Why did my car fail MOT on emissions?

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Why did my car fail MOT on emissions?

Post by warpc0il »

There are various reasons why the RX8 can fail the MOT emissions tests.

Decat, missing cat core (poor man's decat) or failed cat.
Your car can't pass the MOT emissions test without a fully working cat.
The original cat is a massive beast and very good at its job, until something kills it, like bad ignition dumping lots of fuel and soot down the pipe.
Some aftermarket Sports cats can struggle to keep up with the exhaust output of a Renesis, even when new, and few last more than 20K miles before then burn up.
There are some good alternatives to the OEM cat from rotary specialist suppliers.

Premixing, adding oil - usually 2T - to the fuel tank.
Included in this list because many people seem to think this will cause emissions failures, but it doesn't.
At the ratio that you get from adding 100-300cc of oil per full tank of fuel, the difference in emissions is undetectable, both in real time and in the impact on cat performance in the long term.
However, old fuel left in the tank for a year or so, can have an impact on measured emissions, so if the car has been off the road for a while then treat it to a top-up of fresh premium fuel before the test.

Ignition system issues
Anything that prevents all four plugs from producing a good spark will have a bad impact on emissions.
Leading spark failures should be noticeable by a reduction in power, unless this is the first time you've driven an 8.
Trailing spark failures can go undetected by the driver, especially in normal traffic conditions.
Faults can range from anything between the loom, coils, HT leads and the plugs themselves.
See viewtopic.php?f=25&t=36282

Test Conditions
The engine and the cat need to be fully hot at the time of the test.
If you leave your car to be tested and they wait a couple of hours before checking the emissions, then it's likely to fail.
Make sure that the emissions are tested as soon as you arrive, ideally after at least a 20 minute hard drive.

Engine Load
There are two things that increase the load on the engine at idle; the alternator and the air-conditioning.
Make sure that the battery is fully charged and the battery connections are clean and tight, so the alternator is at minimum load. Although the engine needs to be hot, don't test the emissions while the cooling fans are running, as this adds alternator load. Similarly, don't leave any other electrical load on, such as heated seats, HRW, cabin fan, lights etc.
Switching off the climate control reduces the electrical load and prevents the A/C compressor drive from engaging.

Fast Idle Test
In all the UK (except NI) the emissions test as two steps; normal idle and fast idle.
For the fast idle test the operator has to sit in the car and hold the engine revs at around 2k, while a timer runs on the test equipment.
However, if he doesn't start the test quick enough after increasing the revs then the PCM will cut them back (as it knows that the engine is being rev'd in neutral) just before the timer finishes. The tester has to blip the throttle to bring the revs up again and you get a Fail. Some testers know about this and will either get it right first time, or wipe the test result and repeat until they get a clean result. Others might just say "Failed mate", even though it's their fault.

Bad inputs to the PCM
The PCM manages the air:fuel ratio based on numerous inputs and if any of these are "off" then the mixture is wrong, resulting in poor running and emissions failures.
Things to look out for are dirty/oil MAF, poor electrical connection to the MAF/IAT unit, non-original replacement O2 sensors (sometime replaced in an attempt to diagnose an unrelated issue), Barometric sensor issues, intake vacuum leaks, exhaust leaks,....

Internal Engine failures
If the engine internals are significantly worn or damaged, then you're not going to get clean combustion.
The symptoms of poor starting, nasty noises or just lack of power, should already be obvious but will also cause emissions test failures. The combustion will be incomplete and the exhaust gases may also include burnt engine oil (side seals) or coolant (water jacket seal).

Overfilled Oil Level or lack of care when topping-up
The engine cases "breath out" as the temperature increases and the expelled air exits via the breather system and back into the intake manifold, to reduce overall vehicle emissions.
If the engine oil level is left too high then this breath can also include oil droplets or liquid oil, either all the time that the engine is hot or just when you accelerate harder and longer; e.g for that overtake or a motorway on-slip.
If it's bad enough it can blow over the MAF and create false readings (see above) but even at a lower level the increased oil burn can cause an emissions fail.
On the original cars the breather system was also prone to get contaminated with oil if you refilled or topped-up the oil too quickly, as it would back up in the filler neck and overflow into the breather. Mazda changed the layout of the breather from 2006 in an attempt to prevent this, though it's still possible if you don't take care.
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