Petrol models
- The engine should be warmed up, and running well (ignition system in good order, air filter element clean, etc).
- Before testing, run the engine at around 2500 rpm for 20 seconds. Let the engine drop to idle, and watch for smoke from the exhaust. If the idle speed is too high, or if dense blue or black smoke emerges for more than 5 seconds, the vehicle will fail. Typically, blue smoke signifies oil burning (engine wear); black smoke means unburnt fuel (dirty air cleaner element, or other fuel system fault).
- An exhaust gas analyser for measuring carbon monoxide (CO) and hydrocarbons (HC) is now needed. If one cannot be hired or borrowed, have a local garage perform the check.
CO emissions (mixture)
- The MOT tester has access to the CO limits for all vehicles. The CO level is measured at idle speed, and at 'fast idle' (2500 to 3000 rpm). The following limits are given as a general guide:
At idle speed - Less than 0.5% CO
At "fast idle" - Less than 0.3% CO
Lambda reading - 0.97 to 1.03
- If the CO level is too high, this may point to poor maintenance, a fuel injection system problem, faulty lambda (oxygen) sensor or catalytic converter. Try an injector cleaning treatment, and check the vehicle's ECU for fault codes.
HC emissions
- The MOT tester has access to HC limits for all vehicles. The HC level is measured at "fast idle" (2500 to 3000 rpm). The following limits are given as a general guide:
At 'fast idle' - Less then 200 ppm
- Excessive HC emissions are typically caused by oil being burnt (worn engine), or by a blocked crankcase ventilation system ("breather"). If the engine oil is old and thin, an oil change may help. If the engine is running badly, check the vehicle's ECU for fault codes.
Diesel models
- The only emission test for diesel engines is measuring exhaust smoke density, using a calibrated smoke meter. The test involves accelerating the engine at least 3 times to its maximum unloaded speed.
Note: On engines with a timing belt, it is VITAL that the belt is in good condition before the test is carried out.
- With the engine warmed up, it is first purged by running at around 2500 rpm for 20 seconds. A governor check is then carried out, by slowly accelerating the engine to its maximum speed. After this, the smoke meter is connected, and the engine is accelerated quickly to maximum speed three times. If the smoke density is less than the limits given below, the vehicle will pass:
Non-turbo vehicles: 2.5m-1
Turbocharged vehicles: 3.0m-1
- If excess smoke is produced, try fitting a new air cleaner element, or using an injector cleaning treatment. If the engine is running badly, where applicable, check the vehicle's ECU for fault codes. Also check the vehicle's EGR system, where applicable. At high mileages, the injectors may require professional attention.
(You can find the original on the resource: PeugeotBook.ru)