Passing The MOT Smoke Test

This is a little guide on how to pass the MOT smoke test after failing. I provide a description on what I did to pass the test.

Firstly if the plate number sticker happens to be missing or damaged on test test day, a default value will be used instead as a base value for the smoke test. The default values are much less strict! In my case below 1.5 instead of under 0.6! Here is my sticker, you can find it under the hood usually, but it can be on your door sill too. Look for a number in a square box.

How a smoke test is performed (2020)

The vehicle RPM is quickly raised to the limiter, these are known as accelerations. If the opacity value of the exhaust gas is below the limit on the first acceleration the vehicle passes. If not, up to a further 5 attempts will be made and an average taken. My first acceleration was 2.01, by the sixth it was 1.2 Fail. It would have passed if the default value was used, but as my plate number was visible and intact a value of 0.6 used.

Now bear in mind i have no DPF issues at all, and i do drive regularly at a constant speed on highway specifically to ensure regen occurs, i do exactly what the Volvo manual tells you to do, and have never once had a DPF type error. Also my fuel trims and fuel economy are all within spec. My theory was it had nothing to do with DPF or fuel. It is in fact just accumulated soot thats blown out the rest of the exhaust system on the MOT acceleration. No one in their right mind would ever quickly snap open the throttle day to day so it never gets blown out. I also drive in a very fuel efficient manner never above 2k or so, so my theory fits.

Its ironic the smoke test for pollution encourages driving in a less fuel efficient manor to pass the test! its a silly potentially damaging test.

So with all this in mind i needed to clear the exhaust system. I first thought of just holding it to the limiter for 20 or so times. However my cam belt is due to be changed this year, so that idea didn’t appeal. A long run above normal RPM would potentially be less damaging. Plus my theory is just a theory, maybe is is something counter intuitive about how the exhaust gas flows through the DPF, where by the extra heat would be beneficial. So my plan was to do a bit of both.

So the next day morning right before the MOT i did as follows.
I drove in third gear at 4K revs for 18 miles, whilst all the while keeping an eye on coolant temperature. Then a further 8 miles at only 3k revs, due to the road to the mot station is smaller single lane highway. 1 mile before the test centre I stopped in a quiet layby. I then slowly opened the throttle 100% and held it 3 seconds or so for 10 repetitions. I then drove straight into MOT test area, and immediately had it tested whilst hot. The result? It passed on first acceleration 0.56

Take from this what you will. Doing this is not going to guarantee a pass, it depends on why you vehicles particulates are high.

A clogged DPF reason seems to be an illogical answer,as if the DPF is clogged how the hell do addition particulates get past it?

A damaged DPF would be a possible reason for failure, that would be apparent, as the opacity value would remain constant on successive accelerations. Due to a portion of the exhaust gas passing unfiltered.

Another reason fail would be bad fueling or excessive oil in the intake, a DPF is only so efficient, somewhere north on 90% So if the particulates pre DPF are higher than normal, so would the particulates post DPF, again successive opacity values would remain high.

So to conclude. If you fail your MOT smoke test but your opacity values are falling on each successive acceleration, try the above.