928 Intermittent miss fire (electrical fault suspected)

Clare Pascoe
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Clare Pascoe PCGB Member
2018/02/28 16:22:05 (permalink)

928 Intermittent miss fire (electrical fault suspected)

My 928 S4 (D-reg) has been suffering from an intermittent fault for a few months. I'm emailing the forum to see whether others' have had the same issue; and to find out what was the root cause for your car - in the hope it gives us a fast track to check likely causes (rather than investing hours chasing a needle in an 'electrical rats nest' of a hay stack!)
The garage (AW Motorsport, Southbourne, Hants) are Porsche experts, and I cannot fault them - but they rarely see 928's so are becoming stumped.  They confirm the engine runs beautifully, and they are convinced the fault is electrical; but as much of the electrics in the car are original, they're not in great shape - and once tampered with, tend to require replacing.
I'd be very grateful for any thoughts.  Thank you, Clare
-  The fault doesn't occur when the engine is cold;
-  Once up to temperature, it faults every 30mins or so.  
-  The fault presents as a mis-fire, where for a split second it seems like the engine is going to cut out, with a brief loss of power.  Latterly this produced a few impressive back fires!
In a bid to find the fault, the following has been done:
-  Replaced Ignition Module (original fell apart when it was checked!)
-  Replaced TDC Sensor (part no. 94460611500) & connection plug (female part) to wiring loom 
-  Road tested to check the relays (the indicator relay and a heat exchange (?) relay seemed to be faulting?)
The result of the above, is that the car is running better, with no back firing - but it still faults, although less frequently.
I've been advised to:
1) have an electrical specialist spend a day with it, fault finding - but this could lead to a whole can of worms; and cost a lot of specialist time before anything is actually resolved....
2) fit a new fuse board, with new relays; but at £83 a pop for 12 relays, this isn't a cheap option unless I know it will work. Unless there's a way to do this economically?
Thank you again!
post edited by Clare Pascoe - 2018/02/28 17:47:42

2 Replies Related Threads

    PCGB Member
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    tscaptain PCGB Member
    Re: 928 Intermittent miss fire (electrical fault suspected) 2018/02/28 19:33:28 (permalink)
    Hi Clare and welcome to the forum.  Have moved your post to the 928 forum as the original is forum technical and software issues.
    I have a 928 too but I don't know enough about them to advise on your problem although when mine was running badly a couple of years back, the MAF was knackered!  Hopefully someone will be along soon to help or you could email the Register Sec
    He may suggest the best contacts.

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    paulreginalsmith PCGB Member
    Re: 928 Intermittent miss fire (electrical fault suspected) 2018/03/07 16:36:31 (permalink)
    Have you checked the battery positive terminal? There are several cables screwed onto the terminal clamp, make sure they are all tight. Also make sure the positive terminal is covered so it can't short out on the metal lid, and that the battery is firmly bolted down.
    PS you will get more replies if you post your technical questions on the 928UK list :
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