Porsche Repair: !981 Porsche 928, spark plug wires, fuel injection system

Question
Dave,  I have couple of problem questions to ask,  On my 928 when I start it up it idels fine at 850 rpms for about 15 minuites then it compleatly shuts off, I can drive it all day as long as I dont come to a complete stop and when I do,  it shuts off, I can sill start it back up and continue to drive,  could this be a vacume leak or maybe a fuel presure regulator problem? Another question is, When I enguage the clutch to change gears,  It vibrates and the gear shift shakes and it feels like the petal is on the floor, could this be the slave cylinder? My final question is I can completly chaarge my battery and once I park it overnight,  the battery is dead and when I start it up the voltage regular doesnt move but when I drive on the interstate at 50mph,  it starts to charge.  I would appreciate of you could help me with these problems concerning my car,  you have trully been great with the questions an answers that you have given me in the past.  Thanks,  Greg

Answer
Hi Greg,

1.  Idle problem******The part of the problem that is puzzling the most is that apparently within the first 15 minutes you can come to a stop, idle, and drive off...but after 15 minutes, no dice.  I'd expect the 'normal operating temperature' to be reached well before 15 minutes, so heat doesn't seem to be a trigger.  

Here's the laundry list of the most likely culprits:

a.  fuel filter clogged and/or water or impurities in the fuel system.
b.  faulty or incorrectly gapped spark plugs.
c.  faulty spark plug wires.
d.  vacuum leak in fuel injection system, intake manifold, or hoses.

If the problem was more immediate after starting, I too would be suspicious of a fuel pressure problem.  And as for the list above, I don't see one that would logically be a 'come and go' kind of issue.  I don't know anything else to suggest at this point.

2.  Clutch shaky, pedal to the floor*****  This is really two separate problems.  The shakiness is caused by one of the following:
a.  oil on the clutch lining material, or a burned/glazed lining and pressure plate.
b.  loose engine or transmission mounts.
c.  worn splines on the clutch hub.
d.  warped pressure plate or flywheel.

In this case if you find the engine/transmission mounts to be OK, then searching any further for the culprit here will mean dropping the transmission to get at the clutch.

If the clutch pedal is going to the floor and you're feeling little or no resistance, then here are the things to check for:
a.  master or slave cylinder faulty.
b.  brake hose, pipe burst...or a connection leaking.
c.  no fluid in reservoir.
d.  broken release bearing or lever.

Charging problem*****  Your two symptoms, losing a full charge overnight, and questionable charging while driving, are no doubt related.  

There's a whole bunch of testing you can do, but it's description is way more than I can type, believe it or not.  Since you'll be quite busy looking into your other perplexing problems, let's use this time to try to pin down the circumstances and hopefully narrow our search a bit.

If you'd take a couple minutes and answer these questions, I'd appreciate it.

1.  Does this loss of charge happen every night after the car has been driven?
2.  When you park the car at night do you verify that the battery's voltage level is 12.-something?
3.  Is the alternator drive belt sufficiently tensioned?
4.  Are the battery terminals squeaky clean?
5.  When you do see charging above 50 mph, are you referring to the voltage gauge being at 12+.  Before that was the reading toward the middle of the gauge or closer to the lowest possible reading?  Any other comments about your experiences would be helpful.

Use the follow-up feature in Allexperts when it's convenient to get your answers back to me.

Thanks.

Dave