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DME engine management system 200-1

Principles of operation 200-3

Fuel supply and fuel delivery 200-3

Volume air flow sensor 200-4

Cylinder head temperature sensor 200-5

Reference and speed sensors 200-5

Throttle position switches 200-6

Altitude correction switch 200-7

Oxygen sensor 200-7

Idle speed control 200-7

DME main/fuel pump relay 200-8

Ignition timing 200-9

Emission controls 200-9


Mechanical condition 200-11

Preventive maintenance 200-12

Valve adjustment 200-12

Basic settings 200-12

Airflow measurement and vacuum leaks . . .200-13

Electrical System 200-13

Fuel delivery troubleshooting 200-15

Idle speed 200-15


a. Engine Driveability Troubleshooting 200-16


The Porsche 911 Carrera is equipped with a Bosch DME (Digital Motor Electronics or Motronic) engine management system. This section describes operation and troubleshooting of the DME system.

Refer to the following sections for repair information:

• 280 Ignition System.

DME engine management system

The DME system uses electrical signals from the air flow sensor, the air and engine temperature sensors, the speed sensor, and an exhaust-mounted oxygen sensor as its primary inputs to electronically calculate fuel delivery and engine timing.

Ignition timing is electronically controlled and non-adjustable. The ignition distributor contains no mechanical timing-advance components. The only function the distributor serves is to distribute high voltage to the spark plugs. All ignition timing is determined by the engine (or DME) control module (ECM), interpreting signals from sensors that indicate engine crankshaft position and speed.

The ECM (C) is located under the driver's seat, next to the DME main/fuel pump relay (A) and altitude correction switch (B).

Porsche 964 Altitude Sensor


944 Non Turbo Dme Diagram

DME engine management system


1. Fuel pump

2. Fuel tank

3. Fuel pressure test port

4. Fuel injectors

5. Fuel pressure damper

6. Air flow sensor

7. Intake air temperature (lAT)sensor

8. Full throttle position switch

9. Idle position switch

10. Idle air control valve

11 .Cylinder head temperature sensor

12. Fuel pressure regulator

13. Ignition distributor

14. Ignition coil

15. Luggage compartment fusebox

16. Main/fuel pump relay

17. Altitude correction switch

18. DME engine control module (ECM)

19. Engine speed sensor

20. Reference (TDC) sensor A Power (+) from battery

B Fuel from fuel tank C Fuel return to fuel tank D T-fitting to throttle housing

Fuel under pressure is injected via electronically controlled solenoid-type fuel injectors. The ECM electrically controls the opening and closing of the injectors.

The DME system features advanced adaptive control circuitry that actually changes its baseline values in the control module memory based on how that particular car is operating.

Principles of operation

The fuel injection is completely electronic in operation. The DME engine control module (ECM) controls engine fuel delivery and ignition timing using signals from the following sources:

• Air flow sensor with intake air temperature (IAT) sensor

• Cylinder head temperature sensor

• Reference position (TDC) sensor

• Crankshaft speed (RPM) sensor

• Throttle position switches

• Altitude correction sensor

• Oxygen sensor

Fuel supply and fuel delivery

Fuel from the tank is supplied to the engine compartment fuel system loop via an electric fuel pump, a fuel filter, and the connecting fuel lines. The fuel pump is controlled by the ECM via the DME main/fuel pump relay.

Engine compartment fuel system loop

Fuel Pressure Dampers

4. Left fuel rail

5. Fuel pressure regulator

6. Pressure damper

1989 S15 Crank Signal For Fuel Pump

1. Fuel pump

2. Fuel filter

3. Fuel pressure test port

4. Left fuel rail

5. Fuel pressure regulator

6. Pressure damper

7. Right fuel rail

8. Fuel injector



Needle valve


Porsche 944 Pressure Chart



Fuel pressure regulator

Porsche Boxster Intake Vacuum Lines

Vacuum connection to intake manifold Spring Diaphragm From fuel loop Return to fuel tank


Pulsed type fuel injector


Harness connection


Needle valve




Fuel pressure regulator

Vacuum connection to intake manifold Spring Diaphragm From fuel loop Return to fuel tank


^ The DME system precisely meters fuel by changing the opening time (pulse width) of the fuel injectors. The injectors are switched on and off by the ECM at the ground side of the connector. Positive (+) battery voltage is always present at the connector when the car is running. The six injectors are mounted in two common fuel supply rails.

^ To ensure that injector pulse width is the only factor that determines fuel metering, fuel pressure is maintained by the fuel pressure regulator. The pressure is controlled by returning surplus fuel back to the fuel tank. A change in fuel pressure results in a change in the amount of fuel (or fuel mixture) that is delivered to the engine.

Volume air flow sensor

^ Air entering the engine passes through a pleated paper air filter in the air filter housing. Air flow is controlled by the throttle valve in the throttle housing. The throttle housing is connected to the volume air flow sensor by flexible rubber ducts. Air entering the engine is measured by the air flow sensor. An intake air temperature (IAT) sensor is integrated into the air flow sensor.

Air-flow Throttle sensor

Iat Sensor

Air-flow Throttle sensor


Cracked Sensor Plug Porsche 924s

Cylinder head temperature sensor

^ The cylinder head temperature sensor is the main engine temperature input to the ECM. The temperature sensor sends continuous engine temperature information to the ECM. The temperature sensor is of the NTC (negative temperature coefficient) type; as temperature increases sensor resistance decreases.

— The sensor is mounted in cylinder no. 3 (left front cylinder).

Reference and speed sensors

"A The reference (TDC) and engine speed sensors are mounted to the crankcase on the flywheel end of the engine. The ECM needs a signal from the reference position sensor for the engine to start.

The reference sensor responds to a set screw (A) in the flywheel during cranking. The engine speed sensor responds to a toothed wheel on the flywheel (B). Engine speed is determined by the rate at which the wheel's teeth pass the sensor. Two voltages pulses are generated for each tooth on the flywheel.

The sensor clearance (arrows) should be 0.8 ± 0.3 mm (0.03 ±0.01 in.).

Electrical connection


NTC resistor

Porsche 911 Deceleration Valve

Throttle position switches

Two switches in the throttle housing are used by the ECM to determine throttle position.

^ The idle position switch (arrow) provides a signal to the ECM when the throttle is in the rest position. When the idle switch is closed, the idle control valve is activated to regulate idle speed. The idle position signal is also used to for fuel cutoff during deceleration. The injectors are turned back on at 1,080 rpm when coasting from a higher speed.

Ignition Map Rpm Throttle Position

The full throttle switch provides a signal to the ECM when the throttle is in the rest position. When the full throttle switch is activated, the ECM substitutes an enrichment fuel delivery and ignition map for optimum power. The oxygen sensor signal is ignored under these conditions and the A/C compressor will be turned off if in use.

Deceleration Fuel Cutoff Porsche 911 Deceleration Valve


Altitude correction switch

^ Due to lower air pressures at high altitude, the air/fuel mixture will be richer when the car is operated at high altitudes. The altitude correction switch decreases the injection pulse width by 6% at 1,000 meter (3,300 ft.) and above.

Oxygen sensor

The oxygen sensor adapts the air-fuel mixture by sending a varying voltage signal to the engine ECM. The sensor is positioned in the exhaust stream and measures the amount of oxygen in the exhaust gas so that the fuel injection system can correctly adjust the air-fuel mixture.

Rich mixture 1 volt ~ (lack of air)

0 volt

Lean mixture (excess air)

17.66 A/F ratio


A high concentration of oxygen in the exhaust gas indicates a lean mixture and a low content indicates a rich mixture. The signal from the oxygen sensor is used by the ECM to influence engine performance and driveability.

The signal from the oxygen sensor is ignored by the engine ECM until the engine reaches a specified temperature. Therefore, when troubleshooting cold engine driveability problems, the oxygen sensor can be ruled out as a possible cause.

1984 Porsche 928 Idle Air Control Valve

Idle speed control

Idle speed is electronically controlled via the idle speed control valve, which maintains idle speed by bypassing varying amounts of air around the closed throttle valve.

When the throttle is in the rest position, a programmed nominal speed (rpm) in the ECM is continuously compared to the engine's actual speed. If the actual speed varies from the programmed speed, the duty cycle signal to the valve is changed to maintain the desired engine speed. When the engine is warm, the regulated nominal engine speed is 800 ±20 rpm.

1. Air flow sensor

2. Idle speed control valve

3. DME control module (ECM)

4. Engine speed sensor

5. Engine temperature sensor

6. Idle contact

Main/fuel pump relay electrical circuit

Battery to oxygen sensor

Main/ fuel pump relay

control unit

20 fuel pump control

J 1 ^.OBñjl.l battery power with key on





















! Luggage i compartment i fusebox #1

Fuel Pump

DME main/fuel pump relay

The main/fuel pump relay is energized via the DME engine ECM and supplies power to the many of the DME components and subsystems, including the fuel pump.

The fuel pump relay is energized from terminal 20 of the ECM. The fuel pump only runs when the engine running. If this relay is faulty, the engine will not start.

Ignition timing

The ECM uses engine load, engine speed, coolant temperature, and intake air temperature as the basic inputs for timing control. The DME ignition characteristic map represents the optimum ignition point for varying engine speed and engine loads. A map similar to the one shown is digitally stored in the ECM.



Engine speed sensor

Engine speed sensor

Engine position sensor



Ignition coil primary circuit

Engine Management Characteristic Map

Engine load (Air-flow sensor)

^ The initial ignition point is determined by the reference (TDC) sensor during cranking. Once the engine is running, timing is continually adjusted based on operating conditions.

The ignition system is a high-energy system operating in a dangerous voltage range that could prove to be fatal if exposed terminals or live parts are contacted. Use extreme caution when working on a car with the ignition on or the engine running.

Emission controls

The emission control systems used in the 911 Carrera include the oxygen sensor system, the evaporative emission system, and the crankcase/oil tank ventilation system.

Oxygen sensor. This system provides the fuel injection system with feedback information about combustion efficiency.

Evaporative emission controls. This system provides venting for the fuel tank while at the same time trapping the fuel vapors that would otherwise be vented into the atmosphere. The evaporative emission system collects fuel vapors from the fuel tank in a charcoal canister. During certain engine operating conditions, the canister is purged, drawing the fuel vapors into the engine to be burned.

Ignition advance

Porsche Turbo Canister Purge Circuit


Ignition advance


Crankcase and oil tank ventilation. This system provides venting for the crankcase and oil tank while at the same time trapping the fuel vapors that would otherwise be vented into the atmosphere. The crankcase and oil tank ventilation system also includes a secondary venting system to prevent throttle icing in cold weather. See 100 Engine-General for more information.


Driveability—the overall performance of the car, its ability to start quickly, run and accelerate smoothly, and deliver fuel economy as well as power—can only be achieved when all of the engine's major systems are working properly.

Poor driveability or faulty running may have a variety of causes. The fault may lie with the fuel system, the ignition system or a combination of both. Because of the interrelated functions of these systems it is often difficult to know where to begin looking for problems. For this reason, effective troubleshooting must always consider these system as one major system.

This section offers basic troubleshooting information that addresses symptoms of poor driveability. It is a guide to problem solving, intended to be used in conjunction with the other sections in this manual.

Observe the following cautions and warnings when performing any service or repair on the engine management system.

• The fuel system is designed to retain pressure even when the ignition is off. When working with the fuel system, loosen the fuel lines very slowly to allow residual fuel pressure to dissipate gradually. Avoid spraying fuel.

• Fuel is highly flammable. When working around fuel, do not disconnect any wires that could cause electrical sparks. Do not smoke or work near heaters or other fire hazards. Keep an approved fire extinguisher handy.

• There are lethal voltages present at the ignition system when the engine is running or the key is on.

• Before making any electrical tests with the ignition turned on, remove the harness connector from the DME main/fuel pump relay. The relay is located under the driver's seat.

• To prevent damage to the ignition system or the electronic fuel system components, including the ECM, always connect and disconnect wires and test equipment with the ignition off.

• Cleanliness is essential when working with the fuel system. Thoroughly clean the fuel line unions before disconnecting any of the lines.

• When replacing parts, install only new, clean compo-nents.Always replace seals and O-rings.

• Avoid nearby use of compressed air, and do not move the car while the fuel system is open.

• Do not use compressed air above 40 psi to blow out lines or components. Internal damage may result to components.

Mechanical condition

Before troubleshooting a poorly running engine or an engine that will not start, determine the general condition of the engine, especially if it has high mileage. If the engine is severely worn or has mechanical problems, the only remedy is overhaul or repair. If a tune-up or scheduled maintenance is due, it should be done before proceeding to other repairs.

General engine condition can be assessed by checking the compression at each cylinder. Make sure that the valves are correctly adjusted before making the test. The compression check is described in 100 Engine-General.

A compression test requires special test equipment. If the equipment is not available, most automotive repair shops can do these tests quickly and at a reasonable cost.

Carbon deposits on the fuel injectors and the intake valves will affect the way the engine idles and runs. Even a 10% decrease in the amount of fuel that the injectors deliver can cause driveabitity problems. A car that is predominantly driven on short trips around town or in city traffic seems to increase the likelihood of deposits forming.

Light carbon can usually be removed using a fuel additive. Specialized cleaning equipment is required to remove heavier carbon deposits. Check with an authorized Porsche dealer for information on carbon deposits and the best methods used to remove them.

Always follow the manufacturer's directions when using fuel additives designed to remove carbon deposits and clean injectors. The excess amount of cleaner in the fuel can dilute engine oil and accelerate engine wear. Check with an authorized Porsche dealer before using fuel additives together with high detergent fuel.

Best Way Clean Sensor

Preventive maintenance

The condition of the tune-up and emission control components can affect engine performance and driveability. Extending maintenance schedules beyond those recommended by the manufacturer can adversely affect the way the engine runs. When experiencing driveability problems, a good starting point is to perform the manufacturer-specified maintenance. Many driveability problems are eliminated simply by replacing worn out or dirty spark plugs.

Replacement schedules for the spark plugs, spark plug wires, distributor cap, ignition rotor, fuel filter, air filter, oxygen sensor, and oil and oil filter are given in 030 Maintenance.

Valve adjustment

Correctly adjusted valves are necessary for efficient engine operation. If the valve clearances are too small, the valves may not close all the way, resulting in low compression and a loss in power. If the valve clearances are too large, the valves may not fully open causing a reduction in engine efficiency. Procedures for checking and adjusting valve clearances are described in 030 Maintenance.

Porsche 911 Valve Adjustment

Basic settings

In addition to tune-up component replacement, it is important that all of the basic setting and adjustments are within allowable limits. Check throttle plate adjustment, idle speed, idle mixture (%CO), and ignition timing to be sure they are all as specified. All of the models covered by this manual are equipped with a non-adjustable or self-correcting, ignition timing, idle mixture, and idle speed. Basic adjustment are outlined in 030 Maintenance.

^ The idle mixture (%CO) is adaptive via the oxygen sensor feedback system. A CO correction screw is located in the air flow sensor, but is capped off by a non-reusable cap (arrow). If the idle mixture (%CO) is outside the allowable limits and the oxygen sensor has been replaced, the CO level may be corrected at the adjustment screw. This adjustment should be made by an authorized Porsche dealer or other qualified shop with the appropriate test equipment (gas analyzer).

• The signal from the oxygen sensor is ignored by the engine ECM until the engine reaches a specified temperature. Therefore, when troubleshooting cold engine driveability problems, the oxygen sensor can be ruled out as a possible cause.

• As the oxygen sensor ages, it loses its ability to react quickly and it may eventually cease to produce any signal at all. When this happens, fluctuations in idle speed and increased fuel consumption may be noticed. The oxygen sensor should be replaced at the specified mileage interval as described in 030 Maintenance.

Air flow measurement and vacuum leaks

To control fuel injection the engine management system uses an air flow sensor to precisely measure incoming air. The sensor sends an electrical signal proportional to the measured air flow to the ECM, which uses this signal to determine the amount of fuel the engine needs.

Because proper fuel metering depends on accurately measuring the intake air, any unmeasured air entering the system will cause a lean fuel mixture and poor running.

There are many possible places for unmeasured air to enter the engine. Carefully inspect all hoses, fittings, duct work, seals and gaskets. Check the oil filler cap seal on the oil tank. For a thorough inspection, it may be necessary to remove hoses and ducts that cannot be completely checked in their installed positions.

Electrical system

All the cars covered by this manual use engine management systems that rely on precise electrical signals for proper operation. Some of the DME circuits operate on very low current and are sensitive to increased resistance due to faulty or corroded wiring or connectors. If any of these signals are distorted, incorrect, or missing, the car can develop major driveability problems.

In most cases, a faulty ECM is not the cause of driveability problems. These modules are extremely durable and reliable. Actual failures are not that common. Driveability problems are more often caused by missing or incorrect signals to the ECM, or by other faulty components.

First check to make sure all ground connections and harness connectors are tight, free of corrosion and damage-free. In particular, clean battery terminals and ground straps. For a complete listing of ground points, see 971 Electrical Component Locations.

Make sure the battery is fully charged and in good condition. The DME system requires a specified operating voltage to function correctly. Battery voltage can be measured across its terminals with all cables attached. For the battery to maintain its proper voltage level, the charging system must be functioning correctly. See 270 Battery, Starter, Alternator.

A digital voltmeter should be used to accurately measure battery voltage. A fully charged battery will measure 12.6 volts, or more, while a battery only 25% charged will measure 12.15 volts. Using an analog meter may result in inaccurate results.

The electrical system is subject to corrosion, vibration, roadway elements and general wear. Because of this, the integrity and freedom from corrosion in the connections, wires, and switches, including all ground connections, are important conditions for trouble-free operation of the engine management systems.

Loose or damaged connectors can cause intermittent problems, especially at the small terminals in each ECM connector. In most cases, a visual inspection will detect any faults. If a connector shows no visible faults but is still suspect, perform a voltage drop test at the connector. Even a small amount of corrosion in a connector can cause a large voltage drop to the circuit's load. See 900 Electrical System-General for more troubleshooting information.

Poor ground connections are a major sources of driveability problems. There are only a few main ground connections or points for the engine management systems. These ground points are a grouping of many wires crimped or welded into an eyelet that is then bolted to the car's chassis or metal parts. If any of these ground points are faulty, the voltage to the circuit will be reduced or even eliminated.

To thoroughly check a circuit ground, check the voltage drop between the connector and a good ground source. Large voltage drops indicate too much resistance: The connection is corroded, dirty or otherwise damaged. Clean or repair the connection and retest. Also check both battery terminals and all ground straps between the engine and the body for voltage drops.

• For a listing of the DME-related ground points, see 240 Fuel Injection. A complete listing of the ground connections is given in 971 Electrical Component Locations.

• For voltage drop tests and other general electrical troubleshooting information, see 900 Electrical System-General.

Fuel delivery troubleshooting

Fuel from the tank is supplied to the engine via an electric fuel pump, a fuel filter, and the connecting fuel lines. If either the filter or a fuel line is restricted, the engine may not run properly. To check for a clogged fuel filter, a restricted fuel line, or a weak/worn fuel pump, perform a fuel delivery rate test as described 201 Fuel Supply.

Power to run the fuel pump is controlled by the DME main/fuel pump relay. If the relay is not working, the car will not start or run. DME main/fuel pump testing is covered in 201 Fuel Supply.

Idle speed

Erratic idle speed is one of the most common driveability problems encountered on the cars covered by this manual.

Be sure the idle position switch is correctly functioning as described in 240 Fuel Injection. If no vacuum leaks can be found, the idle speed control valve may be worn out. The best way to check for a faulty valve it to substitute with a known good part.

Table a lists additional probable causes and corrective actions for driveability problems. The boldface type indicates the section of the manual where the applicable test and repair procedures are found.

Table a. Engine Driveability Troubleshooting

Symptoms or Faults

a. Engine does not start or starts hard

b. Erratic idle

c. Poor acceleration

d. Engine misfiring

e High fuel consumption

f. Low engine power output

Probable Causes of Fault

Corrective Action






Ground or plug connection

Check ground connections at intake runner 1, behind fuel filter. Check connectors at fuel injectors, other fuel injection components. 240 Fuel Injection.



No power to ECM or fuel pump

Test DME main/fuel pump relay. 201 Fuel Supply






Speed sensor

Test sensor with oscilloscope for 2.5 V sine


Reference (TDC) sensor faulty

Test sensor with oscilloscope for 2.0 V sine





Ignition system fault

Test coil. 280 Ignition System.







Air flow sensor faulty

Test air flow sensor. 240 Fuel Injection.






Fuel pressure incorrect

Check fuel pressure/pressure regulator. 240 Fuel Injection.



Idle air control valve faulty or no control signal to valve.

Check for 12V at center terminal of valve with ignition key on. Check idle position switch. 240 Fuel Injection.



Full throttle position switch faulty

Check for continuity across switch terminals with throttle in wide open position.







DME cylinder head temperature sensor (Temperature sensor II) faulty

Test resistance value at room temperature = 2.2 to 2.8 k-ohms







Fuel injector(s) not working

Check for at least 10 V with key on at injector connector. Check for pulsed ground signal with engine running/cranking. 240 Fuel Injection.




CO level and idle speed adjustments incorrect

Check for vacuum leaks, replace oxygen sensor, check air flow sensor. 240 Fuel Injection.





Intake air leak

Check for loose intake air ducting connections, splits or cracks in air boot, other sources for vacuum leaks.




High altitude switch faulty

Bridge connectors at switch connector with engine running. CO should drop slightly (1% drop)



Low system voltage.

Check battery voltage with engine running. Test alternator output 13.2 to 14.2 volts. 270 Battery, Starter, Alternator.

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