Appendix B Exhaust Emmisions

A. Non-catalyst test
4. Unstable readings
Single exhaust systems
1. The Exhaust Emission Test
A check of vehicle exhaust emissions is part of the MOT test for all 4-stroke spark ignition engined vehicles with 4 or more wheels in Class IV and VII. Two of the exhaust gases are included. Carbon Monoxide (CO). Hydrocarbons (HC)Assessment on most vehicles is straightforward, but a number of factors should be borne in mind. Some vehicles give unstable readings due, for example to their carburettor or fuel injection system design.  Before failing a vehicle, it is important to establish that a particular limit has been exceeded constantly for a period of 5 seconds. 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.
2. Conducting the test
5. Holed exhaust
Dual exhaust systems
The test should be conducted with the engine warm. Testing a cold engine could lead to an unjustified failure.It is important to ensure that any enrichment device is not operating.The engine should be idling normally during the test and should not be subject to significant electrical loading such as heated seats or heated rear windows.If an engine will not idle, an assistant may apply light throttle pedal pressure.To assess that these conditions are met, MOT Testers can either. Use their own judgement, or . Refer to manufacturer's or other reliable data A holed exhaust can allow air to be sucked in, causing artificially low readings.Where a vehicle has an exhaust holed to the extent that MOT failure is warranted, the emissions should be rechecked when the exhaust is repaired even if the vehicle does not leave the testing station in the meantime.  Owners should be made aware that any emission readings taken with a leaking exhaust might be incorrect.Holes not justifying MOT failures do not normally have a significant effect on the exhaust gases at the tailpipe and can be ignored. 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 till considered a dual exhaust.
3. Electric engine cooling fans
6. Total gas emitted
Many modern vehicles are fitted with electric engine cooling fans which can cut in during an emission test.  The extra load on the alternator reduces the idle speed which causes the engine management system to react.  This gives rise to highly variable readings.

If this happens during a test, wait until the fan switches off and the readings stabilise before continuing.
The MOT limits prescribed relate to the total exhaust gas being emitted by the vehicle.If a vehicle has a dual exhaust system, then the emissions from the tailpipes should be averaged.  This is done by adding together the readings and dividing by two, eg  

1st pipe emits 6% CO, 400 ppm HC
  2nd pipe emits 4% CO, 500 ppm HC

Average CO reading is


Average HC reading is

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