Troubleshooting Porsche 928 Starting Problems
These notes apply to the LH equipped 928 cars.
For any car, there are 4 essentials for the engine to run - fuel, spark, timing and compression of the air/fuel mixture.
All LH equipped 928s operate on a similar principle - a sensor (flywheel speed sensor) detects engine speed and crank position from a toothed gear at the rear of the engine. The flywheel speed sensor output is fed to the EZK/EZF (ignition) ECU. This gives the EZK the information it requires to time the spark.
A modified version of the rpm signal is fed from the EZK to the LH ECU (and also the rev. counter). When the LH (fuel injector ECU) receives this rpm signal it will energise the fuel pump relay. The rpm pulse also times the injector signals from the LH ECU. Information on engine air intake mass (load) comes from the MAF (mass airflow sensor. From rpm and laod data, plus engine temperature and other sensor paramters, the LH ECU determines the correct fuel injector opening time.
3 relays are vital for the car to start - fuel pump relay, EZK relay, LH relay. The fuel pump relay is a common source of problems, as it passes a high current to the fuel pump (and also the O2 sensor on later cars).
1) Does the car have fuel ?
2) Do you hear a regular clicking noise from the engine when the igntion is switched on, but the engine not cranked? If so, this is a sign of LH ECU failure (see LH ECU FAQs). If no, proceed as below.
3) Is there a spark ? If not, swap out the EZK relay for a known good one. If still no spark, check if there is no spark at each distributor. If spark at one only, then check out final amplifiers. For a Euro S2, check that both distributors are turning when the engine turns. There is a belt coupling them which can break Then suspect flywheel speed sensor or its connections. This sensor is located under the engine airfilter, on top of the engine.
4) If there is spark, but no start, then check that the fuel pump relay is being energised when the engine is turning. Alternatively, you can "jump" the fuel pump relay. But take great care to link the correct pins on the relay socket, or damage can be caused to the LH ECU. The contacts to be linked are 30 and 87. When these are linked, the fuel pump should be heard to run, even with the igntion switched off. If the car now starts, then remove the link and fit a known good fuel pump relay.
5) If there is still no start after fitting a known good fuel pump relay,but the car starts with the bridge in place, then the LH ECU is not energising the fuel pump relay and may be faulty and so requires testing and repair if necessary.
6) If no start with the fuel pump bridge in place, check that a type "53" relay has not be fitted to the kckdown relay position by mistake, this will result in a no start. 5 speed cars should have no relay at this position, Automatics should only have the correct kickdown relay fitted.
6) If still no start, then LH failure is a possibility if car ran well before the no start. If car stopped suddenly at speed, and now will not start, consider the following possibilities - a) Cambelt snapped. Remove both intake airtubes and check if both camshafts are still turning. b) Check fuel pressure at the test point on the fuel rail. A fuel pump can fail if fuel tank level is low and/or very hard sustained cornering momentarily starves the pump with fuel. Note that there are often two pumps, the external one, and often an in tank pump. Both may be damaged through fuel starvation. The intank pump hose inside the tank has been known to fail, and debris from it can block the external pump.
7) If there is fuel, but the car does not start, bear in mind that a 928 engine can be easily flooded with fuel if cranked repeatedly with a no start. Remove a spark plug to check if it is wet. If wet, try removing the fuel pump fuse and with accelerator pedal fully depressed crank until the engine fires. Then replace the fuel pump fuse, and again crank to start with throttle depressed.
JDSPorsche can check out your LH ECU, EZK ECU and MAF for a nominal charge if you believe any of them are responsible for a non start.