Porsche ECU and DME Systems Explained

A few weeks ago I opened myself up to questions. More specifically, I noted that I’m not a Porsche Expert, but I play one on the Internet and offered to become your Porsche research assistant free of charge. I felt this would be a great opportunity for me to interact with my growing reader base as well as an excellent learning experience for me. As the saying goes, be careful what you wish for. To date I have received almost 20 emails with a variety of technical questions regarding Porsche cars and even one question on purchasing Porsche stock. I have responded individually to each answer seeker, but will now post a few of the more interesting questions and their answers over the next coming weeks.

The very first question(s) I received came from Carlos who just recently acquired a 2002 996. His exact question is below:

“I bought a 2002 996 with an x51 engine. The original engine was replaced around 22k to 24k miles (not sure exactly when). Can you give me a more detailed difference between this engine vs the regular 996 engine?”

“I understand with the x51 comes PSE (exhaust) and ECU flashed. Is this true? Kindly elaborate if possible.”

“Also, what do ECU and DME stand for? Are they different and if they are, what’s the difference?

Thanks in advance for whatever advice / comment you have.”

So, as you can see I got hit with a great question (really three questions) right from the get-go. Carlos and I have been emailing back and forth ever since. As I was researching the answers to his questions, one of my other readers, from the “Babbler’s Forum” on Facebook (one of the many social groups regarding Porsche and a topic for another post), actually took a stab at answering the portion of the question with regard to ECU and DME (thanks Dede!). Here’s what she had to say:

Dede’s answer to the ECU/DME question

In automotive electronics, an electronic control unit (ECU), also called a control unit or control module, is an embedded system that controls one or more of the electrical subsystems in a vehicle. Some modern cars have up to 70 ECUs, including:

Engine Control Unit

also called Powertrain Control Module (PCM)

Transmission Control Unit – TCU

Telephone Control Unit – TCU

Man Machine Interface – MMI

Door Control Unit

Seat Control Unit

Climate Control Unit

Speed Control Unit

Digital Motor Electronics (DME) is a microprocessor based system that controls the ignition, fuel injection, oxygen sensor, and numerous ancillary functions of an automobile. The DME provides raw data to an on-board computer that uses an alphanumeric display to provide you with information such as the average fuel consumption, the driving distance available with the fuel in the tank, average speed, outside temperature, etc.

In the automotive industry, DME is often referred to as the Engine Control Unit (ECU), under the heading of Electronic Engine Management Systems.

DME operates by continually monitoring such factors as engine temperature, speed, intake airflow, exhaust gas composition, and even altitude. DME can fine-tune the engine hundreds of times a second to provide maximum performance and efficiency. DME has a fail-safe program in the event of certain electrical faults. Current DME versions also have on-board diagnostics (OBD).

The two main tasks DME performs are (1) the injecting of the proper amount of fuel and (2) providing a spark at the correct time. In order for this to happen, the system requires information about the engine’s current state. DME can track dozens of different sensors, however, every system needs to know three basic things:

How much air is coming into the engine.

The position of the throttle.

How fast the engine is running.

Using the information about how much air is flowing through the engine and how fast it’s turning, DME uses a fuel map to determine how long each injector should stay open each cycle to inject the right amount of fuel. During part-throttle operation, the injector pulse-width is also modified by the readings from the oxygen sensor, a device that sits in the exhaust collector and determines how much oxygen is left over in the exhaust. Each cylinder is constantly adjusted to maximum operating efficiency under virtually all conditions.

In the event of an electrical fault, DME can reconfigure itself to bypass the problem and it can diagnose itself for quick and efficient trouble-shooting.

Now Carlos, that’s a fairly comprehensive answer and should hopefully start you on the way to a better understanding,. I know it helped me. More answers to follow from the rest of Carlo’s question along with others. Don’t forget to check out the original post and feel free to ask away.