Exhaust gas temperature

Exhaust gas temperature measurement
VAG-COM blocks 034 and whatever else only display an internally calculated O2 sensor temperature, which is what most people think are real EGTs. That value may or may not have anything to do with the real EGTs or temperature at the primary O2 sensors; in fact, those temperature values are stored in 2 maps where the inputs are load and RPM.

Block 112 is the only block where a real EGT value is displayed, and only when the actual EGTs exceed 945&deg;C. This is only due to the design of the sensors and their electronic modules, which makes that they can only "read" 945-1035&deg;C and that's all. So basically if all you see is 945 all the time, it means your actual EGTs never exceed 945.

If you see block 034 EGTs hitting 750 or 800 or 850, those values have never been, are not and will never be actual EGTs. The only real EGTs are in block 112. If you want to know what they are when they're below 945, then you have to use RS6 sensors with adequate programming, so you get a range of -40 to 1100&deg;C and then you will know what your real EGTs are.

EGT enrich
EGT enrich occurs at 980&deg;C from the factory and AFAIK not many people play with it. It can be set lower, because 980 is a lot for any car. When you do actually exceed 980, you'll see some percentage values (also displayed in block 112), which shows how much extra fuel the ECU is dumping into the combustion chambers to keep the EGTs at 980 or as close to that as possible.

Exhaust gas temperature control
For turbocharged engines, the maximum permissible exhaust gas temperature is a key design criterion.

To protect the exhaust gas turbocharger and the exhaust manifold, the exhaust gas temperature should not exceed 1000°C for a lengthy period of time.

Since many of the components which influence the exhaust gas temperature have tolerances, thermodynamic adaptation previously took place at 950°C for safety's sake.

This was achieved by enriching the air/fuel mixture.

The exhaust gas temperature is recorded in a cylinder-bank-specific manner by the two exhaust gas temperature senders G235 and G236.

The Motronic controls the exhaust gas temperature to 980°C by enriching the air/fuel mixture.

It is therefore possible to largely dispense with the prophylactic enrichment process that has been standard practice until now.

The mixture is only enriched
 * when necessary and
 * to the extent necessary.

This means that engine operation with lambda = 1 is possible up to high load and engine speed ranges.

To facilitate exhaust gas temperature control, the exhaust gas temperature must be recorded to a high degree of accuracy.

An accuracy of ±5°C is achieved in the measurement range from 950°C to 1025°C. The exhaust gas temperature sender is located inside the exhaust manifold upstream of the exhaust gas turbocharger.

It comprises a measuring sensor and evaluation electronics.

The measuring sensor and the control unit are permanently connected by means of a shielded, heat-resistant wire.

The evaluation electronics convert the signal which the measuring sensor generates into a pulse-width-modulated signal (PWM signal). This is a square-wave signal with a fixed frequency and a variable pulse duty factor. The pulse duty factor is expressed as a percentage. The measurement range extends from 10% to 90%. A specific pulse duty factor is assigned to each temperature (refer to diagram).

Substitute function and self-diagnosis
A pulse duty factor of <1% or >99% is recognised as a fault. A fault is detected as of a certain enrichment quantity. If a sender fails, the charge pressure is reduced to a safe level and an emergency enrichment characteristic (engine speed-dependent) is used.