Injectors
A modern diesel engine does not inject fuel once per power stroke. It does so three to seven times.
First come one or two pilot injections of 1–2 mm³. Then the main injection. And finally, often, a post injection, used to regenerate the particulate filter.
It ignites a very small quantity of fuel just before the main injection. That gives a gentler pressure rise in the cylinder when the main quantity burns.
The pilot injection is what separates a modern diesel from the shaking, booming diesels of the eighties. Without it the engine sounds as though something is wrong — because the pressure rise becomes too abrupt.
1–2 mm³ is a vanishingly small quantity. At quantities that small, even a few per cent deviation is clearly felt — as diesel knock, especially when the engine is cold.
And here is the point: an injector can deliver perfectly correctly at full load and still be faulty. The fault sits in the smallest test point, where there is least margin.
A test bench that only measures at full load will never find it. The vehicle has full power, the fault codes are empty, and still the engine knocks.
If your vehicle knocks more than it used to — particularly on cold start and at idle — it is worth having the injectors measured before you start replacing parts.
We test at several points across the whole operating range, precisely to catch this kind of thing. It costs DKK 250 per injector incl. VAT, and you get the measured data in writing.
