MOT emission failure advice

edited March 2018 in Fuel

Emissions issues and failed test, help is here.
To keep it simple, we will assume the engine is running just fine, no misfires that could be a result of faulty plugs, leads or the coil pack. No vacuum leaks and the injectors are all seating properly and not leaking fuel in at the wrong time.
HC is hydro carbons. Limit for MOT/IVA 200 parts per million.
CO is carbon monoxide. Limit for MOT/IVA is 3% at idle and 2% at fats idle
Lambda reading limits for MOT/IVA are 97 – 1.03.
Let’s look at possible causes of emission test failure.
You have probably removed it or disturbed fittings or replaced it during your build. The gas analyser probe sniffs for un burnt oxygen and a tiny leak could cause air to be drawn in thus exiting the tail pipe un burnt resulting in a high Lambda reading.
The Lambda (Oxygen sensor) test fail is also known as incomplete combustion and high CO.
We assume you have one fitted and it does not rattle. If it rattles it is probably dead.
Continued running rich will poison a CAT.
If faulty there will be a reduction of the conversion of carbon monoxide to carbon dioxide thus high CO readings will be measured resulting in a fail of emission tests. HC may also read high if CAT is bad.
Some cars have post and pre CAT oxygen sensors known as lambda’s. If faulty the ECU will receive incorrect info and create a rich or weak mixture known as stoichiometric ratio.
MAF (mass air flow meter).
Again if faulty can cause high or low mixture.
The exhaust gas recirculating valve allows exhaust gas to be re burnt to reduce NOx (nitrous oxide).
They are designed to reduce combustion temperature and remain closed at idle. Therefore if CO is OK at idle and fails at fast idle it maybe the EGR that has failed.
The idle air valve should let a tiny amount of air in to keep the engine running when the main butterfly valve in the throttle body is shut tight. The IAV can stick though but can be cleaned out. If CO is high at idle but OK at fast idle a faulty IAV may be the cause. Try carb cleaner.
Bad emissions tend to reduce when the engine is hot, if you suffer a marginal fail try giving it some stick and getting it very hot and try again. At IVA they may well put a large fan in front of the radiator, this can cause the engine to run cooler than intended so if you suspect this ask the inspector to pull the plug on his fan, inspectors don't have many fans !

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