This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
In 1984, Porsche replaced the ten-year-old Continuous Injection System, or CIS with a new type of computer controlled fuel injection from Bosch AG. This new type of injection system, called Motronic, is technologically leaps and bounds above the CIS system in terms of performance and emissions control.
The CIS system sprays fuel into all the manifolds at the same time, relying on the opening and closing of the cylinder valves to regulate the amount of fuel that enters the combustion cylinder. While very effective at delivering fuel, the CIS system is not the most efficient. When the valve to the cylinder head is closed, the injector still injects fuel onto the closed valve. This extra fuel has a tendency to cause multiple backfires, especially when the engine is cold. Even properly tuned CIS systems are infamous for their backfires that can cause damage to the plastic CIS airbox (See Pelican Technical Article: Pop-Off Valve Installation).
The Motronic system that was implemented on the 1984 911 Carrera is what is commonly called a pulsed injection system. The fuel injection system only sprays fuel into the intake manifold when it is required in the combustion cycle. In this manner, the system can more accurately control the fuel levels and fuel delivery, and help to better maintain the ideal stoichiometric air/fuel ratio of 14.6.
The Motronic system takes fuel injection one step further, and incorporates an engine management system that also controls the ignition system. The marriage of ignition and fuel injection is a natural fit because they are completely reliant on each other for proper engine operation. The Motronic system not only times and meters the fuel, but it also decides when to fire the spark to make combustion happen.
The central Motronic computer controls the ignition and fuel injection system. The computer takes input from a variety of sensors placed about the engine. An engine RPM sensor indicates how fast the engine is running. An airflow sensor measures the amount of air that is being drawn into the engine, and thus the amount of load on the engine. An oxygen sensor measures the exhaust gases exiting the engine in order to correctly meter the mixture. Timing sensors and cylinder head sensors also provide additional information for the computer.
The computer itself is able to take all the input from the sensors, and calculate the best ignition timing and fuel delivery to create the most horsepower with the least amount of emissions. The computer has a Read-Only-Memory (ROM) computer chip inside that contains what is known as a data map based on engine dynometer and emissions tests performed at the factory. At any split second, the computer can read in all the input from the engine, compare it to the map in the ROM chip, and decide how much fuel to deliver and when to fire the ignition.
The advantages of such a system are numerous. Gone are the mechanically complex methods of advancing the timing with counter weights and vacuum advance units. Points inside the distributor are a thing of the past as well. Instead the engine relies on the computer to control all fuel and ignition related decisions. In milliseconds, the computer can gather input data, access the ROM chip, and interpolate the best settings for the engine.
Storing all the computer data on a ROM chip also allows for the chip to be swapped out for different applications. Certain aftermarket manufacturers sell chips for the 911 Motronic system that are not as conservative as the factory chips. They promise gains of about 10-15% more horsepower at various RPMs. While some of these chips may not be legal in all 50 states, they do seem to work. Check with your local auto repair shop to see which chips are legal in your area.
TUNING AND ADJUSTMENT
Because the Motronic system is so integrated within its computer, there aren't many adjustments that need to be made to the system. It should basically run by itself. The only two major adjustments that might need to be made are the CO level and the idle adjustment. The CO level should only be adjusted by a qualified mechanic equipped with a CO meter. The CO adjustment is made by turning a small screw located in the bottom, rear, left-hand corner of the air flow meter. This screw is normally covered by a plug that needs to be removed prior to the adjustment.
The other common adjustment to make is to the idle speed. While the computer should properly regulate the system on its own, it may be necessary to adjust the speed manually. On the left side of the engine compartment, in-between the coil and the rear regulator plate, there is a small test port with three small sockets. In order to adjust the idle, you need to first disable the idle volumetric control. Bridge a wire from terminals B and C. Then, using a small screwdriver, adjust the bypass screw on the throttle housing until the car reaches the desired idle speed.
In general, the Motronic system is very reliable. Problems arise when the sensors to the system are not functioning properly. In general, the debugging of the Motronic system is not for the home mechanic, as the system requires a handful of specialty tools and knowledge that are not easily acquired. Most problems associated with the Motronic system are electrical in nature, and involve complex testing of these electrical devices, or the less complex method of simply replacing parts until the problem is fixed.
Sometimes the relay for the system fails. This relay is actually two relays in one; one for the Digital Motor Electronics (DME) system, and the other for the fuel pump. If you find that the system is acting erratically, or the fuel pump is not getting power, try replacing this relay. The relay is located next to the engine control module (ECM) underneath the driver's seat.
For a more complete understanding of the Motronic system, take a look at the book, "Bosch Fuel Injection & Engine Management" by Charles O. Probst. It provides an excellent overview into the theory and practice of implementing the Motronic system.
In order to properly adjust the idle on the Motronic system, you need to disable the idle volumetric control. The two white arrows in the photo show the two jumper plugs that must be connected in order to disable the control system prior to adjustment. This plug is located on the left side of the engine compartment.
In this photo, the yellow arrow indicates the airflow bypass screw which regulates the idle for the engine. The green arrow points to the location of the mixture adjustment screw, which is located under a protective cap. This cap must first be removed prior to adjusting the mixture. Make sure that you have a CO meter on hand to properly adjust the mixture.