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Diesel injector spot colour meaning?

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I've noticed on my SDIs, there are tiny little paint colour spots between the bleed pipes on each diesel injector. On one car they are green, the other yellow and on two spare sets I have white and also light blue. I have a hunch that they are flow matched at the factory. The lift or pressure when they open has been measured on each injector during production and then they've been colour binned? Does anyone know what each colour means?

When you use diagnostics to read the idle injection amount trimming, it may say -0.2 0.0 +0.5 -0.3 in milligrams of diesel (the ECU obviously averages out the amount to be zero). The differences by my reckoning are the small variations in the injector opening pressures. From my experiments, I've noticed that you can remove the shim in an injector to up the pressure at which it opens. When I mixed colours, the ECU really didn't like it- I got readings like -2.1 -0.3 -1.2 3.6 where the 3.6 injector was a yellow one in a batch of greens. It ran rough at idle. So, is that all it is? Just a matched set?

Begs the question, which colour gives the biggest punch....? Which colour for which pressure? Or is it flow? Just a wild guess right now....

Has anyone ever got a straight result of zero across the board? I'd guess that it would be the same for AHU and 1.9 engines with turbines (all non common rail, non PDI systems, with a timing sensor on one injector). 

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