Diesel Engine Control ECU Maps Analysis

If you are new to the idea of playing with ECU maps. Read this guide before you start.

Do not play with ECU maps until you understand what they do. Mistakes can be expensive!



To control a modern diesel engine you need to control:

Fuel injected quantity (IQ)

Fuel injection timing (SOI)

Fuel injection duration


In order to control fuel injection you must know how much air is flowing into the engine and you must know the engine speed.

So you have 5 factors very closely linked.

  1. Mass of Air Flowing (MAF)
  2. Fuel Injected quantity
  3. Fuel injection timing
  4. Fuel injection duration
  5. Engine speed (rpm)

The following guide  based on a 1.9 tdi pd VAG engine.

This engine has a capacity of 1.9 litres or 1900 cubic centimetres (cm3).

The exact figure is actually 1897 cm3.

The engine has FOUR cylinders so each cylinder is 474 cm3. (1897/ 4)

All four cylinders are identical so we only need to deal with one.

If one cylinder has a volume of 474 cm3, the maximum amount of air & fuel it can hold is 474 cm3.


Fuel request (Drivers wish).

The electronic accelerator pedal sends a signal to the ECU showing how much the driver is pushing the pedal down.

The measurement is usually a percentage.

0 % is no push so the engine idles. 100 % is full or Wide Open Throttle.

So at 0 % throttle the injectors must give a fixed QUANTITY of fuel, for a fixed DURATION starting at a fixed TIME. This results in a pre-set idle speed. e.g. 900 rpm.

So the idle speed has been Mapped to a specific value for MAF, QUANTITY, DURATION and TIMING.

At 100 % throttle the injectors must give a fixed quantity of fuel, for a fixed duration starting at a fixed time. So 100 % throttle has been Mapped to a specific value for MAF, QUANTITY, DURATION and TIMING.

This means that every other percentage from 1 % to 99 % will also need to be Mapped to a specific value for MAF, QUANTITY, DURATION and TIMING.


How much fuel should be Injected?

The ECU knows how much fuel to inject because it knows how much air is in the cylinder. So we need to know about AIR before we decide about FUEL.

If our cylinder has a volume of 474 cm3, the maximum amount of air & fuel it can hold is 474 cm3.

If we ignore fuel for a moment, it means the maximum amount of air that the cylinder can hold is 474 cm3

If air was a liquid, life would be easy. The amount of a liquid that fits into 474 cm3 is 474 cm3.

Air is a gas and so you can fit different amounts of air into the same space.


So how much air fits into 475 cm3 ?

This is determined by the density of the air and the density of the air depends on the surrounding temperature and pressure.

The density of air at sea level and on a warm day is between 1mg/cm3 and 1.2 mg/cm3.

Let’s assumes air density is 1.0 mg/cm3.


So our 474 cm3 cylinder will hold 474 x 1.0 mg of air, which is 474 mg of air.

So every stroke of one piston will suck in 474 mg of air. This is referred to as 474 mg/stroke.

So now we know how much air is in our cylinder (474 mg. We can inject some fuel.


Injecting fuel (IQ)


Our cylinder holds 474 mg of air.

Diesel burns at maximum efficiency at roughly 14.6 mg of air to 1 mg of fuel. So 474 mg of air can efficiently burn 32.5 mg of diesel fuel. (474 / 14.6)

So we inject 32.5 mg of fuel and off we go. Not really.


This doesn’t mean the injectors inject 32.5 mg of fuel per stroke (mg/stroke).

32.5 mg/stroke is the ideal maximum, assuming a normal air supply (EGR shut)

If the injectors inject more than 32.5 mg/stroke, some of the fuel won’t burn properly and will come out of the engine as black smoke. (This is often described as the smoke limit).


The injectors can inject any amount of fuel less than 32.5 mg/stroke and that’s what they do.

At idle the injectors may be injecting as little as 6.0 mg/stroke.

To make the engine speed rise the INJECTION QUANTITY is increased


The injection quantity is controlled by a map in the ECU often called Drivers Wish.


At idle the accelerator pedal will be set at 0 %, so no EXTRA injection will occur because idle speed is controlled by an idle speed map, not the Drivers Wish map.

When fully pressed down (wide open throttle. WOT) the accelerator pedal will be 100 %.

So the ecu receives a signal varying between 0 % and 100%.

If you apply 30 % accelerator pedal the ecu consults the built in DRIVERS WISH MAP checks the required INJECTION QUANTITY and injects that amount.




Unfortunately this is not simple because diesel engines don’t really measure how much fuel they inject.

Fuel injection is very complicated these days, so this is a very simple explanation.

Imagine a fuel injector is like a doctor’s syringe loaded with 100 mg of fuel.

The driver presses the accelerator pedal and WISHES for 30 %. The ecu consults the DRIVERS WISH MAP and decides to inject 16 mg of fuel. SIMPLE.



When do you inject the fuel and how long will the injection take?

Engine designers measure time in degrees of rotation of the CRANKSHAFT. That is why you hear people referring to engine timing.

The ideal point to inject the fuel is generally taken as Top Dead Center.(TDC). This is the point when both valves are usually shut and the air has been squashed to its maximum.

TDC is often referred to as Degrees Before Top Dead Center (BTDC) or Degrees After Top Dead Center. They are both the same thing, just opposites of each other.

So 4°BTDC is the same as -4° ATDC. (Only BTDC is used here)


Injecting 16 mg of fuel will take time (DURATION) and because the piston is going up and down, you need a START OF INJECTION point.


Lets assume 2 mg of fuel takes 1 degree of crankshaft rotation (°CR) to inject.

Assuming that the injection best time is 0°BTDC and 16mg will take 8°CR to inject. (DURATION)

Injection will need to start at 8°BTDC instead of at 0°BTDC.

Start of Injection (SOI) has to be 8 degrees BTDC so that all the fuel has been injected by 0°BTDC.


So the ecu needs maps to decide on;

INJECTION QUANTITY as requested by the accelerator position.

INJECTION DURATION as calculated from the injection quantity

INJECTION START (SOI) as calculated from the injection quantity.


Assuming that the engine is in perfect condition, the maps for Injection Quantity, Injection Duration and Start of Injection will be accurate.

So a precise amount of fuel will be injected for the correct amount of time (DURATION), starting exactly on time.(Start Of Injection)


The ecu can be sure of this because the crankshaft and camshaft sensors give precise details of the piston positions. These measurements end up on the dashboard as engine speed measured in Revolutions Per Minute (rpm).


Below is a graph showing the amount of fuel (IQ) being injected into a cylinder to cause the engine rpm to rise.

IQ vs RPM graph 19 pdplusnote

On this graph you can see that;

At idle (850 rpm) the ECU is injecting 23mg of fuel.

32mg IQ raises engine speed to 1050 rpm.

45mg IQ raises engine speed to 1400 rpm.

52mg IQ raised engine speed to 1900 rpm.

The ECU could carry on injecting 52mg to make engine speed rise higher (As per the added RED line)

BUT it doesn’t.

52mg IQ raises engine speed to 2550 rpm but after that IQ is reduced by the ECU.

50mg IQ is used to achieve engine speed 3200 rpm.

48mg IQ is used to achieve engine speed 3750 rpm.

44mg IQ is used to achieve engine speed 4200 rpm.

38mg IQ is used to achieve engine speed 4550 rpm


So the ECU is LIMITING IQ as the engine speed rises.  Why is IQ being limited?

The amount of fuel being injected (IQ) must be limited for a number of reasons.


  1. We can only inject fuel if we have enough air to allow it to burn.

This is the Air supply limit known as MAF limit or smoke limit. (See smoke map)


  1. We only want to produce engine power in a smooth controlled manner to protect the clutch, gearbox etc.

This is the Torque limit. (See Torque map)


Lets look at limiting factors.




We have established that our 474 cm3 cylinder will hold 474 x 1.0 mg of air, which is 474 mg of air.

So every stroke of one piston will suck in 474 mg of air.


The ECU can measure the Mass of Air Flowing thanks to a Mass Air Flow (MAF) sensor.

This sensor constantly monitors the air flow and sends a signal to the ECU. Typically the air flow varies from 0 to 1000 mg/stroke.  The MAF needs to read above the normal atmospheric value of 474mg because of the addition of a turbocharger.


The MAF sensor plays a big role in EGR function but I won’t go into that here.

So our smoke map or IQ limit by MAF is based on information coming from the MAF sensor on the engine. Obviously the MAF sensor must be in good condition and give accurate results or the ECU will make the wrong IQ calculations.




Air pressure and temperature vary depending on where you live in the world, weather etc.

The following assumes an air temperature around 20 °C and an air pressure of 1000 millibar (mbar).


Let’s assume our engine cylinder is receiving air at 1000 mbar pressure and 20 °C temperature at a rate of 474 mg/stroke. (To keep things simple, I am assuming no EGR is involved)


A typical turbocharger boost value adds an extra 1000 to 1500 mbar of air pressure. So a typical turbo boost pressure graph against rpm will run from 1000 mbar (no boost) up to 2500 mbar max boost. (That’s an extra 1500 mbar boost)


The extra air pressure means extra air so if we have 474 mg of air at 1000 mbar we can have 948 mg of air at 2000 mbar. (2 x 474) So with twice as much pressure we have twice as much air in the cylinder and can burn twice as much fuel at the same efficiency as before.

This results in the engine developing more power.


The engine ECU needs to know the boosted air pressure so the engine has a boost pressure sensor (manifold absolute pressure (MAP)sensor.).

The ECU also needs to know the air temperature so the engine has an Intake Air Temperature (IAT) sensor.

The desired Boost for engine speed and IQ is controlled by the Boost map. This map tells the ECU how much boost is required for a specified engine speed and IQ.


So we stamp on accelerator, get maximum IQ, maximum boost and off we go.  Not really.


The turbocharger doesn’t instantly change its turbine speed and boost pressure. It needs to spin up to speed. So the ECU needs to allow for the ‘spin up’ time.

Once up to speed the turbocharger will give maximum boost as per the boost map, which is fine for acceleration but most drivers don’t accelerate all the time, they cruise. So at 70 mph on the motorway the engine may have 2500 rpm thanks to high gearing.

IQ may have dropped to 32mg/stroke so the air needed is less than during acceleration so we don’t need lots of boost.

So the ECU needs to be able to control boost levels and make decisions about them.



Turbocharger boost control.


As explained earlier the turbocharger needs to be controlled because the engine design and fuelling maps assume a certain BOOST level under certain engine speed and IQ conditions.

The engine ECU therefore uses the boost pressure sensor (MAP sensor.) and Intake Air Temperature (IAT) sensor to gather data about current boost conditions.


These sensors allow the ECU to compare current boost pressure with boost pressure maps stored in the ECU.

(The ECU also has a Single Value Boost Limiter (SVBL) which acts like an emergency cut off for boost.)

The turbo boost map controls the boost level inline with the required IQ.


The actual control of boost is via an electrical signal that controls the opening of a vacuum valve called the N75 valve.

The engine ECU varies this electrical signal to vary the amount of opening of this valve.

The ECU contains a map for N75 Duty cycle. The map ensures that the correct amount of boost is available as set by the Boost map.


Boost control has a limiting map known as the boost limit map.  This map is to protect the turbocharger. It is based on the measurement of atmospheric air pressure.

Remember we decided to think of air pressure as 1000 mbar at 20 °C.

If the atmospheric air pressure and temperature never change we won’t need a boost limit map.

In real life, air temperature changes all the time and atmospheric pressure changes with the weather and when we drive up and down mountains so our cars will need a boost limit map to protect the turbocharger and stop the ECU raising IQ when the turbocharger can’t provide enough air. (Like when you drive up a mountain…If you do

If the boost stays outside the range of the boost limiter for too long, the ECU will switch the boost OFF. (Limp mode)


The engine ECU also contains a Single Value Boost Limiter just in case the turbo control fails. The turbo will be switched off if the actual boost goes above the Single Value Boost Limiter. (SVBL).


Hopefully the above information will give you a clue about your turbocharged diesel engine and how it works.

Lots of things in the ECU are inter-linked and changing one ECU map can have unexpected effects on other maps so it is vital that you think before you act.

The most basic changes in the ECU mapping will require changes to

  1. Drivers wish map
  2. IQ limit by MAF (smoke map)
  3. Boost map


If in doubt…Don’t do it.

Think before you make changes.

Mistakes can be expensive and even dangerous.