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1989 928 S4 will not run

31 Jul 2018 22:38 #20557 by Jjburns15
My 1989 928 S4 has been in my garage for 7 years without being run due to a family tragedy.
I changed the plugs, oil and filter and fogged (fog oil) the cylinders. It cranks very strong but will not start. Sounds as if it will for about a second or two but then dies.
I ordered a new fuel pump relay.
Does anyone have a suggestion ?
Thank you !

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02 Aug 2018 11:36 - 02 Aug 2018 11:43 #20560 by Rich928
Replied by Rich928 on topic 1989 928 S4 will not run
Hello John,

If you don't have them, download (no charge) the complete Porsche 928 factory workshop manuals. There is a great deal of information that will assist you to understand the car. Go to www.ligeti.com/928/

I hope a fuel stabilizer like Staybil was added to the fuel during storage. If not, the fuel can gum up and clog everything from the fuel pump to fuel filter and fuel injectors. I suggest draining the old fuel and replacing it with several gallons of fresh premium fuel.

If you have some mechanical expertise, there are several diagnostic steps to take to determine the nature of your problem. Since you're planning to replace your fuel pump relay, I suggest a test prior to plugging in the replacement.

1. Confirm that the fuel pump fuse #42 is not burned out. If so, replace.
2. Do a "jumper test" to listen for the fuel pump operation

The photo above shows a "jumpered" relay, look to the base and you'll see the pin positions 30 and 87. If you wrap a small wire around the 30 and 87 pins, you now have a jumper. This bypasses the low current control path and turns on the circuit. By inserting the jumper into the fuel pump relay socket, you'll turn on the fuel pump. You should hear a light buzzing sound at the center of the rear of the 928. The fuel pump is located above the metal shield plate. If you don't hear a buzzing the fuel pump may be frozen by the stale fuel.

Suggestion: solder the wire to the leads of the 30 and 87 of the old 141.951.253.B relay. Keep it in your glove box so if anytime in the future you suspect a failed 53 relay, it can be replace by this jumper relay to diagnose the problem. This jumper relay cannot be let in place as it doesn't turn off when the ignition is turned off, use it only for diagnostics. The three relays needed for the engine to run are LH, EZK and fuel pump. I suggest that all 928 owners replace these relays with a QUALITY relay as the originals are getting old and relays fail with age. The EZK relay may the special PN 928.615.203.00 type with fuse tester. It can be replaced with the standard and lower cost 141.951.253.B relay.

3. With the jumper still in place, if you hear the fuel pump running, crank over the engine. If it starts, then good news. Turn it off, remove the jumper and install the new relay. If it doesn't start with the new relay in place, the LH fuel injection module isn't "turning on" the fuel pump and requires service.

If you still have a no-start condition, you need to make a more specific diagnostic path. I've attached a document that outlines the theory behind the Porsche 928 fuel and spark system with some hints. If you have further specific questions feel free to ask.

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Last edit: 02 Aug 2018 11:43 by Rich928.

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