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PRE MOT CHECKS

Avoiding MOT Failure

Emissions - Diesel Engines

Make sure that the engine is in good working order and tuned. Ignition timing, clean air filter etc.

Start the engine and hold the revs at around 2500 rpm for about 20 seconds.

Allow the engine to return to idle and watch for any smoke from the exhaust pipe.

If dark smoke comes from the exhaust then the car will fail and need engine repair or further tuning.
Blue smoke usually means oil is being burnt. This usually occurs when worn seals allow oil into the cylinders. (Valve stem oil seals or piston rings.)
Black smoke usually indicates unburnt fuel. This usually occurs with faults in the injection or carburettion system or a blocked or dirty Air Filter.

Check that the idle speed is not too high (Usually between 600 - 1000 rpm)

For any further tests you will need the use of a gas analyser capable of measuring Carbon Monoxide and Hydrocarbons. These can be hired or you could take your car to a garage for a check.
Levels permitted for each car vary by model but the amount of hydrocarbons and level of Carbon Monoxide produced from your exhaust must not exceed the levels set by the manufacturers for model and age of car.

Visual Inspection

For all private cars and light goods vehicles up to and including 3500kg design gross weight first used before 1 August 1979, a visual test will be carried out. The Tester will check the smoke emissions by raising the engine speed to around 2500rpm or half the maximum engine speed if this is lower. This speed will be maintained for 30 seconds to ensure that the inlet and exhaust system has been fully purged. The Tester will then allow the engine to return to idle. Once the engine has stabilised at this speed, the emissions from the exhaust tailpipe will be assessed. If the exhaust is emitting dense blue or clearly visible black smoke for a period of 5 seconds or more the vehicle will fail the test.

The Tester will then rapidly increase the engine speed to around 2500 rpm or half the maximum engine speed if this is lower and assess whether the smoke emitted from the exhaust is likely to obscure the vision of other road users. If it is likely to do so, in the Tester.s opinion, the vehicle will fail the test.

However, cars and light goods vehicles manufactured before 1960 will not be failed if the smoke is unavoidable due to the engine design.

Metered smoke test

For all private cars and light goods vehicles up to 3500kg design gross weight first used on or after 1 August 1979, a metered smoke test will be carried out.

Before checking the smoke emissions the Tester will firstly ensure that the engine inlet and exhaust system is fully purged, and the engine speed governor is functioning correctly. The engine speed will be raised to around 2500rpm or half the maximum engine speed if this is lower1. Upon reaching this speed, the Tester will hold the engine speed steady for 30 seconds to purge the inlet and exhaust systems, then the engine speed will be slowly increased to maximum to check the operation of the fuel pump governor. Where the engine speed stabilises at its maximum speed indicating that the governor is working, the engine will then be returned to idle speed.

Where it is clear that the governor is not working, the engine will be returned to idle speed and stopped, the smoke test will not be carried out and the Tester will be unable to pass the vehicle.

Provided the preliminary checks are completed satisfactorily, the Tester will prepare the smoke meter and insert the sampling probe into the exhaust tailpipe. Having restarted the engine the tester will start the smoke test.

Diesel Fast Pass
Since 1 August 2002 a metered „fast pass. procedure has been used to help simplify and reduce the time taken for the test, allowing vehicles with low smoke emissions to pass the metered test after one acceleration of the engine. If, after the first acceleration, the meter reading is at or below 1.5m-1, the vehicle will pass the meter test.

The smoke meter will indicate to the Tester to accelerate the engine. The accelerator pedal will be depressed quickly and continuously but not violently, to reach the full fuel position in less than 1 second. After the release prompt is given, the Tester will immediately release the throttle. The meter will calculate the maximum smoke emission during the acceleration and display the result. If the smoke level reading is at or below 1.5m-1, the vehicle has passed the metered test, and a pass result will be displayed on the meter.

Standard diesel test
If the smoke level reading is greater than 1.5m-1, a further two accelerations will be requested by the meter. Provided the average of the 3 tests is at or below the appropriate limit in Table 3, and the three results are within a specified tolerance of each other, the vehicle will have passed the test. The Tester will stop the engine and remove the smoke meter probe from the tailpipe.

Where the average smoke emission at the end of the third acceleration exceeds the limit in Table 3, the meter will request further tests. This will continue until either the average of the three preceding accelerations is at or below the limit in the Table or a maximum of six accelerations have been completed. Once the vehicle has either passed the test or a maximum of six accelerations have been completed the Tester will stop the test and remove the smoke meter probe from the exhaust tailpipe.

As a final check the Tester will assess visually whether the smoke emitted from the exhaust, regardless of measured smoke density, is likely to obscure the vision of other road users. If it is likely to do so the vehicle will fail the test.

Test procedures may be amended at any time. You are advised to refer to the relevant VOSA Inspection Manual for details of the test procedure which will be applied to a vehicle undergoing examination.


Fuel & Exhaust
Emissions - Petrol
Catalytic Converters
Diesel Particulate Filters
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