The Mot test Inspection Manual now moves onto Fuel and Emissions and is divided into four sub-sections: exhaust system, fuel system, exhaust emissions-spark ignition and exhaust emissions-compression ignition.
As with almost all of the Inspection Manual, the tester is not obliged to follow any set examination order as long as the test is carried out to the correct standards with no omissions. In this section I will deal with each subject in the order it appears in the manual.
It would be fairly obvious to most people that a vehicle has a blowing exhaust, however, the tester must also inspect all other parts of the exhaust system looking for the less obvious, for example missing or ineffective mountings, excessive noise caused by deliberate modification or repairs which do not effectively seal exhaust gas leaks. Minor gas leaks such as those around joints or from small pin holes anywhere in the system would not normally fail, they can however, have an adverse effect on the result of the exhaust emission test carried out on petrol engine vehicles. When assessing noise levels generated by a deliberately modified exhaust the nominated tester can only use his or her discretion aided by the wording in the Inspection Manual which states “the noise emitted from the vehicle is clearly unreasonably above the level expected from a similar vehicle with a silencer in average condition�. The term average condition means a silencer fitted to a similar vehicle in an unmodified state.
Whilst under the vehicle the tester must examine all visible fuel system components for leakage and/or insecurity, should the tester feel, in his or her opinion, that the fuel lines are heavily corroded the presenter should be informed and an advisory notice issued, the basic rule is no leak, no fail. The security of the fuel system components includes any fixings applied to fasten parts to the body and/or chassis, ineffective clips, mountings etc can cause premature component failure and fuel leakage. For a complete system examination the tester must also inspect system components in the engine bay and also assess the condition of the fuel filler cap and its mounting flange. It is a common mis-conception that the filler cap must be of the lockable variety, this is not the case as the test regulations only specify that the cap and flange joint must seal such that it maintains tank pressure and fastens by positive means and not a simple and wholly ineffective push fit. This cap/flange joint is subject to a detailed check ensuring that any sealing ring, washer etc is present and in good working condition, this is to assess its capability of preventing fuel loss should the vehicle overturn as a result of a traffic incident.
The last part of this section deals with exhaust emissions, both spark ignition and compression ignition engines, and for the sake of expediency I shall leave out dual fuel and hybrid powered vehicles.
Like the section dealing with seat belts, the Inspection Manual has differing requirements for spark ignition emissions dependant on the age of the vehicle being tested; a brief outline is as follows:
- First used before Aug 1975
- Visual check only                                                                          Â
- First used between Aug 1975 & July 1986
- CO max 4.5%, HC max 1200ppm
- First used between Aug 1986 & July 1992
- CO max 3.5%, HC max 1200ppm
- First used between Aug 1992 & Feb 2002
- CO max 0.3%, HC max 200ppm, lambda 0.97 and 1.03 (during fast idle test)
- CO max 0.5% at normal idle.
- First used after Mar 2002
- CO max 0.2%, HC max 200ppm, Lambda between 0.97 & 1.03
Looking at the figures above we can see how much the effect of legislation and engineering has had on reducing exhaust pollution especially since 1992 and the introduction of catalytic converters and powerful electronic engine management systems.
Prior to the vehicles first used before August 1992 emissions testing comprised of a 5 second gas analysis or prior to first use August 1975 a visual check for dense blue or black smoke. For vehicles with first use since August 1992 the tester must carry out a detailed gas analysis measuring carbon monoxide (CO), hydrocarbons (HC) and the value for lambda at two engine speed ranges, idle (450rpm to 1500rpm) and fast idle (2500rpm to 3000rpm). Fortunately for the tester this task is carried out by a computer controlled gas tester which will instruct the user when to increase the engine speed, when to allow it to idle etc and the test results will be displayed on screen and on a paper printout.
So what about compression ignition engines? Again the age of the vehicle will dictate the test procedure but the gases emitted by these engines are very different to their petrol cousins so a different type of analyser is used to measure, in crude terms, the density of exhaust smoke during engine acceleration or for the more technically minded, the co-efficient of obscuration.
- First used before July 1979
- First use after Aug 1979
- 2.5m-1 maximum for non-turbocharged engines
- 3.0m-1 maximum for turbocharged engines
The visual check is basically an assessment by the nominated tester that the vehicle does not produce dense blue or clearly visible black smoke during a period of 5 seconds at idle, this assessment is repeated during acceleration. The Inspection Manual definition of dense smoke is emissions which largely obscures vision. For vehicles which are subject to a metered smoke analysis the same rules apply together with the maximum limits above.
Remember if you need repairs to your vehicle you can get parts for any vehicle including Vauxhall Vectra parts from Carsparefinder and have a garage fit them for you, by supplying the Vauxhall parts yourself the garage will cut the bill as they will not have to pay for those expensive new parts.