I took the now repaired Fiesta back for another go at the MOT. Regular readers will recall that in addition to rear suspension bushes and a rusty brake pipe, it also failed the last test with truly awful emissions. The natural idle CO reading was 5.39% against a limit of 0.5%.
With such a huge discrepancy I was seriously concerned that something very expensive had failed but I couldn’t see how since it had only drive 6 miles since the previous failure when the CO reading had been 0.3%
Remembering the garage’s opinion that maybe it needed running hard to clear out the engine, I decided to run it for a while just before the MOT. I started the engine and then turned on as many electrical items as I could to load the alternator. Then I slowly increased the revs to 3000 and held it there for five or six minutes. The engine temperature went well above its normal running range and I hoped that this would encourage any gunge to burn off. I also hoped it would help the engine management computer to re-learn the engine after the battery had been disconnected when it was off the road.
Anyway, I took it to the MOT centre and left it whilst I went in to town to buy new glasses. When I got back I was shocked to discover that it had passed the MOT and had sailed through the emissions test first time. When I say sailed through, I mean that it had almost no emissions whatsoever. The CO reading had dropped from 5.39% to 0.00% (yes, zero) for both fast and natural idle and the HC reading was a miserly 1ppm. It wasn’t that good when it was first tested at three years old!