Tuning IP: Overall Fueling screw. Where and how.

[TRUTHautomotive]

Here's what I've been working with over the past month on my 94 L300 4D56T

Symptoms:
Bad fuel mileage
black smoke on full boost and kickdowns under load
black smoke at mid boost (2000rpm) on full throttle

So I advanced the pump 1.5mm and noticed a quieter engine, better performance and economy. Good. But still smoke at the before stated ranges.

Thus I've been incrementally making small adjustments to the boost compensator. Increasing preload of the spring by adjusting the star nut up, and rotating the diaphragm in 45 degree increments towards the minimum fuel position, noticing small improvements. So far I have improved my economy from 13L/100km to 11L/100km. Good. But I think this thing can do better, like 9 or 10L/100km.

Well today I just hit the total minimum on my diaphragm, still smoke and so excessive fueling.

I just learned of an overall fuel adjustment screw the adjusts just that. I feel like this is the last adjustment I need to make to bring this thing into line. Question is where is it and what direction do I turn it in?

I found a screw in between the four fuel lines on the back of the pump. I suspect this is it. Any thoughts?

[FalcoColumbarius]

If you adjust that screw ~ make sure you make a note of the position it is originally in. When you adjust the screw, you do so at small increments. A quarter turn is too much, you should be thinking along the lines of degrees, like five or ten degrees, max. There should be a cover over the screw to protect it, this cover should be replaced after you've adjusted it. The screw has a locking nut around it that requires a 13mm deep socket to loosen. You must re-tighten that nut when you've adjusted the screw. What that screw does is adjust how much fuel is being distributed into the cylinders, in relation to how much air is going in. The more fuel you add the hotter the engine will run, however, if you restrict the fuel too much your engine will also run hotter as with the more air to fuel ratio the combustion will be hotter. There is an optimum setting, a sweet spot if you like. It's all about balance.

Honestly, before screwing with your settings ~ the first place I would look at is the air filter element. See what colour it is. it should be soft white as the driven snow. the grayer it becomes the less air going through. take the element out of the air filter system and shine a trouble light against the side and look down the middle and see how much light is getting through. This is a good indicator to how clean your filter is and how much air is getting to the cylinders. A simple replacement of a dirty air filter element will cure many issues.

One other thing. If you turn that screw too much the engine can take off on you (runaway diesel engine) and turning the key off won't necessarily stop the engine. If you find yourself in this situation ~ block off the air intake to starve the combustion of air. If that doesn't work then get away from the vehicle and break out your calculator to determine the cost of a new engine and turbo.

This is one of many YouTube vids about runaway diesels:


I'd recommend further research before toying with your settings. Having said that I have adjusted my fuel screw.

Falco.

P.S.: The fuel itself can be a determiner, check out "Diesel Moot" ~ not all diesels are created equal. There's also your fuel filter to consider.