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 the plate number sticker. This number determines the maximum smoke value permitted for you vehicle. The MOT tester will look for this sticker. However, if it happens to be missing or damaged on test day, a default value will be used instead. The default values are much less strict! In my case 1.5 instead of 0.6! Here is my sticker, you can usually find it under the bonnet, 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 then taken. My first acceleration was 2.01, by the sixth it was 1.2. Fail, it would have passed if the substituted value was used, but as my plate number was visible and intact, a value of 0.6 used.

Now usually when you search google for reasons to why you have failed, many will blame the DPF. A damaged DPF would be a reason for failure, one sign of this would be the opacity values remaining the same on successive accelerations due to a portion of the exhaust gas passing unfiltered.

Some will suggest a clogged DPF. This is an illogical answer, as if the DPF is clogged how do additional particulates get past it?

Another reason for failing is be bad fuelling 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.

In my case I know my car inside and out, my fuel trims and fuel economy are all within spec, nd there is no excessive oil in my intake. My theory was it had nothing to do with DPF or fuel. It was in fact just accumulated soot that’s blown out of the exhaust system during the MOT accelerations. No one in their right mind would ever quickly snap open the throttle day to day so the accumulated soot never gets blown out. I also drive in a very fuel efficient manner never above 2k or so, so my theory fits.

So with all this in mind i needed to clear out the exhaust system. I first thought of just replicating the test by holding it to the limiter for 20 or so times. However if your cam belt is due for renewal (Which mine was) There a potential for it to snap writing the engine off. This is such a problem that a tester will refuse to test your vehicle if its not been serviced. I decide a long run above normal RPM would be less damaging.

So the next day morning right before the MOT i did as follows.
I drove in third gear at 4000RPM 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 being single lane highway. 1 mile before the test centre I stopped in a quiet layby. I decided to due some accelerations after all, just in a gentler manner. I 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.

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

UPDATE March 2021

Immediately before MOT, Drove 4 miles at 3500-4000 RPM, did 10 accelerations (I changed cam belt this year) Passed on first acceleration with a value of 0.27! Also because my sticker was damaged, they used a substitute value of 1.5.

This all goes to show how silly the MOT smoke test is. A potentially engine damaging test procedure which is falsely fails vehicles.