Porsche Repair: intermittent electrical? problem 928S4, fuel injector cleaners, fuel injection system

Question
Hi Dave i have a late 89 S4 that intermittently runs on just 4 cylinders (exhaust note deepens and auto shifts become quite harsh) but if you stop the car and then restart it it's o.k. (for a while anyway) the perplexing thing is that it can be fine for months and then for no reason the problem reappears?

i enquired about this problem to a "Porsche specialist" who claimed it could be related to poor battery voltage, but after having recharged my battery overnight it happened 4-5 times on my way to work this morning (and appeared when decelerating each time) but then after the 5th restart has been fine (got me to work) and felt stronger than ever on the way home today not missing a beat!

i've checked battery & body earths (seem fine & tight) is there a wiring loom connector somewhere that can cause this problem due to poor electrical contact? and if so how do i locate it?
i also have a (new) idle hunting problem? any ideas? thanks George P.S. car has 129k on it & great service history.  

Answer
Hi George,

What a lucky guy you are!  An S4!

Unfortunately, I don't have a 928 repair manual, but I suspect the problem is completely in the fuel injection system.  Nothing electrical at all.  Not in the way we usually thing about wires and grounds etc.

The fuel injection system, Bosch's LH-Jetronic, was originally designed for and used on four cylinder engines.  To get the system to work on eight cylinders the engineers (and I'm simplifying this a lot) made some arbitrary divisions inside the fuel distributor and the spark distributor (at the trigger points) so that parts of the system feed half the cylinders and another part the other half.  The design works fine usually.  

Combined with your new idle issue, I suspect it's actually crud in the mechanical aspects of the system.  Have you used fuel injector cleaners before?  Oftentimes the problem of crud and crap arise from draining the gas tank down to it's last drop before refilling...then the fuel pump sucks the nasty little floatie things out of the bottom of the tank and into the fuel filter.  The filter cannot catch everything, escpecially if it is already full of previous crud and crap because it's too old.

To make a long story short...change the fuel filter, get your mechanic or your parts guy to recommend a fuel injector cleaner that works... and buy a couple bottles.  Use them over a couple tanks of gas and see what happens.

Dave