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Car MOT Manual 2012 onwards






Exhaust, Fuel and Emissions - 7.4 Exhaust Emissions Compression Ignition

Information
This inspection applies to all compression ignition engine vehicles with four or more wheels except quadricycles and electric/combustion engine Hybrid vehicles.
T he smoke test must only be completed when:
there is sufficient oil in the engine
the oil pressure is not too low
there is no abnormal engine noise
the governor has not been tampered with
the camshaft belt is in a satisfactory condition
the engine is at its normal operating temperature.

Note: It is not normally sufficient to run the engine with the vehicle stationary to warm it up to temperature.

The engine oil temperature measured by a probe in the oil level dipstick tube is to be at least 80C or normal operating temperature if lower 60C minimum. Testing an engine below normal operating temperature may affect the test result. If owing to vehicle configuration temperature measurement is impractical, the establishment of the engines normal operating temperature may be made by some other means, for example by the operation of the cooling fan or the engine block temperature measured by the level of infra-red radiation to be at least an equivalent temperature.

An approved diesel smoke meter DSM will be needed to perform this inspection. Do not carry out a metered smoke test if the engine is not in a safe condition to do so. This will involve questioning the vehicle presenter and a brief examination of the vehicle condition. The reason for not conducting a smoke test must be clearly shown on the VT30. See Introduction 2h.

The probe on some types of smoke meter must be correctly aligned with the exhaust gas flow. Reference to the smoke meter manufacturers instructions may be necessary. When testing vehicles fitted with automatic transmission care must be taken to avoid overheating the transmission system. Do not carry out unnecessary engine acceleration or prolonged high revving of the engine. Reference to the vehicle manufacturers instructions may be necessary.

Maximum engine revs cannot be achieved on some vehicles due to design features. Where this is the case, the vehicle must be tested as presented.

The MOT limits prescribed relate to the total smoke level being emitted by the vehicle. If a vehicle has a dual exhaust system, then the smoke test must be repeated and the emissions from the tailpipes averaged. This is done by adding together the readings and dividing by two, e.g. 1st pipe emits smoke level of 3.50m-1. 2nd pipe emits smoke level of 2.00m-1 3.5 2.0 Average smoke level is: 2 = 2.75m-1.

A dual exhaust system has two separate pipes from the engine manifold all the way back to the tailpipes. An exhaust system with a balance tube between separate pipes is still considered a dual exhaust.
A single exhaust system has at least one point in the system where all the exhaust gases from the engine travel through the same pipe, even though the system may split at some point to separate silencers or tailpipes. Only one of these need be checked.

Some low emission diesel engines mainly Euro IV and those which will not reach maximum revs due to design features may fail to trigger a reading due to the very low levels of smoke produced. If the DSM does not register a reading or shows an error, the printout should be marked accordingly showing that the emissions limits were tested and met but the DSM could not register the reading. If the DSM will not produce a printout then the tester must make a written note of the following: -
Test Station number
Testers name
Date time
Test number
Vehicle Type
Vehicle VRN
That the vehicle passed the emissions test
No print out was produced due to low emissions.

This information must be recorded and kept with the emissions records for audit purposes for 3 months.
Method of Inspection
A. Vehicles first used before 1 August 1979 1. If the engine checks are satisfactory and having removed any oil temperature probe:

raise the engine speed to around 2500 rpm, or half the maximum engine speed if this is lower
hold this speed steady for 30 seconds to ensure that the inlet and exhaust system is fully purged
allow the engine to return to idle and the emissions to stabilise
assess the smoke emitted from the tailpipe.

2. Rapidly increase the engine speed to around 2500rpm or half the maximum engine speed if this is lower and assess the smoke emitted from the tailpipe during acceleration.

B. Vehicles first used on or after 1 August 1979 1. Check that the smoke meter probe can be inserted fully into the tailpipe

Fast Pass
2. If the engine checks are satisfactory and having removed any oil temperature probe, raise the engine speed to around 2500rpm, or half the maximum engine speed if this is lower, and hold for 30 seconds to fully purge the inlet and e xhaust system.

Raise engine speed slowly to maximum to check the operation of the governor. Once the engine speed has stabilised or if it becomes clear that the governor is not working, release the pedal, return to idle and stop the engine. Prompt the meter to carry out a zero check. Insert the probe fully and securely, in line with the gas flow. Restart the engine. Following the meter prompts, depress the accelerator pedal quickly and continuously but not violently, to reach full fuel position in less than 1 second.

Hold it there until a release prompt is given, then immediately release the pedal. Allow the engine, and any turbocharger fitted, to return to i dle speed. At the end of the 1st acceleration read the smoke level displayed on the meter. If it is at or below 1.50m-1 the vehicle has passed this part of the test and a pass result will be displayed on the meter. Go to Method of Inspection 4.

3. If the1st acceleration smoke level is greater than 1.50m-1 carry out two further accelerations following the meter prompts.

At the end of the 3rd acceleration, read the mean smoke level displayed on the meter. If it is at or below the appropriate limit, the vehicle has passed this part of the test. Go to Method of Inspection 3.

If the mean smoke level is too high, carry out further accelerations up to a maximum of 6 in total.

After each acceleration, check the mean reading. This part of the test is complete when either:
5625656451 the mean of any 3 consecutive smoke readings is at or below the appropriate limit, or
5625656451 six accelerations have been performed.

4. Assess whether the smoke emitted from the exhaust, regardless of measure density, is likely to obscure the vision of other road users.
Reason for Rejection
1. The exhaust emits dense blue or clearly visible black smoke for a period of 5 seconds at idle.

2. The exhaust emits dense blue or clearly visible black smoke during acceleration which would obscure the view of other road users. Note: The criterion is density and not volume of smoke. The description dense smoke includes smoke or vapour which largely obscures vision. Older vehicles, particularly pre-1960, sometimes emit unavoidable smoke due to their design. Such smoke is not a Reason for Rejection.

1. The emissions cannot be measured because a tailpipe accessory is fitted or a deliberate modification has been made which prevents insertion of the smoke meter probe.

2. If the vehicle does not meet the fast pass criteria, go to MoI 3. Note: there is no Reason for Rejection for vehicles that do not meet the fast pass criteria.

3. After 6 free accelerations, the mean of the last 3 smoke levels is:
a. for vehicles first used on or after 1 July 2008, more than 1.5m-1 for both turbocharged and non-turbocharged engines
b. for vehicles first used before 1 July 2008:
for a non-turbocharged engine, more than 2.50m-1
for turbocharged engines more than 3.00m-1.

4. Exhaust emits excessive smoke or vapour of any colour to an extent likely to obscure the vision of other road users.

Note: The criterion is density and not volume of smoke. The description dense smoke includes smoke or vapour which largely obscures vision.





Reproduced from the MOT Inspection Manual by kind permission of THE VEHICLE INSPECTORATE.
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