Porsche 1986.5 problem starting but turns over

I have a unusual problem with my 1986.5  Porsche 928 5 speed manual. car is super clean and has had no problems. Recently the car takes more engine turnovers to start over a day or two. then by third day it turns over but does not start. My mechanic is baffled. He says when it sits with the  battery disconnect turned off and he goes to turn the battery back on the car starts fine.
when the battery is left turned on after a day or two and he goes to start it wont start. any suggestions? joe@festivalsofspeed.com

]Hello Joe,

One of the most overlooked issues with many cars, including the 928, are relays.  After decades of 928 ownership, I suggest that all owners of 928 replace the critical relays needed for the engine to run.

1. In the early 1978 – 1979 cars the fuel pump relay is a special relay that uses the input of the RPM signal from the spark box to turn on the fuel pump.  It is part number 928.615.113.01.  A "standard" relay cannot be used, this is a special relay.

2. The 1980 – 1984 US spec cars use the same fuel pump relay as the earlier cars, but now have the Fuel Injection or AFC (air flow control) relay part number 928.616.119.00.          

3. Starting with 1985 the critical relays are the more standard 141.951.253.B type.  The relay location numbers may vary by model year, but the functions of the relays that should be replaced are spark (either EZF or EZK), fuel pump and LH relays.  Even if the relays haven’t currently failed, with age they will and could leave you with a no-start or hard start condition.  In some of the later model year 928 Porsche, one of these relays had a custom fuse tester built into the top of the relay.  A 141.951.253.B relay can be used as a substitute.  Also, in a bind if one of these relays fail and you're left stranded with a no-start condition, the horn relay is the same part number and can be swapped in ... a toot of the horn indicates that that relay is functioning.

Removing relays can be difficult, so I suggest using special relay removal pliers such as shown here.  It will save you a lot of trouble as it is easy to pull the caps off relays when attempting removal with regular pliers.
 

that is great information, thank you!  we will do that right away.  Joe