How will removing a DPF affect the smoke test?
A DPF is designed to filter out soot from diesel exhaust gasses, therefore a DPF will reduce the opacity of the smoke.
So why does a car not fail the smoke test if the DPF is removed?
Removing the DPF will not cause the opacity level to rise high enough to fail, in fact, it is very difficult to get a diesel vehicle to fail an MOT smoke test, the fail mark is very high.
It is worth remembering that DPF’s were not designed to help cars pass the MOT smoke test, they are a result of recent European legislation and the continued pressure on car manufacturers to clean up their emissions. In fact the diesel smoke test was implemented before DPF’s were even applied to cars.
Is it impossible to fail the smoke test?
If a vehicle is in good mechanical condition and has been well maintained then removing the DPF will not result in a smoke test failure. However, a mechanical engine issue which results in the engine running too rich and producing excessive smoke could lead to a failure. In fact, it is quite common for a simple mechanical issue such as a split intercooler pipe or jammed EGR to cause the engine to produce excessive smoke and block the DPF, once the DPF is removed the underlying engine issue often becomes apparent.