6.4 Exhaust Emissions - Spark Ignition - General

Emission test and limits selection
 
 
This inspection applies to all spark ignition engined vehicles with four or more wheels in Classes IV and VII.

On the following two pages is a flowchart. Use this flowchart carefully and accurately to establish which type of emission test is applicable to the vehicle being tested.
  
Vehicles fitted with modified engines
If an engine has been modified in any way, it still has to meet the exhaust emission requirements according to the age of the vehicle.  
Personal Imports
The vehicle will be tested according to its age from first use. The only exemption to this is if the vehicle owner can present to the tester at the time of test a letter from the vehicle manufacturer stating that that particular engine as originally installed could not meet the equivalent British emission standards. If this is the case, then test to the next lower emission standard.

eg.  A 1995 car first used in Africa has a letter from the engine manufacturer stating that the particular engine (engine number to be stated) cannot meet catalyst emission limits, then use the pre cat limits of CO 3.5%, HC 1200ppm.
  
Vehicles fitted with a different engine
Test according to which is older , engine or vehicle.

eg.  A 1995 car fitted with a 1991 engine (of whatever make), test to 1991 standards for emission purposes.

Note: The onus is on the vehicle presenter to prove engine age.
  
The following notes are referred to in the flowchart on pages 5a(i) and 5a(ii)
(1) - Wankel rotary engined vehicles first used before 1 August 1987

 - Kit cars, and

 - Amateur built vehicles
(2) - Applies to four stroke engines only- two stroke engines do not require an emissions test unless they are subject to the catalyst test. (3) <= less than or equal to
(4) HC not applicable to CNG (compressed natural gas) fuelled vehicles (5) For emissions purposes, a 'passenger car'

- is constructed or adapted for the carriage of passengers

- has no more than 5 seats in addition to the driver

- has a maximum gross weight not exceeding 2500kg

- is not a 'goods vehicle' (eg. car derived van, van, pick-up or truck).
Advice on establishing whether the design gross weight of a large car exceeds 2500kg

i. it may be shown on the manufacturers VIN plate (example shown in section 3.9, page 27)

ii. it may be listed only in section 2 of the current emissions data book

iii refer to any readily available data, eg. handbook or data book

iv. if still unsure, assume it to be over 2500kg d.g.w.


Vehicles which have been the subject of specialist conversions are to be treated,  for emissions purposes, as if they had not been converted, eg. a motor caravan or ambulance converted from a goods vehicle is still to be treated as not being a 'passenger car' an ambulance converted from a 'passenger car' or a 'passenger car' with seats added is still to be treated as being a 'passenger car'.

A vehicle originally built with 6 or more seats, in addition to the driver, which has had seats removed is still to be treated as not being a 'passenger car'.
(6) To find an exact match in the current emissions data book, you will need make and model and other data such as engine size, model code, engine code, VIN code or serial number. Appendix F of this manual gives more detail of where to find these codes.

If after normal cleaning and/or scraping processes a particular code is unreadable
or inaccessible , carry on as if no exact match can be found. If the flowchart leads to a non-catalyst test then carry out that test.

If the flowchart leads to a catalyst test to default limits, then carry out that test, but use the less demanding of either the default limits or the specific limits for any vehicle which is an exact match in everything but an unreadable code.
(7) The full title of the Department of Transport Emissions book is 'In-Service Exhaust Emission Standards for Road Vehicles'. The latest edition must be used.   
6.4 Exhaust Emissions - Spark Ignition - Non-Catalyst
Information
Method of Inspection
Reason for Rejection
This  sub-section should be read in conjunction with Appendix B (section A)

Engine speed and temperature

When checking exhaust emissions, the engine must be at its normal idle speed and normal operating temperature. Engine speeds and temperature can be assessed either subjectively or by reference to manufacturer's or other reliable data.

Gas analyser probe
It is important to ensure that the gas analyser probe is inserted as fully as possible into the exhaust pipe and is secure

Suitable Exhaust Gas Analysers
Suitable exhaust gas analysers are those accepted by the Vehicle Inspectorate and calibrated and maintained in accordance with the inspectorate's instruction (see appendix D).  Mot stations with 1996 specification equipment may use the automated routine to complete the non-catalyst inspection (after completing Method of Inspection 1(c), Section A).

Early catalyst vehicles
The flowchart and notes on pages 5, 5a(i)  and 5a(ii) must be carefully followed. Just because a vehicle is fitted with a catalyst does not necessarily mean a 'cat' test.
A. All vehicles

1. Raise the engine speed to around 2500rpm or half the maximum engine speed if this is lower. Hold this speed steady for 20 seconds to ensure that the inlet and exhaust system is properly purged. Allow the engine to idle and the emissions to stabilise.

a. assess the engine idle speed



b. assess the smoke emitted from the tailpipe at idle.

c. 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. Allow the engine to return to idle.
A. All vehicles1. The enginea. is idling at a speed clearly above its normal idling speed

b. emits dense blue or clearly visible black smoke for a continuous period of 5 seconds at idle

c. emits excessive dense blue or clearly visible black smoke during acceleration which would obscure the vision of other road users.

Note: Older vehicles, particuarly pre-1960, sometimes emit unavoidable smoke due to their design. Such smoke is not a reason for rejection
When to do the test

It is  recommended that the engine is tested as soon as possible after driving on the road. Because of this it is acceptable for this item to be checked separately from the remainder of the test if this allows the test to be carried out on a fully warmed up engine.

For example, the test may be conducted as soon as the vehicle arrives, even if the remainder of items will not be tested until later the same day, provided that both parts of the test are carried out by the same tester.

Working Environment

To prevent the build up of fumes, the test should be carried out in a well ventilated area.
B. Vehicles first used on or after 1 August 1975 B. Vehicles first used on or after 1 August 1975
1. Check that the analyser probe can be inserted into the tailpipe 1. The emissions cannot be measured because a tailpipe accessory is fitted which prevents insertion of the analyser probe
2. Use a suitable exhaust gas analyser to determine the proportions of carbon monoxide (CO) and hydrocarbons (HC) in the exhaust gas over a period of at least 5 seconds at idle
Note: Any residual hydrocarbons (ie those indicated by the analyser when it is sampling only clean air) should be deducted from the HC reading obtained from the vehicle.
a. a carbon monoxide content exceeding the limit (see page5 of this section) for a continuous period of  5 seconds
Note: If a vehicle meets the CO requirement at its normal idling speed but fails the HC check, re-check the HC level at a high idle speed of 2000rpm. If the HC reading is 1200ppm or less, then the vehicle will meet both the CO and HC requirements.

the CO requirement must be met with the engine running at its normal (low) idling speed

do not use a cold start/cold running mechanism to achieve a high idle speed, instead, apply light pressure to the throttle pedal.
b. a hydrocarbon content exceeding the limit (see page 5 of this section) for a continuous period of 5 seconds.
6.4 Exhaust Emissions - Spark Ignition - Catalyst
Information
Method of Inspection
Reason for Rejection
When to do the test

It is  recommended that the engine is tested as soon as possible after driving on the road. Because of this it is acceptable for this item to be checked separately from the remainder of the test if this allows the test to be carried out on a fully warmed up engine.

For example, the test may be conducted as soon as the vehicle arrives, even if the remainder of items will not be tested until later the same day, provided that both parts of the test are carried out by the same tester.
1. A suitable exhaust gas analyser will be needed to perform this inspection (see information column). Check that the analyser probe can be inserted into the tailpipe 1. The emissions cannot be measured because a tail pipe accessory is fitted which prevents insertion of the analyser probe.
Suitable Exhaust Gas AnalysersExhaust gas analysers must either be approved by the Vehicle Inspectorate's 1996 specification or alternatively be accepted by the inspectorate for 'manual' catalyst testing. Both types of analyser must be calibrated and maintained in accordance with the inspectorate's instructions (see Appendix d). 2. 
a. Ensure that the analyser's daily leak check has been performed.
a. The engine speed is clearly above the vehicle specific limit

Note: If the engine speed is clearly above the vehicle specific limit and this can be easily adjusted, a tester may perform the adjustment and complete the test- the adjustment is not, however, part of the MOT test.
b. Identify the vehicle specific test  limits using the flow chart on page 5. b. in the 2nd fast idle test, one or more exceeds the vehicle specific limits continuously for the last 5 seconds of the 30 second countdown:

- Carbon monoxide (CO)

- Hydrocarbons (HC)

- Lambda
1996 Specification Analysers

All 1996 specification analysers will prompt when to carry out specific operations, eg 'run the engine at fast idle'. You must follow these prompts closely. The method of inspection opposite is given for reference purposes only.

'Manual' Catalyst Testing

Where 'manual' catalyst testing is permitted, the  Vehicle Inspectorate's standard check sheet must be used for every test. Refer to the method of inspection opposite for additional guidance if necessary.
c. Connect the engine speed measuring device and insert the engine oil temperature measuring probe into the dipstick hole.

Note: Engine speed and engine oil temperature must be measured whenever possible. If engine speed cannot be measured then the vehicle's tachometer should be used if fitted. Otherwise, a subjective estimate should be made. If engine oil temperature cannot be measured, see note in paragraph d.
c. In the idle test, the following gas exceeds the vehicle specific limit continuously for the last 5 seconds of the 30 second countdown:

- Carbon monoxide (CO)
Printouts and check sheets

The 1996 specification analysers will produce two printouts. One printout must be retained with the VT20/VT30 records and the other printout must be given to the vehicle presenter. In the case of 'manual' catalyst testing, the check sheet should be retained with the VT20/VT30 records. A duplicate of the check sheet need only be produced if the customer specifically requests it.
d. Engine pre-conditioning: Check the engine oil temperature, if it is below the vehicle specific requirement, raise the engine speed to between 2000rpm and 3000rpm and maintain this speed until the minimum engine oil temperature has been reached. Remove temperature measuring probe and replace dipstick.

Note: Where, in exceptional circumstances, the engine oil temperature cannot be measured (eg in the case of a dry sump engine), check one of the following: (i) temperature gauge showed warm engine; (ii) cooling fan had cut in; (iii) coolant pipes were hot.
 
Gas Analyser Probe

It is important to ensure that the gas analyser probe is inserted as fully as possible into the exhaust pipe and is secure.
e. Perform a HC hang-up check and ensure that HC<20ppm before continuing. Insert the analyser sample probe. 
Working Environment

To prevent the build up of fumes, the test should be carried out in a well ventilated area.
f. 1st Fast Idle Test: Raise the engine speed to the vehicle specific fast idle speed and maintain for 30 seconds. If the engine speed drifts outside the fast idle speed range, begin the 30 second countdown again. During the last 5 seconds note the readings for CO, HC and lambda, and record the results. 
Cosmetic engine covers

Where engine speed can only be measured by the removal of a cosmetic engine cover, the engine speed must be measured if the cover can be easily un-clipped. Otherwise, engine speed measurement must be by-passed.
g. if the vehicle has passed the first fast idle test, then go to paragraph (j), otherwise go to paragraph (h). 

 

h. Additional engine pre-conditioning: Run the engine between 2000-3000rpm for 3 minutes or until all the emissions are within limits. If the engine speed goes outside the fast idle range, then freeze the countdown until the engine speed is within range once again.

 

  i. 2nd Fast Idle Test: Repeat the procedure as laid down in paragraph (f), then go to paragraph (j).

 

  j. Catalyst stabilisation: Raise the engine speed to the vehicle specific fast idle speed and maintain for 30 seconds. If the engine speed drifts outside the fast idle speed range then begin the 30 second countdown again.

 

  k. Idle test: Allow the engine to idle during a 30 second countdown. During the last 5 seconds, note the CO reading and record the result. 
  l. Remove analyser sample probe and engine speed measuring device. 
6.4 Exhaust Emissions - Compression Ignition (Diesel)
Information
Method of Inspection
Reason for Rejection
This inspection applies to

All compression ignition engined vehicles with four or more wheels.
A. Vehicles first used before 1 August 1979

Where possible check that:
. there is sufficient oil in the engine.
. the oil pressure is not low.
. there is no abnormal engine noise.
. the govenor has not been tampered with.
. the engine is at normal operating temperature (see information column)
A. Vehicles first used before 1 August 1979


Reasons for not carrying out a smoke test
Do not carry out a 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 engine A smoke test must not be carried out if the oil temperature is below 60deg C. The reason for not conducting a smoke test must be clearly shown on the Notification of Refusal. (VT30)

When to do the test


The smoke test must only be completed when the engine is at normal operating temperature . 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 80 deg C or normal operating temperature if lower (60 deg C 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 engine's 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 infa-red radiation to be at least an equivalent temperature.
. Raise the engine speed to around 2500rpm or half the maximum engine speed if this is lower

.Hold this speed steady for 20 seconds to ensure that the inlet and exhaust system is fully purged

.Allow the engine to return to idle and the emissions to stabilise.

1. 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.
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 would obscure the view of other road users.

Older vehicles particularly pre-1960, sometimes emit unavoidable smoke due to their design. Such smoke is not a reason for rejection.
Smoke meter probe

With some types of smoke meter, care must be taken to ensure that the smoke meter probe is correctly aligned with the exhaust gas flow. Reference to meter manufacturer's instruction may be necessary.
   
Smoke meter probe

With some types of smoke meter, care must be taken to ensure that the smoke meter probe is correctly aligned with the exhaust gas flow. Reference to meter manufacturer's instruction may be necessary.
   
Suitable smoke meters

Suitable smoke meters are those accepted by the Vehicle Inspectorate and calibrated and maintained in accordance with the Inspectorate's instructions.


Suitable engine temperature measuring devices


Suitable oil temperature measuring devices are only those accepted by the Vehicle Inspectorate.
B. Vehicles first used on or after 1 August 1979


Where possible check that:
. there is sufficient oil in the engine.
. the oil pressure is not low.
. the camshaft belt is in satisfactory condition
. there is no abnormal engine noise.
. the govenor has not been tampered with.
. the engine is at normal operating temperature (see information column)
B. Vehicles first used on or after 1 August 1979

Automatic transmission

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 vehicle manufacturer's instructionn may be necessary.
If appropriate remove the 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 exhaust system.

Raise the engine speed slowly to maximum to check the operation of the governor. Once the engine speed has stabilised or it becomes clear that the govenor is not working, release the pedal, return to idle and stop the engine.
 
 
prompt the meter to carry out a zero check .

Check that the smoke meter probe can be inserted into the tailpipe

Insert the probe fully and securely , in line with the gas flow and restart the engine.
 
  Fast Pass

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 idle speed.
 
  At the end of the 1st acceleration, read the smoke level displayed on the meter. If it is at or below 1.5m-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 3 . 
 
If the 1st acceleration smoke level is greater than 1.5m-1 carry out two further accelerations following the meter prompts

At the end of the 3rd acceleration, read the smoke level displayed on the meter. If it is at or below the appropriate level the vehicle has passed this part of the test and a pass result will be displayed on the meter. Go to Method of inspection 3

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

After each acceleration, check the mean reading. This part of the test is complete when either:

- the mean of any 3 smoke levels is at or below the appropriate limit, or

- six accelerations have been performed
 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

Note: There is no reason for rejection for vehicles that do not meet the fast pass criteria.

2. after 6 free accelerations, the mean of the last 3 smoke levels is :

- for a non-turbocharged engine more than 2.5m

- for turbocharged engines more than 3.0m
 
3. Assess whether the smoke emitted from the exhaust, regardless of measured density, is likely to obscure the vision of other road users. 3. Exhaust emits excessive smoke or vapour of any colour to an extent likely to obscure the vision of other road users.
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