Diesels & The MOT

Introduction

The MOT seems to scare most drivers of older cars, even those who keep their car in fine fettle

I’ve taken several cars through their MOT. In fact, all of them apart from Kermit have had at least one MOT under my care. The Cinquecento was the newest car I ran - she had her first MOT the summer before I sold her. In order to get through, she needed some work doing to the rear drum brakes, but was otherwise in fine condition.

It is drivers of diesel cars that seem to worry the most, usually on the grounds of the “soot test.” This involves running the engine up to the governor and checking the opacity of the exhaust. Turbocharged engines must score under 3·5 whereas “atmo” diesels have a limit of just 3·0.

The Mondeo TD flew through the soot test. She scored an average of just 0·66 on her last MOT - the first run was around 0·9, and the second and third tests were just over 0·5. This compared to an average of 0·72 for the previous MOT. The injectors were never replaced or reconditioned.

The best time to spot soot is when another car is following at night. You might not see the soot during the daytime, but you’ll usually see it in the lights of a following vehicle. Extend the engine: if you see a whopping big cloud of smoke, don’t immediately assume that your engine is smoking like a trooper! The lights of the following vehicle exaggerate the smoky trail.

If you can hardly see the car behind, day or night, then something is probably wrong.

What causes diesel soot?

First, it’s important to make sure that the soot (or smoke) is indeed black. White or light grey smoke is unburnt diesel and is most commonly seen when starting the car, and perhaps the initial running when it’s very cold.

Most black, smoky soot is caused by incompletely burnt hydrocarbons. Petrol cars can also produce black soot (look out for an older car running with the choke, or a modern car that’s over-fuelling). Generally, a diesel will produce black soot when you boot it, as the engine cannot burn all of the fuel before it is chucked out.

Another point to bear in mind is that some of the soot may be dislodged from the exhaust during the MOT - this is especially true if you are gentle with the car.

This can be caused by a number of things, one of which is inherant engine design! However, running with a dirty, partially clogged air filter increases soot, as the engine cannot take in enough air to burn all of the diesel. Partially clogged injectors can also cause big problems, since if the spray pattern is disrupted, the diesel fuel does not form into a fine mist, and so not all of it can burn as well.

Pre-test Advice

There are a number of simple things that you can do to ensure that your engine is at its best ready for the MOT. Firstly, if it is due a cambelt change, get it replaced. You don’t want the cambelt snapping either at the MOT, or leading up to it, because that may be a new engine job.

If the car is due a service, it’s a good idea to get this done either on the day (before the MOT), or a week or two before. That should give you a cleaner air filter (unless the garage didn’t bother to change or clean it!).

If you are worried about leaving a sooty trail behind when you accelerate, use some injector cleaning fluid - but use it in the tank before the test. If you are using it when the test is performed, and it is working properly, you engine will be cleaning itself during the soot test and this will increase the soot content!

When you come to fill up for the tank that the car will run the MOT on, use diesel from a busy filling station. The last thing you want to do is be running stale, old diesel. Some people advocate using low sulphur diesel, or the “quality” stuff from Shell and the like, but I have never noticed any difference myself.

If you are not having a service first, make sure that the engine is at normal operating temperature when you arrive at the MOT testing station. This is because catalytic converters and EGR (exhaust gas re-circulation) systems need to be at normal temperature to be working at their peak efficiency.

Finally, on your way to the MOT test centre, do a bit of Italian Tuning yourself. Stretch the engine’s legs. If you never find the engine speed governor all year, make the day of the MOT the time when you do. It’s important that your engine is in fine health for this, of course - running the revs up to maximum does stress it. It’s also wise to ensure that the engine is up to normal operating temperature, too! It’s is best to put some load on the engine, and my preferred gear would be second or third, subject to the speed limit in force on that road and the performance of the car in any one gear. Run the engine up to the maximum engine speed and hold it there for a brief moment, and perhaps do this several times during the trip. This will help clean out the exhaust system of any gunk that would otherwise be cleared out during the soot test.

Okay, I failed, what now?

If the car failed by a little bit, you’ll find that some more Italian tuning, a clean air filter, and maybe some injector fluid will see it through the test.

If the result is higher than the specified limits, you should see a cloud of smoke behind the car when you accelerate hard. If you don’t, there is a chance that the equipment in use at the MOT centre is not working, or the operator misread / doesn’t understand the reading. Query the result!

If, however, the car is leaving big black clouds of soot wherever it goes, you may like to consider having the injectors reconditioned. I have never had to have this done to any of my cars - I was expecting the Mondeo’s injectors to need it doing since she had covered over 100,000 miles. If the injectors are clogged up, the chances are the car is not performing as it should, and will also be using up more diesel than normal.