1984 Porsche 944 Car hesitates

THILTONNC

MEMBER
1984 PORSCHE 944
4 CYL
2WD
MANUAL
112,000 MILES

I have a '84 944 that hesitates and backfires sporadically. It's worse when the car is cold and the ambient temperature is lower. It's also worse at lower RPMs. The car has a recent tune-up and fuel filter. The fuel pressure has been checked at the rail. I read on Pelican Parts forum that a DME temp sensor could cause this but was told by the parts person that my model doesn't have one. When I bought the car, I was told the computer was bad. I bought another one but don't remember what model it came out of, could that cause the problem? Also, all the gauges act erratically, are they tied in somehow? Wiring diagrams for this car aren't easy to decipher. As I said, sometimes it runs halfway decent, other times it'll barely move. Thanks for any help you can offer.

1 Reply

DAVE H

MECHANIC
13,384 POSTS

Hello .. thanks for the donation .. much appreciated

If the correct computer is not fitted to the car it could be sending incorrect data to contol systems for correct running settings ?

The 911 Carrera and 944 models are equipped with the Bosch digital motor electronic (DME) engine control system. The DME system uses various data sensors which monitor intake air volume, speed, crankshaft position, coolant temperature, intake air temperature, and throttle position.
Signals from these sensors, as well as a start signal and oxygen sensor signal, are sent to the control unit.
The Control Unit (CU) is a microcomputer. It is the "brain" of the DME system. Using information obtained from data sensors, the CU determines the correct amount of fuel and optimum ignition timing.
The 944 DME control unit switches from open loop to closed loop operation when the coolant temperature is above 113 °F (45 °C), and when the oxygen sensor temperature is above 480 °F (250 °C).
Fig. 2: Schematic of 944 DME Engine Control System The DME system consists of 4 sub-systems.

OPERATION
The DME system consists of 4 sub-systems: Fuel Control, Data Sensors, CU and Spark Timing.
FUEL CONTROL
The 911 Carrera and 944 are equipped with the Bosch Air Flow Controlled (AFC) fuel injection system. The AFC system is electronically controlled by the CU, which is programmed to regulate fuel injection based upon information received from various data sensors. It also compares the specific data (stored in computer memory) for the engine.
The CU generates control signals for the fuel pump relay, auxiliary air valve and the cylinder port injectors. These devices control cold idle, curb idle speed and mixture, air/fuel ratio and fuel supply. Additional fuel required for cold starting is supplied by the fuel injectors.
The 911 Carrera models also use an idle speed control system. This system consists of an air regulating valve which opens an air by-pass around the throttle. When necessary, the CU operates the air regulating valve to stabilize the idle speed.
SPARK CONTROL
Spark control allows the CU to determine the exact instant that ignition is required, based upon information received from data sensors.
At the optimum time, the CU breaks the primary circuit of the ignition coil, producing a high voltage at coil center tower. This voltage surge fires the spark plug at the proper time for most efficient combustion, eliminating the need for vacuum and/or centrifugal advance.
DATA SENSORS
Each sensor furnishes electronic impulses to the CU. Using this information, the CU computes spark timing, and correct amount of fuel necessary to maintain proper engine operation.
The function of each sensor is closely related in maintaining proper engine operation. Operation of each sensor is as follows:
Oxygen Sensor
This sensor is mounted in engine exhaust stream, in front of catalytic converter. It supplies a low voltage (under .5 volt) when fuel mixture is lean (too much oxygen) and a higher voltage (up to 1 volt) when fuel mixture is rich (not enough oxygen).
The oxygen sensor must be hot (over 480 °F/250 °C, on 944) to function properly, and to allow CU to accept its electrical signals. The oxygen sensor measures quantity of oxygen only.
All Calif. models are equipped with a special electrically heated oxygen sensor. This oxygen sensor reaches operating temperature sooner and also begins to function earlier. The heated oxygen sensor has 3 wires, 2 for the heater element (power & ground), and a single wire for the oxygen sensor signal.
The heating begins with ignition on (via fuel pump & DME relay terminal 87). The plugs from the sensor to the wiring harness are located near the flywheel sensor plugs (speed, reference mark).
All Calif. 911 Carrera models also mount a bridge between CU connector terminal 10 and ground. This bridge modifies the oxygen sensor control setting.
NOTE:No attempt should be made to measure oxygen sensor voltage output. Current drain of conventional voltmeter may permanently damage sensor, shift sensor calibration range and/or render sensor unusable. DO NOT connect jumper wire, test leads or other electrical connectors to sensor. Use these devices only on CU side of harness after disconnecting sensor.

Reference Mark Sensor
The reference mark sensor is located on crankcase flange. This sensor detects crankshaft position in relation to top dead center, and sends this signal to the CU. It is triggered by a bolt cemented into the flywheel.
Speed Sensor
The speed sensor is mounted on an adjustable bracket with the reference mark sensor. The speed sensor measures engine speed by counting the teeth on the starter ring gear. The speed sensor sends 2 voltage pulses to the control unit for each tooth that passes.
Temperature Sensor II
This sensor is located in cylinder head No. 3 (911 Carrera) or in the coolant stream of the intake manifold (944). This sensor supplies engine temperature (911 Carrera) or coolant temperature (944) information to the CU. This information affects the following engine systems: Air/fuel ratio (as engine temperature varies with time during a cold start) and spark timing.
Intake Air Temperature Sensor I
This sensor is located in the air stream of the air flow meter, and supplies incoming air temperature information to the CU. The CU uses this along with other information in regulating the fuel injection rate.
Air Flow Sensor
This sensor is located in the air stream of the air flow meter, and supplies air volume information to the CU. The CU uses this and other information in regulating the fuel injection rate.
The air flow meter incorporates an air flow measuring plate. The air flow plate opens when the engine draws in air. The plate is connected to a potentiometer. The potentiometer transmits an electrical signal determined by position of the measuring flap, to inform the CU of engine load.
The potentiometer within the air flow meter prevents loss of engine power during sudden acceleration/deceleration by signaling the CU of necessary fuel enrichment requirements.
Throttle Switch
A contact-type throttle switch is located on the throttle body. The 911 Carrera has one contact for wide open throttle position. The 944 has 2 switch contacts. The idle contact signals the CU to control idle stabilization and coasting fuel cut-off. The full throttle contact signals the CU for optimum power demand.
Idle Contact
The 911 Carrera uses a micro-switch to sense closed throttle position. It is mounted on the throttle housing opposite the throttle switch. The idle contact signals the CU to control idle stabilization and coasting fuel cut-off.
High Altitude Switch
Switch is mounted under the driver's seat (911 Carrera) or behind left dashboard (944). The high altitude switch closes over 3300 ft. (1000 m), signaling the CU to lean the fuel mixture.
Idle Speed Stabilization
The 911 Carrera uses an air regulating valve to provide both auxiliary air valve operation and idle speed stabilization. It is located near the throttle body. The air regulating valve is controlled by the CU. When necessary, the air regulating valve will adjust the air by-pass opening to maintain both cold and warm engine idle speeds.
Auxiliary Air Valve
The 944 uses an auxiliary air valve to provide additional air during cold engine starts and warm-up. It is located next to throttle body. The valve consists of an electrically heated bi-metallic strip, movable disc and air by-pass channel. The heater coil on the bi-metallic strip is energized by the fuel pump relay.
Control of the valve is based upon engine temperature. The air by-pass channel is open when engine is cold and gradually closes as temperature rises. At predetermined temperatures, air by-pass channel is blocked and additional air flow stops.
Fuel Pressure Regulator
The pressure regulator is located at the end of the injection collection line. Pressure regulator maintains constant fuel pressure to the fuel injectors.
Pressure Damper
The pressure damper is located at the end of right fuel collector (911 Carrera) or between fuel tank and injection collector tube (944). The damper absorbs the pressure oscillation caused by the injection cycle.