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Fuel pump relay issue

06 Jan 2019 19:00 #20724 by Jjburns15
Took my 928 to a foreign car shop and learned that the alternator was fine but the battery was bad after Sears assured me twice that the DieHard battery was good!
The car was stored for 10 years in my garage so I ended up replacing the fuel pump, fuel filter and fuel injectors because the gas had turned to varnish on the parts!
The problem now is the car cranks but doesn’t start.
Someone suggested that it was the fuel pump relay and that I should create a bypass relay with a wire soldering the contacts together to turn on the pump which it did and the car started!
Purchased a new fuel pump relay and inserted it and the problem came back again where it cranks but won’t turn over and start !
Put the bypass relay in and it starts again !
Purchased a second new relay and car again wouldn’t start!
Put the bypass relay back in and the car started !
Anyone gave an opinion !
Thank you

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11 Jan 2019 09:04 #20726 by Rich928
Replied by Rich928 on topic Fuel pump relay issue
Hello John,

You didn't mention the year of your Porsche 928, that makes a difference in specific instructions. I assume that your 928 is a 1987 or newer S4, am I correct?

To test your new relays, unplug the horn relay (relay number XII on 1987) and plug in the new relay. Honk the horn. If it sounds, the new relay is working.

So, what are the other potential causes of the no fuel-pump running problem? The fuel pump relay is energized by the LH fuel injection module, which provides ground to the low current switching circuit via connector pin 21 to relay socket position 85. Either there is an open circuit somewhere in this circuit, or the LH fuel injection module isn't providing the ground to energize the relay.

This type of failure of the LH fuel injection module is a fairly common experience, the modules fail with age. I can test the module and confirm operation (free service to 928 OC Members). If the module has failed I provide a rebuild service.

I also suggest that the calibration of your mass airflow sensor (MAF) be tested. After decades of operation (there is a burn off cycle each time the engine is switched off) the hot wire thins and the MAF reports the incorrect amount of air entering the engine - so the engine will not run properly. I also provide no-cost MAF testing for 928 OC member.

I can provide further testing details and module removal instructions, please drop me a line at This email address is being protected from spambots. You need JavaScript enabled to view it.

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