Tuned Japanese cars and emissions

exhaust emissions

Evos can struggle come MOT time

One of the main issues, come MOT time with tuned Japanese cars is often the emissions. Often when your car goes for it’s MOT that you’ll receive a call from the garage to say you’re over the legal limit for emissions – Don’t panic. There are many reasons why the emissions may be reading over the limit.

Causes of high emissions can be:

  • Catalytic convertor not fitted. In order to having a hope of passing the emissions test, you’ll need to fit a Catalytic converter, even if just for the test. Many Japanese imports come into the country without a cat, and many owners remove them to gain a healthy increase in power output. Replacement cats can be sourced online for around £130, with a genuine Mitsubishi part costing a lot more. If you have a decat fitted then check beforehand that your exhaust is not a single welded length from the manifold back, as swapping a cat over will then be a lot harder requiring the old exhaust to be cut apart.
  • Faulty coolant temperature sensor. A fault coolant temperature sensor often fails in the ‘cold’ position meaning the ECU believes the engine is cold and is therefore adding more fuel than it should, when the engine is warm. This is cheap to replace and should show up on a diagnostic check.
  • Faulty lambda/oxygen sensor. A faulty lambda often means the fuelling on part throttle will be incorrect, with the ECU receiving incorrect data, it may overfuel incorrectly. Again, this is relatively inexpensive to fix.
  • Wrong temperature grade spark plugs. Many problems can arise from fitting the wrong plugs. Run too hotter grade and you risk detonation and sever engine damage. Too cool and there will not be a proper burn and you may again be running too rich, causing emissions problems.
  • Exhaust leak. Rare on Evo’s as most have stainless steel exhausts but it does happen with older joints rusting and leaking. This is usually an easy fix with a weld or new exhaust section.
  • Aftermarket ecu mapped too rich to pass. Tuned Jap turbo cars are often mapped to a certain specification and/or badly mapped. If the ECU is running too rich due to a poor map, there is little to resolve it other than remapping or replacing the standard ECU system. A overly rich map will of course require a remap to lean out the mixture. A map that is too rich is not good for the engine anyway, and will destroy a cat in a short period of time. Fortunately this is rare.

If the emissions result is marginal, you can often get the car through by ensuring the cat is nice and hot for the test, i.e. take the car for a hard drive beforehand.