Check for Spark. Use caution, the 928 spark system produces high voltage and can cause death or serious injury. Use a spark jump test tool. If there is spark, then the ignition system is working, there is no need to test the engine RPM speed sensor or EZK output stage. Focus on fuel related issues.
Check Power and Ground connections. These connections are key points to inspect and clean. Start with the battery terminals, the multiple wires on the "+" connector must have a clean and tight connection. Remove all the fuses and clean the spade connectors. Check the ground strap between the engine and chassis. The LH fuel injection module is grounded via Pin 17 to chassis ground point VIII, measure the resistance between LH connector Pin 17 and a chassis ground point (see LH diagram). Resistance should read as close to zero as possible. If it reads a high resistance, a corroded ground connection at MP VII or cut in the wire could be the problem. Anywhere you see a brown (ground) wire bolted to the chassis, remove the bolt and clean the connection.
Check the Temp Sensor II. [applicable to all LH systems, beginning with Euro/ROW 1984 to 1995]. If the ambient engine temperature is too warm to check the Ohm reading at colder temperatures (0C/32F), remove the Temp II sensor from the engine and immerse it in cold ice water. A failure mode of the sensor is to go open circuit (infinite resistance) at cold temperature. A cold test is necessary to confirm functionality of the temperature sensor for cold start.
To check the Temp Sensor II, you will need an ohm meter. Remove the connector to the Temp Sensor II. There are two prongs on the sensor. There are two separate temperature sensors housed in the one sensor body (in LH fuel injected 928s, one sensor is for the LH and the other for the spark module).
Prong 1 is the sensor for the Electronic Ignition and prong 2 is the sensor for the LH fuel injection. Clip one of the ohm meter leads to prong 2 (LH) and clip the other lead to a ground point. Do NOT connect the ohm meter leads between the two prongs of the sensor. Repeat the resistance measurements for prong 1 (Electronic Ignition).
The proper Temp Sensor II resistance measurements are:
0 C / 32 F: 4.4k to 6.0k Ohms
15 - 30 C / 59 - 86 F: 1.4k to 3.6k Ohms
40 C / 104 F: 0.9 to 1.3k Ohm
60 C / 140 F: 480 to 720 Ohms
80 C / 176 F: 250 to 390 Ohm
If the resistance readings are not correct range, the temperature sensor must be replaced. If the resistance readings are in the correct range at the sensor, there could be a problem with wiring to the LH controller or the connector itself could be corroded. Measure the resistance from the Temp II sensor connector to the LH and spark controller connectors to confirm the circuit is good.
The Mass Airflow Sensor (MAF) [applicable to all LH systems, beginning with Euro/ROW 1984c 1995].
"Limp-Home" mode. The engine will run with the MAF disconnected. However, if the engine is cold, you may need to hold the throttle open when cranking to start the engine. This allows the car to be driven to a repair shop. The LH defaults to the follow pulse width:
RPM < 2000 RPM: 3.5 mS
RPM > 2000 RPM: 6.3 mS
With the limp home mode, you can imagine there are just a few RPM regions where the injection pulse provides the proper fuel mixture. However, unplugging the MAF provides a good test to see if your MAF is dead. If the engine runs better with the MAF disconnected, it is likely your MAF is not functioning.
Electronik Repair, Inc. is not affiliated with Porsche AG, Saab, Volvo, or Robert Bosch GmbH. All brands and marks are property of their respective owners.
Copyright 2020. Electronik Repair, Inc. All rights reserved.