[Diy_efi] 928/749

Kevin Murray kevmurray
Thu Aug 16 13:08:17 UTC 2007


Hi Bill, I understand your frustration as I've been there once before. In my 
case it turned out to be a bad ignition coil. Firing the plugs in open air 
is much easier than when under compression. When the impedance at the plug 
is too high (in the cylinder under compression) the coil energy will 
discharge internally. To make things worse, this situation wears the coil 
out faster.
I'll admit I'm not familiar with Porche ignition systems but perhaps this 
will give you some direction.

Kevin Murray

----- Original Message ----- 
From: "Bill Shaw" <b.shaw at comcast.net>
To: <diy_efi at diy-efi.org>
Sent: Wednesday, August 15, 2007 11:23 PM
Subject: [Diy_efi] 928/749


I've been chasing a long series of problems with my 928 since I
installed the new motor this spring .  Right now it won't start,  it's
having some weird electrical problem.

A little background...
I ran it in a similar configuration for 3 years... Porsche 4.7L v8,
Vortech supercharger, GM ignition module, sensors, throttle body, and
749 ecu.  Ran great,  had loads of fun flogging the crap out of it on
all the road courses in the northeast.  Last fall it ate 2 spark plug
tips and scored 2 cylinders at Lime Rock.  New motor was in the works so
I finished it up and got it in the car.  The new one is a 5.0L Porsche
v8, '87 big dish pistons,  16 valve ported & extrude honed heads all
adding up to about 8.5:1 cr,   bigger cams, bigger throttle body, bigger
exhaust.  It ran pretty good for about 2 days,  good enough to set me
back in my seat and put a grin on my face,  then it all went to he!! again.

I've swapped ECUs, ignition modules, plugs, cap & rotor, been through
all the plug wires with an ohmmeter, wrung out the wiring harness from
the sensors to the ECU.  I'm getting fuel,  I can hear all the injectors
clicking,  and have 43 lbs of fuel pressure.   The REF signal from the
distributor makes it through the ignition module to the ecu and the ecu
is putting out EST pulses, verified by o'scope.  #1 is firing at TDC
with bypass unplugged.  Everything is great,  it just won't start.
Tonight we checked the cam timing and retensioned the timing belt.  I've
been through all the basics I can think of, all is good but no fire in
the hole. I've reverted to my last known good bin while doing all this
testing.

The only weird thing I can put my finger on is with spark.  I move my
inductive timing light from wire to wire checking for spark on all 8.
I'll find one not firing (it changes some times),  pull the wire off the
spark plug,  insert another spark plug in the wire and ground the tip on
the block and it will fire,  the timing light will light up.  Move the
wire back to the plug in the block and it won't fire.  Pull the plug
from the block,  put it in the plug wire and ground the electrode on the
block and it will fire again.  WTF???  This is driving me nuts.  I've
pulled one plug wire off 3 times to check resistance, wiggled both ends
and the whole wire looking for an intermittent break,  everything is
great,  3.1k ohms per spec.

I'm running out of ideas.  Anyone have a thought??

Thanks,

Bill
928s 5.0L Vortech/749
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