Non-Intercooled L series Tuning

Tuning the non intercooled L series diesel can be done manually without any laptops, chips or soldering irons. Just a toolbox can see considerable increases in performance and drivability.

People have attempted to describe how to tune the engine with various claims of “half-a-turn” here and “2 turns” there. Well sometimes that works but often it doesn’t. What is suitable for one car may not be suitable for another which is why you’ll see I don’t give any definate instructions. Sorry if you were expecting them but you won’t get them in this guide as all cars are different. What you will hopefully get is an understanding of what adjusts what and how to tune the pump to give the performance you are after.

The Basics

Before you start tweaking you need to have a basic understanding of how a diesel works. The sole engine speed and power control is the amount of fuel that you add which is controlled by the fuel pump.

The throttle pedal is connected to an arm on the pump and then it adds an appropriate amount of fuel through the injectors per engine revolution. The appropriate amount of fuel is the key thing here. To little and you’ll get poor performance. Too much and you get too much smoke, high exhaust gas temperatures, excessive fuel consumption and again less than optimal performance.

Because our engines are turbocharged the fuel pump also has to use a boost pressure feed so that it increases the amount of fuel to keep the air/fuel ratio the same.

A well tuned diesel engine will emit a constant (acceptable) amount of smoke under full load from idle speed right up to its rev limiter. This is a key bit of information and what you are aiming for in an ideal world.

A constant amount of smoke equates roughly to a constant air/fuel ratio which means no lack or excess of fuel at any points in the rev range which will equate to a loss of performance.

The Adjustments

The Max Fuel Screw

This is the basic control for the fuelling level. It controls the maximum amount of fuel that the pump will deliver per stroke when you open the throttle fully.

Think of it as the most basic adjustment you can make and can have a huge effect on the fuelling. Adjusting it affects the fueling over the entire boost range. The graph below shows the effect of turning up the max fuel screw. You can see it increases fuelling everwhere.

The X axis of the graph is engine speed, the Y axis is fuel quantity. You can see the increase in fuelling as the turbo comes on boost and so there is more air in the cylinder so more fuel is needed.

N.B. These are not real graphs they are made up data to try and get my point across otherwise most people get confused.

Red line = Standard
Orange line = Max fuel screw screwed into the pump

As you turn the screw inward you’ll notice the fuelling increases everywhere which includes at idle which has the effect of increaseing idle speed, which you will have to reduce to compensate. This is simply a screw that props the throttle arm open a little on the pump so its pretty obvious what to do with it to increase and decrease idle speed. You’ll also notice that there is a locknut to prevent both the idle and max fuel screw from accidentally turning. To turn the max fuel screw inward you may have to remove the anti-tamper collar on the screw. This is best done with some stout pliers and a sharp tap is often needed to break the spot weld on it. If you have trouble removing the anti-tamper collar then the whole screw can be removed from the pump (some diesel will leak out of the pump). It is best to leave the locknut in position so you know how far to screw it back into the pump. Small adjustments make big differences so 1/4 turn at a time is advised until you get used to the adjustments.

Smoke Screw

This name is slightly confusing to many people, but ignore the name for a moment. What it does is adjust the preload on the boost compensator part of the pump, i.e. the bit of the pump that increases the fuel in proportion to the boost pressure. So in effect screwing it inward makes the pump think that the turbo is producing boost pressure when it isn’t. Its effect is to increase off boost torque. Many people will suggest screwing this inward, don’t be too eager to follow their advice initially! When tuning this is the last adjustment that I make but we’ll go into the whole process later. The graph below shows its effect if the screw is screwed inward. The further in you screw it the better the off boost performance will be but the more smoke you’ll create hence its name, its job is to control off boost smoke. The more perceptive of you may notice that the difference between off and on boost fuelling is decreased and this can become an important and undesirable factor when tuning vehicles substantially above their design performance.

Red line = Standard
Orange line = Smoke screw screwed inward


This device alters the spring pressure on the boost compensator. This means that if you increase the spring pressure the boost will increase more slowly as the turbo comes on boost. Conversely if you decrease the spring pressure the fuelling is increased more quickly as the turbo comes on boost. This is a very difficult adjustment to get right, and most people won’t ever delve this deeply into the pump.

Red line = Standard
Orange line = modified (left = decrease in spring pressure, right = increase in spring pressure)

Limit Shim

This is a mod again not needed for the majority of people out there. I’d go as far to say if you have a standard turbo then you don’t need to worry about this. The boost compensator increases fuelling according to boost pressure. As boost pressure gets higher it may reach the a limit stop. This limit is a small plastic circular shim inside the pump. If you make the shim thinner or even remove it entirely you may gain a little extra travel in the compensator mechanism and so more capacity for increaseing fuel at higher boost pressures. The only one I’ve removed entirely was on a car running 27psi so as I say unlikely to be needed for most cars. Before thinking about removing it you need to make certain that you still have travel left on the boost compensator. If you don’t pump damage may occur or even a decrease in fuelling at high boost levels.

Red = Standard
Orange = Shim reduced in thickness or removed

The way I do it

I’m not going to say this is how you have to do it, or this way is the best way, its just the way I’ve founf works best for me. Before starting the car should be up to running temperature, have a boost gauge fitted, have the boost set at an appropriate level and be in a good general condition, e.g., a good cambelt, working cooling fans etc. The boost level should be checked during the tuning process as it may change. If it increases too high then decrease it back to a sensible level before proceeding. You will also do many, many test drives so an area of unrestricted road where test drives can occur is also needed. Ideally an assistant can drive the car whilst you watch how much smoke is produces from the exhaust.

First note down where the original adjustments are so that you can return them to this if necessary. The easiest way is to count the number of threads on the max fuel and smoke screws. You also need to make the decision if you are brave enough to open up the pump top. If you do then also count the starwheel screw threads too.

Then I start by unscrewing the smoke screw as far as I can without it falling out. You need to loosen the locknut on this before unscrewing it. Then tighten the locknut back up. You’ll adjust this again later.

Next you adjust the max fuel screw, screwing it in 1/4 a turn at a time and test driving it until the smoke level becomes too high at some point in the rev range, and then back it off until it becomes acceptable again. You’ll need to reduce the idle speed during this process, but its pretty obvious how to do it as it is simply a threaded rod that props the throttle arm open a little. Make a mental note where this excess smoke is in the rev range if you are delving deeper into the pump adjustment as you’ll need to make adjusments based on whether you are getting too much smoke off boost or on boost.

If you aren’t delving deeper into the pump then you have one final adjustment to make. Remember at the start we wound out the smoke screw. Well now it is time to wind it back in. As you do this you’ll find that the off boost performance increases a lot however you’ll increase the smoke. You have to judge for yourself where the balance lies between smoke and performance off boost. Adjusting this screw won’t affect the on boost performance at all.

Once you are happy with this adjustment then thats it, you have finished and I hope you are pleased with the results.

If you are delving deeper then this is where it becomes a little more complicated potentially, as the adjustments you make will be dependant on where you see smoke in the rev range. It’ll soon become obvious why I spent some time explaining exactly the effect of each adjustment earlier as without this it is very easy to get lost and make a right mess of things!

If you are getting smoke under low boost conditions before the turbo is producing full boost, but the off boost smoke is fine and smoke at full boost is ok, you may wish to increase the spring pressure on the diaphram by adjusting the starwheel. This has the effect of delaying the fuel enrichment as the turbo comes on boost. You want to balance the smoke at low boost levels with how quickly the turbo comes onto full boost. More fuel makes the turbo achieve full boost sooner but also generates more smoke.

If you are running high boost levels then you may find that the amount of extra fuel available with the standard fuel enrichment is not sufficient as there isn’t sufficient travel in the enrichment mechanism. One possible solution to this which I have tried quite sucessfully on one car (145bhp SD – see the hall of fame for pics), is to remove or reduce in thickness the plastic shim that limits the travel of the diaphram and plunger unit. This effectively give the mechanism more travel and so potentially more fuel at higher boost levels. However a word of warning here, you must make sure that the boost mechanism doesn’t travel too far as you can actually decrease the fuelling if the plunger travels too far or worse still cause damage to the fuel pump, so this mod is best left to the pro’s for now.

Once you are happy with the star wheel adjustment and any alterations in shim thickness then you are almost done. Remember at the start we wound out the smoke screw. Well now it is time to wind it back in. As you do this you’ll find that the off boost performance increases a lot however you’ll increase the smoke. You have to judge for yourself where the balance lies between smoke and performance off boost. Adjusting this screw won’t affect the on boost performance at all.

Once you are happy with this adjustment then thats it, you have finished and I hope you are pleased with the results.

As you can see lots of test driving is needed to get the results right and experience helps in judging how much to adjust things just right and to judge how much smoke is acceptable or otherwise but hopefully this article will give you some helpful hints and tips and save you a lot of time. I had to work it all out from scratch!