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My usual comparative description of the tuning of a diesel uses ‘Grandma’s Kerosene Lantern’. In fact, the two items are not too dissimilar in the way they work. As you wind up the wick on the old lantern, you will noticethe light getting brighter. Once it reaches its maximum level of light for the available air, if you continue to turn up the wick the light doesn’t get much brighter and smoke will start to be produced.
Turn up the wick even further and the smoke coming out the top of the lantern starts getting very black, to the point where the light produced will actually reduce. The same goes for a non-turbo diesel. There is only so much fuel you can put in before the fuel level overcomes the air available and it starts smoking. This point is at its maximum level of tune.
So why are turbo diesels so easy to over-tune? By introducing a turbo, you have massively increased the available air to burn the diesel with. You have essentially
‘raised the bar’ before that warning sign of smoke will appear. Without the visible smoke, the safety zone for the engine is no longer apparent until it’s too late.
So many stories have I listened to of people telling me how ‘the turbo’ blew up the engine. In reality, the guy who adjusted the fuel level actually blew up the engine – not the turbo. Diesel engine failures that happen and contain the words ‘cracked pistons’, ‘cracked cylinder head’, or ‘fried motor’ are really the result of over-tuning for the level of work the engine will be doing.
In summary, over-tuning may not damage an engine immediately. It will really depend on how long that engine is under full load whilst being over-tuned. Remembering that most diesels are towing, it is very important that when your mechanic tunes your engine they take into consideration higher-than-usual outside temperatures and sustained full engine loads that are found commonly on a long towing journey.