This guide applies to most VAG pd tdi engines.

It is a good idea to log your engine when it is healthy so that you have something to use for comparison when your car is not healthy.

The information here is only a general guide as there is variation between engines.

Pre CAN control engines tend to need more diagnosis while CAN control engines do more self diagnosis.

The following measuring block checks will help you investigate an engine problem that hasn’t logged a fault code.

It takes you through:

Engine temperatures

EGR and MAF diagnosis.

Injector diagnosis (You can't diagnose injectors using VCDS)

Charge air control (Turbocharger).

It is advisable to fit a new air filter and fuel filter before running any checks.

This assumes you know how to engine measuring blocks.

What to do.


A good, first check, is the engines coolant, fuel and air temperatures as these are very important for fuelling.

Find the block that displays temperatures.


007. 1 Fuel temperature

007. 3 Air Intake temperature

007. 4 Coolant temperature.

Start the engine from COLD and select log START.

Go for at least a 20 minute drive to bring the engine up to working temperature.

When you get back, save the log you have produced.

This will be a very long log which has Time Stamp as one of the columns.

You need to turn this log into an excel graph. (How to…is on this site).

The excel graph will look something like the one below.

I have only included Coolant temperature to make it easier to read.

Pre CAN vehicles like the B5 passat cannot self diagnose a cooling problem so you have to do a test like this.

CAN vehicles like the B6 passat will flag a fault code if the engine ECU detects that the warm-up graph is incorrect.

A Fuel temperature graph should look the same as the coolant graph. It is likely to be a few degrees lower.

If the fuel temperature graph looks odd or goes too high/low, change the fuel temp sensor.

Air intake temperature should be similar to the outside temperature on the day of test and should stay constant. (flat line on a graph).

If the air intake temperature is considerably different to ambient temperature, change the Air Intake Sensor.

The Fuel and coolant temperatures are far more important than you might think. The engine ECU may be relying on 6 Duration maps and 10 Beginning of Injection maps for Injection Quantity. The over riding factor for adjusting these maps is Fuel temperature. (Coolant temperature if fuel temperature sensor fails)

Adjustment temperatures are approximately:

-20, -10, 0, 11, 20, 25, 40, 55, 70, 89 Degrees Celsius.

So if the ecu thinks the temperature is below 89 degrees C, it will not run on it's optimum economical settings.


To evaluate Air flow you need to look at EGR (exhaust Gas Recirculation) and MAF (Mass Air Flow).

As far as VCDS is concerned, these are the same thing because most engines can only log data for the AMM (MAF) sensor. (The EGR measuring block values are MAF values)

Find the block that displays EGR values.


003. 1, Engine Speed, (G28), 819 /min, RPM

003. 2, Exhaust Gas Recirculation (spec.)

003. 3, Exhaust Gas Recirculation (actual)

003. 4, Exhaust Gas Recirculation Duty Cycle

Log the four EGR sections as you accelerate through the gears up to 4000 rpm.

Use Excel to produce a graph of your log.

It should look something like this;

This was taken on a cold day and it took a while for the engine to warm up to normal.

The important bits are the shape and height.

The gradual increase is what you should expect to see.

At roughly 88 °C the thermostat should open and the temperature should level off.

This MUST happen above 80 °C, preferable above 85 °C and ideally at 88 °C.

I have not included EGR duty cycle on the graph.

The EGR specified value at idle is 300-400 mg/stroke so the blue line starts low at 1300 rpm.

When the accelerator is pressed the specified EGR rises rapidly to 800 mg/stroke at 1400 rpm.

This hardly surprising as the faster engine must suck in more air.

You need the EGR specified trace on the graph so that you can compare it with the EGR actual trace.

The EGR actual trace shows you the Mass Air Flow into the engine recorded by the AMM (MAF) sensor.

You are expecting 300-400 mg/stroke at idle and 800-900 mg/stroke at high revs.

So the EGR actual trace should look similar to the EGR specified trace.

The pink EGR actual trace on the graph roughly follows the EGR specified trace.

If the AMM (MAF) fails completely the engine ECU reverts to an in built guess of air flow. This will log as the EGR actual figure constantly being 470 mg/stroke (approximately).

If the AMM (MAF) has failed and is giving silly results, the engine ECU won’t know and the engine will run rough, feel under powered etc. You can only spot this sort of failure by comparing the EGR specified trace with the EGR actual trace on a graph.

If the EGR valve is stuck open the EGR actual figures will constantly be lower than specified.

If the EGR valve is stuck shut the EGR actual figures will constantly be higher than specified.

Exhaust Gas Recirculation Duty Cycle %

This value is not much use for fault finding but it gives you a clue to what should be happening with the EGR valve.

The engine ECU is opening and closing the EGR valve according to the duty cycle.

100 % means the EGR valve is shut so NO EGR gases going into the Intake air.

0 % means the EGR is fully open so MAXIMUM EGR gases going into the Intake air.

During constant idle or acceleration the EGR valve should be shut so the duty cycle will be high.

Typical value being 95%

Between idle and acceleration the engine ECU will constantly open and close the EGR giving duty cycle values between 30 and 70 % typically.

It is worth logging EGR duty cycle during a spell of normal driving to see if it behaves as expected.

High value during acceleration or constant idle and middle values in general driving.

Note: EGR duty cycle is what the engine ECU is telling the EGR valve to do…It doesn’t mean the EGR valve is actually doing it.

DIESEL INJECTION (These blocks tell you what the ecu wants the engine to do....NOT what the engine is doing)

Don’t confuse information for Bosch solenoid injectors with information for Siemens piezo injectors.

Injection should be simple to investigate.

When you accelerate you inject more fuel for a longer duration and advance the timing of injection as the rpm rise. Simple. That is pretty much what happens but the designers have to balance performance against emissions regulations which means the control of injection timing, duration and quantity is very precise.

P.D engines have an extra complication because they have 4 injector pumps. One built into each injector.

Find the blocks that displays injection values.


001. 1 Engine Speed, (G28)

001. 2 Injection Quantity

001. 3 Injection Duration (specified)

001. 4 Coolant, Temperature (G62)


004. 1 Engine Speed, (G28

004. 2 Injection Start (specified) Ign. Timing

004. 3 Injection Duration (specified)

004. 4 Torsion Value (Idle Stabilization)

Log the 001 and 004 blocks as you accelerate through the gears up to 4000 rpm.

Use Excel to produce graphs of your log.

It should look something like this;

Block 013 can be logged at idle or any engine speed. It doesn’t have to be an acceleration log.

013. 1 Injection Quantity, Deviation Cyl. 1

013. 2 Injection Quantity, Deviation Cyl. 2

013. 3 Injection Quantity, Deviation Cyl. 3

013. 4 Injection Quantity, Deviation Cyl. 4

A graph of these results can be done against the time stamp column but it will look a mess.

It is simpler to average the results for each injector and look for any really odd figures.

All 4 injectors should have similar figures as close to zero as possible.


13, 1, Injection Quantity, Deviation Cyl. 1. -0.28 mg/str

13, 2, Injection Quantity, Deviation Cyl. 2. -0.49 mg/str

13, 3, Injection Quantity, Deviation Cyl. 3. 0.56 mg/str

13, 4, Injection Quantity, Deviation Cyl. 4. 0.21 mg/str

If one injector is well out of specification compared to the others it could be faulty or it could be a faulty cam lobe.

If all figures are all over the place it suggests worn camshafts.

It is hard to tell if an injector oddity is due to an injector fault or a camshaft fault.

Apparent multiple injector faults are usually worn camshafts.


Known as charge pressure control in measuring blocks 010 and 011.

Most of the important turbocharger information is found in block 011


11. 1 Engine Speed, (G28)

11. 2 Boost Pressure (specified)

11. 3 Boost Pressure (actual)

11. 4 Charge Pressure, Control Duty Cycle %

At idle the boost pressure specified and boost pressure actual should be the same at about 1000 mbar.

Log the 011 block as you accelerate through the gears up to 4000 rpm.

Use Excel to produce graphs of your log.

It should look something like this;

The engine has no way of measuring Injection Quantity, injection duration or start of injection so the figures are fairly meaningless.

They just show you what the engine ecu is telling the injectors to do.

This graph is for a 105 bhp 1.9 pd tdi.

The boost pressure specified shows a steep curve upwards from 1100 mbar at 1300 rpm to 2000 mbar at 1900 rpm. It then remains stable at 2000 mbar as the rpm continue to rise.

The engine ECU for this engine is mapped to keep the boost at 2000 mbar.

The boost should remain at 2000 mbar because the ECU should be closing the vanes inside the turbo as the rpm rise. (See duty cycle)

The boost pressure actual rises steeply from 1100 mbar at 1300 rpm to 2400 mbar at 2100 rpm.

This slight over boost is normal and is followed by a boost reduction between 2100 and 2500 rpm, as the turbo vanes close. Boost actual then follows that of boost required.

A 2.0 pd tdi will produce a similar shape graph but shifted to the right as turbo boost does not start until 2500 rpm.

An over boosting turbo will show a boost actual trace rising above 2500 mbar and immediately falling back to 1000 mbar. (Boost off).

An under boosting turbo will show a boost actual trace that either stays flat or doesn’t rise very much.

Boost faults usually set a fault code corresponding to “too much boost” or “too little boost”.

Over boost usually causes the engine ECU to put the engine into limp mode by disabling the turbo.

Switching the ignition off and on again resets the turbo function until it trips the ECU again.

Information about turbo boost is measured by the Manifold Absolute Pressure (MAP) sensor.

A faulty turbo or faulty MAP sensor will give the same faults.

If in doubt, change the MAP sensor. It is cheaper and easier.

Be careful if you look at Measuring block 11. 4 Charge Pressure, Control Duty Cycle % as it can be confusing.

B5 and B6 passats will produce different duty cycle graphs because they use different Engine Control Units. (ECU)

The Passat B5 uses one known as EDC15 series and the B6 uses the EDC16 series. (EDC stands for Electronic Diesel Control).

The boost duty cycle for an EDC16 will look like the graph below.

The figures start at about 80% (maximum boost) and fall to about 25% as rpm rises.

This stops overboost by reducing boost as the rpm rise.

The figures start at 80% because the duty cycle is the OUTPUT signal to the N75 valve telling it to stay open and apply vacuum to the turbo actuator to give MAXIMUM boost. As the engine rpm rise, the Output duty cycle for the N75 will be reduced in order to lower boost.

The same graph for a B5 Passat will be the exact opposite of this in terms of duty cycle. It will start at about 20% and rise steadily to about 80%.

So for a B5 Passat (EDC15), duty cycle follows boost. Higher boost means higher duty cycle. This is because the duty cycle is NOT N75 valve output signal.

Duty cycle is MAP sensor input signal. MAP sensor measures boost, so more boost equals more signal so the duty cycle % rises.

The graph should be a smooth descending line from maximum turbo vane angle ( about 85%) at idle to about 25% at 4000 rpm.

Note: The duty cycle is rarely of use in diagnostics unless all other factors have been checked and fixed. A fault is far more likely to be faulty MAP, N75 valve, turbo etc, than an ECU duty cycle problem.