combustion temperatures preventing the post combustion oxidation of CO to CO2 in the exhaust system. The CO level at the 14.7:1 air fuel ratio is normally just under 1%.

CO is probably the most damaging of exhaust pollutants emitted. It is a colourless, odourless gas which when inhaled prevents the blood from carrying oxygen around the body. A concentration of just 0.3% CO in the air that we breathe would cause death in 30 minutes. When you are testing/working on a vehicle try using the gas tester sniffer to measure the CO level around the rear of the car. Now start worrying about your health.


Hydrocarbons in the exhaust system are once again a result of incomplete combustion. It is a measure of unburnt or partially unburnt fuel. It could be translated as a measure of the efficiency of combustion. Under ideal conditions, which are not currently technically possible, this hydrogen combines with the atmospheric oxygen and is emitted as water or water vapour and the carbon combines with the oxygen to form harmless carbon dioxide.

Increased HC levels are not purely dependent on air: fuel ratios, as the CO level, but many other factors, which have an influence, should be taken into consideration. The ignition system plays a big part in HC levels; a modern trend in ignition timing is to advance the ignition to increase engine output. Advancing ignition timing results in lower exhaust gas temperatures, which reduce the after-burning effect of HC’s. Retarding the ignition timing reduces the amount of time available to completely burn the fuel and also results in increased HC levels.

Other factors, which increase HC levels, include; ignition inefficiencies i.e. Misfires, low spark voltage or duration, mechanical condition i.e. Excessive blow-by or low compressions, sticking valve, engine breather faults. Any change from standard spec. I.e. High lift/duration cams, big carburettors, a blocked or restricted exhaust, will all cause an increase in HC levels.

The air: fuel ratio for the lowest HC reading is slightly weaker than 14.7:1 at around 15:1. The actual HC reading at this level varies car to car.

Exhaust gasses contain varied hydrocarbons; namely; saturated, unsaturated and aromatic. Saturated hydrocarbons are odourless, and cause drowsiness, headaches, eye and nose irritations. Unsaturated hydrocarbons are a sweet smelling gas which as well as causing the same physical symptoms as above they are an essential factor in the formation of smog and ozone. Aromatic hydrocarbons have a characteristic pungent smell and are in part carcinogenic (cancer forming) and part nerve toxin with a slight narcotic effect even at low concentrations.

Oxygen (O2)

Oxygen in the exhaust is once again the result of incomplete combustion. In ideal conditions, as above, there should be no unburned oxygen in the exhaust, all unburned oxygen should combine with unburned hydrocarbons to form carbon dioxide (CO2). In practice this is not yet technically achievable.

An over-rich mixture will result in a low or zero oxygen emission; an excessively weak mixture will result in a high oxygen reading.

The same factors that affected the HC readings will also affect the O2 readings. I.e. low combustion efficiency; engine breather faults etc. but also intake air leaks allowing un-metered air into the engine will cause a high O2 reading. A post combustion air leak i.e. exhaust air leak, will cause a high O2 reading but will dilute the other (CO2, Co and HC) readings and cause a lower than normal reading.

At the correct 14.7:1 air: fuel ratio you should normally expect an O2 reading of less than 0.5%. Oxygen, on it’s own, is not at all harmful.

Oxides of Nitrogen (NOX)

During the combustion process the nitrogen in the air drawn into the engine combines with oxygen to form nitrogen monoxide (NO) and nitrogen dioxide (NO2), which are generally grouped together as oxides of nitrogen (NOX).

Oxides of nitrogen are a feature of the combustion process and generally increase in line with combustion temperatures. High compression ratios, slightly weak mixtures, advanced ignition timing all increase the NO2 emissions. A reduction of NO2 emissions can be achieved with lower compression ratios, a slightly rich mixture and retarded ignition timing. If you have been paying attention you might realise that these are completely the opposite of that required reducing other emissions. The catalytic converter is the biggest advance in reducing NO2 emissions, Exhaust gas re-circulation valves also reduce the NO2 emissions, and this is due to a reduction of the combustion temperature.

The peak NO2 emissions usually occur at slightly weaker than the ideal 14.7:1 air: fuel ratio.

NO is a colourless gas, it is considered a severe hemotoxin which combines with the blood to cause a rapid paralysis. NO2 is a reddish brown gas with a sharp pungent odour. It causes respiratory irritation and damage to lung tissues. Oxides of nitrogen also combine with unsaturated hydrocarbons to form smog and ozone.

There are other gasses/pollutants in exhaust gasses, including lead compounds, but they are in even smaller percentages and not effected by tuning/maintenance settings.

Engine Design

Engine design plays a major part in exhaust emission. The most critical factors are; compression ratios, combustion chamber shape, valve timing and intake/exhaust design. Manufactures have placed great importance on reducing emissions to meet both political and legal requirements. It must be remembered that every engine represents a compromise between power, economy, noise and emissions. Various devices have been introduced over the years to improve this compromise and reduce emissions. Catalytic converters obviously have played a major part in reducing all emissions.
Let’s just quickly reiterate what the gasses mean:

  • Carbon Dioxide, is an indication of the completeness of burn, the more complete the burn, the higher the CO2 reading.
  • Carbon Monoxide, is an indication of the air: fuel ratio, too much fuel results in a high CO reading.
  • Hydrocarbons, is an indication of unburned fuel exiting the exhaust, the more incomplete the combustion, the higher the HC reading.
  • Oxygen, is an indication of either incomplete combustion or an air leak. Air: fuel ratio too weak, low combustion efficiency and a pre or post- combustion air leak will cause a high O2 reading.
  • Oxides of Nitrogen, is an indication of combustion temperature, low compression pressure, retarded ignition timing and rich mixture will all result in lower NO2 readings. When considering carburettor cars or basic injection cars, emission faults are relatively straightforward to diagnose.

It’s often good experience to have a play with the settings and note the responses. I.e. pull off a plug lead and note the increase in HC but the decrease in CO. Over-richen the idle mixture and note not only the increase in CO but also the decrease in O2. Lean off the idle mixture excessively and note initially the drop in CO and increase in O2 but then note the slight increase CO as the mixture leans off still further. Change the ignition timing and note the changes in CO, O2, and HC. Experience is, as always, a great teacher. It is important to note that if experimentation is attempted with a car equipped with a catalytic converter then irreversible damage may occur.

The main problem encountered with catalyst/ lambda equipped cars, besides the panic attack we all suffer from when the customer says can you fix it, is the extra components which can have an effect on the overall emissions compared to the conventional carburettor or basic injection car. The cars should be running in closed loop, which basically means the injection ECU is constantly monitoring the cars exhaust emission levels and will the modify the fuelling/ ignition levels accordingly, this process takes place many times a second and in a perfect world should never need further attention. The problem with this system is identifying the failed component as all components in the system work together Most modern ECU’s with this system are intelligent ECU’s with the built in facility to ignore specific sensors which can compound the problem.

The next problem that you will be faced with, with many emission related problems, is that a catalytic converter in good condition can and will disguise the actual fault. This will only happen for a relatively short time before the cat becomes polluted and causes a more pronounced fault (see technical tips on catalysts). Garages faced with this scenario will normally condemn, and replace the converter without checking any further. Although the fault will still be there, this will once again disguise the fault for a short while, but be warned the customer will be back. Probably with a warranty claim against the new cat wrongly believing it to be the cause of all his problems, including his flat spare tyre.

Some cars are equipped with a pre-cat emission test pipe or port into which you can plumb you gas tester. This simplifies matters greatly.

Because of these problems it can be very difficult to differentiate faults with emission control systems. Work through the system logically testing and investigating all avenues. To run the risk of repeating ourselves it is important to check the basics out first.

Check the basic engine tune; check plugs, leads, cap, rotor, air filter etc. Make sure the engine condition is ok (see vacuum testing tech tips). Ensure that the proper testing procedure is carried out, i.e. ensure the engine is fully warmed up and the sniffer pipe is fully inserted into the exhaust etc. All basic stuff, but you would be surprised at how many people change ECU’s etc without even taking a look at the plug condition. Don’t take the customers word for it check it for yourself. You could save yourself a lot of time. You should ensure that the data that you are using as a comparison is accurate. Ensure that it is correct for the car, inc. year, cc model etc. We’ve all, at one time or another spent far too long chasing our tails as a result of a simple oversight or misread number/spec. Test the car, check the emissions at all the required rpm levels and either print out the results or write them down. As a rough guide the expected levels of emissions with a catalyst cat with closed loop injection system are as follows; CO 0.3% max. HC 75 PPM max. CO2 14.5% min. O2 Probably the most common mistake made is when the lambda reading is incorrect. Lambda refers to the air fuel ratio, it is a reading which is calculated from the other emission readings (lambda (l) comes from the Greek word for balance). When faced with a car that has failed the emission test on lambda reading the most common response is to condemn the lambda sensor. This guesswork will sometimes pay dividends but more often than not you will have made an expensive mistake. The lambda reading on a gas tester is, to repeat, an indication of the air to fuel ratio, too high a lambda reading relates to too much oxygen. Too low a reading relates to too much fuel. Check the other readings before condemning the lambda sensor.

The voltage reading from the lambda sensor is often a very good place to start with your diagnose of an emissions failure fault. The lambda sensor can be used as a pre-cat emission tester when used with either an accurate voltmeter or an oscilloscope. With the engine up-to normal working temperature, check the output of the sensor wire, normally black with Zirconia sensors, you should expect an output of 0.2 – 0.8 volts fluctuating between these reading approximately 2 – 3 times per second. If the output voltage is lower than this, i.e. 0 – 0.2 volts there will be a lean running or excess oxygen problem. If the voltage is higher than this, i.e. 0.8 – 1.2 volts then there will be a rich running or excess fuel fault. A point to note is that conventional Zirconia sensor is not capable of producing a voltage above about 1.3 volts; if the voltage exceeds this then there will probably be an earth fault with the sensor. Some cars notably Rover have different requirements in respect to internal lambda connections, should you measure a voltage above 1.3 volts it might be worth checking for a lambda sensor mismatch fault.

If the fault is a high lambda reading, with a high O2 reading but with normal CO and HC readings, the most common cause is post combustion air leaks. A small exhaust system air leak will leak air into the exhaust before any blows can be detected. Only a small volume of clean air introduced into the exhaust will cause a car to fail the MOT test.

Various manufactures have had their problems with exhaust air leaks. The Ford Mondeo is a notable example, with various modifications to the manifold sealing ring having been carried out to prevent such an occurrence. Other cars such as Nissans are fitted with a drain hole in the rear silencer. You may need to temporarily bung this drain hole up to prevent a MOT failure. Check these first before condemning or even checking other emission parts.

An air leak after the lambda sensor, i.e. exhaust silencer/joint etc leak, will cause the O2 reading to be high and all the rest to be normal. If the air leak is before the lambda sensor, i.e. manifold/ down pipe, then a diluted exhaust gas will be monitored by the lambda. This will cause an inaccurate reading at the ECU, which will, in turn, cause extra fuel to be injected by the injection system, which will effect the CO, HC and lambda readings. The amount that these readings are out will obviously depend upon the quantity air being introduced into the exhaust. Often you will find that the CO and HC readings can remain normal due to the catalytic converter cleaning them up. This can be very difficult to diagnose, as above a pre-cat emission test pipe would be a great help but is not always available.

We mentioned earlier that a cat will often clean up a rich running fault at the expense of its life span. It should be noticed that the cat of a rich running car would run much hotter. The customer will often pick this up by reporting a strong egg like smell which they will be experiencing much more frequently than the normal egg like smell emitted when a cat is warming up. A discoloration of the actual cat body may also be noticed, this will be similar to the discoloration experienced when heating up a piece of steel. I.e. it will start with a light straw colour turning to a blue colour in extreme cases. Should this discoloration be noticed then you could be certain that the car is either suffering from a rich running fault or there has been neat fuel passing through the engine (possibly bump starting the car?). Moving on, when presented with a cat, with this discoloration, returned for warranty, Fuel Parts UK would reject the claim for this very reason. So please be careful.

Rich running (high CO) problems are caused by many faults and can be relatively easy to diagnose (famous last words?). Start again with the lambda sensor, measure the output voltage (remember a high voltage indicates a rich mixture). If the voltage is high you can be sure that the fault is before the cat. If the voltage is normal but the CO emissions are high you should suspect a cat fault.

If the fault is before the cat, check to see if the vehicle is equipped with an ECU capable of self-diagnosis. If so is the warning light on? Interrogate the ECU and check the faults logged. Should no faults be logged, and there’s a high lambda voltage, then you should suspect an emissions component that is not monitored directly by the self-diagnostic system. Examples are the fuel pressure regulator, or a fuel injector that has drifted away from the O.E.spec, but not necessarily failed. An ECU on a car in this situation with self-diagnosis will often show a lambda sensor failure fault purely because of the high voltage being relayed back to the ECU. Once again the instant reaction to this would be renew the lambda sensor, after all the “ECU says it’s faulty”. This would be a costly mistake; a faulty lambda rarely gives too high an output voltage, unless there is a mismatch fault as above. You need to check further to find the route cause of the problem.

If the lambda voltage is normal, the self-diagnosis has no faults stored, the exhaust is free from air leaks but the CO emissions are still high then you should suspect a cat fault (see earlier technical tips for details).

You should also check the lambda sensor signal response. With the car up to operating temperature and the car ticking over, connect a voltmeter/ oscilloscope and monitor the voltage, blip the throttle and check how quickly it responds to the change in mixture strength. It should be almost immediate. Lambda sensor response time slows down over its life; this is the main reason why some vehicle manufactures recommend a specific mileage change. Carbon build up, heat stress and corrosion all play a part in the reduction time of the sensor. The reaction time of a lambda governs the mixture control throughout the range, and it is imperative that it is checked. A slow response results in delayed mixture control. If you have any doubts, renew the sensor, this will not only help to clean up the emissions can also rejuvenate a cars performance in the same way that renewing a worn set of spark plugs can. This will be discussed further in a future technical tips issue.

Whilst all the above should be taken as a guide the overall moral to the story is not to be blinkered by the first fault you come across. Check, check and double check, explore all avenues before renewing expensive parts. All faults have a logical conclusion; it’s just finding them.

found this on a link on google explains so very well how the systems work and why the work and how they fail etc

7,172 Posts
Discussion Starter #2
this part is very much the prince engine with a incorrect timed timing chain system ie by 30-40k they are already running stretched timing chains and the increase in emissions follow more as they stretch more
Increased HC levels are not purely dependent on air: fuel ratios, as the CO level, but many other factors, which have an influence, should be taken into consideration. The ignition system plays a big part in HC levels; a modern trend in ignition timing is to advance the ignition to increase engine output. Advancing ignition timing results in lower exhaust gas temperatures, which reduce the after-burning effect of HC’s. Retarding the ignition timing reduces the amount of time available to completely burn the fuel and also results in increased HC levels.
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