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The Motronic Engine Management System
 
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Pelican Technical Article:

The Motronic Engine Management System

Applicable Models:

Porsche 911 (1984-89)
Porsche 930 Turbo (1986-89)

Parts Required:

-

Hot Tip:

The Motronic system should be mostly maintenance free

Performance Gain:

Better running engine

Complementary Modification:

Add a performance chip
101 Projects for Your Porsche 911

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).

OVERVIEW

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.

TROUBLESHOOTING

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.
Figure 1

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.
Figure 2

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.

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Comments and Suggestions:
Par five Comments: I have a 1986 911 and sometimes it starts and sometimes it does not. It will start if I pop the couch and it will run fine, I swapped the computer a few years ago think it needs to be swapped again? Thanks
March 24, 2016
  Followup from the Pelican Staff: Sounds like a voltage issue when cranking. I can't help without knowing what is missing from the system when you are trying to start it. When your engine doesn’t start you’ll want to check the basics. Check spark, fuel injector pulse and fuel pressure, volume, quality and engine compression. Are there any fault codes? Once you figure out what is missing, it will be easier to diagnose.
- Nick at Pelican Parts
 
Bill Comments: Recently, my 1984 Carrera, after warm-up, will "quit" from time to time. If I keep the RPM's higher than normal idle, it runs quite well, then when stopped at a stop light, it will quit again. Could this be chip problem or idle adjustment? Thank you, Bill Elliott
November 28, 2015
  Followup from the Pelican Staff: Sounds like an idle issue. I would check the idle adjustment. - Nick at Pelican Parts  
boxleyan Comments: Hello 1990 C2 964 No Spark No start !! it has a new dme.
what can it be
October 18, 2015
  Followup from the Pelican Staff: Does it have injector pulse and fuel pressure? - Nick at Pelican Parts  
dakota boy Comments: I have a problem with my '85 carerra. It runs very poorly on partial throttle,but goes like hell if full throttled, and then when backing out of the full it runs OK until the next time the driver asks for less than full tilt.
It bogs and spits and farts and is not driveable in any normal way.
We have checked a lot of the sensors and have not found any to be out of spec. This problem is most frustrating. We will change the cylinder head temp sensor tomorrow, as it is fairly easy to do and not too expensive if that is not the problem. The really bizarre thing is that all of the sensors are checking out ok according to the information provided by the books. I am still trying to find enough information to make up a complete end to end drawing of the system so that it can be better understood. Since i am capable of understanding the electronics involved as well as the mechanics, but without more information it is impossible to understand all of the inputs to the computer.
Yes, i have changed the computer, to no improvement, so it must be a sensor that is at fault or some suddenly bad wiring -?? This car is quite clean and not at all rusty or majorly corroded so it seems not likely that the wiring could be at fault.
May 27, 2015
  Followup from the Pelican Staff: I would start with the basics. Maintenance items, spark plugs distributor caps and rotors, air filter, fuel filter etc. Then check and set the fuel mixture. If your problem persists, check fuel pressure and volume. Clean the fuel injector ground on the left side of the intake manifold. Make sure that your ground to the body from the gearbox is intact, making good contact. AFTER all of that is covered, and your problem persists, scope test the airflow meter, and verify that the O2 sensor is not driving the system lean. When you hit the full throttle switch the DME goes to full enrichment "autobahn power mode" and ignores the O2 sensor input. A quick test would be to leave the sensor unplugged after you set the fuel mixture, if your problem goes away, replace the O2 sensor. - Casey at Pelican Parts  
beardogg1 Comments: I was thinking about buying a K&N CO analyzer to where I could adjust fuel air mixture since I live at 8600 ft. and travel to sea level..Motor is 84 3.2.. Has anyone that you know of installed this unit and is it any good? GK
January 23, 2014
  Followup from the Pelican Staff: I don't have any experience with it. I opened a post in our forums. A Pelican community member may be able to answer your question.
- Nick at Pelican Parts
 
Boxman Comments: 88 Cab only owned it a year never a problem. Was driving along today and out of the blue it just quit. Almost like ignition system shut down. Any thoughts
May 29, 2013
  Followup from the Pelican Staff: If your engine doesn’t start you’ll want to check the basics. Check spark, fuel delivery and fuel pressure, volume and quality. Once you figure out what is missing, it will be easier to diagnose.

For a quick test of the ignition system, check if the coil has power and if the ground side is being pulsed.- Nick at Pelican Parts
 
Canuck Comments: i HAVE A GREMLIN IN MY 1990 964.Car worked fine last year, before removing engine.Only change I have made is the secondary muffler with a bypass pipe.The problem is when the car warms up after five minutes,it wants to stall as soon as the throttle linkage breakes contact with the idle control micro switch. No faults showing,durometric.Any suggestion,s greatly appriciated.Not a lot of porsches in my part of the world.
May 15, 2013
  Followup from the Pelican Staff: You have to find out what is happening to the mixture when the idle contact switch opens. Is the car going lean or is it going rich. Do you have a vacuum leak? Try unplugging the O2 sensor and see if the vehicle still stalls off idle. Also you can spray clean your throttle plates and idle air motor. - Nick at Pelican Parts  
MM2000PP Comments: I have a 1984 Carrera 3.2. Plug #3 continually fouls. I heard that it could be a 'leaky injector' that keeps wetting the plug and therefore preventing it from firing. Are the injectors replaceable?
October 30, 2011
  Followup from the Pelican Staff: Yes, a professional mechanic can do it in about an hour and a half. Did you pull out the spark plug to verify the plug is gas fouled? You can have a bad valve guide seal and the plug may be oil fouled. - Nick at Pelican Parts  

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