[Diy_efi] 928/749

Thu Aug 16 12:46:10 UTC 2007

good point, it takes more energy to fire the plug under compression than
in the air.  You didn't mention testing/replacing the coil.

--steve 

> -----Original Message-----
> From: diy_efi-bounces at diy-efi.org 
> [mailto:diy_efi-bounces at diy-efi.org] On Behalf Of dgilbert78 at juno.com
> Sent: Wednesday, August 15, 2007 10:49 PM
> To: diy_efi at diy-efi.org
> Subject: Re: [Diy_efi] 928/749
> 
> Hello: Had a problem like that , everything was there , but 
> would not run. final fix, spark wasn't hot enough. Go buy 
> chep spark energy tester at parts store. If it does not troow 
> a healthy gap, fix that. I went absolutely nuts. I had spark, 
> everything, just not enough and the damn thing would not run.
> good luck
> Darryl..
> On Wed, 15 Aug 2007 20:33:37 -0700 "Steven P. Donegan"
> <steve at donegan.org> writes:
> > Only takes 3 things to go bang - gas, spark and compression 
> - you've 
> > verified 2 of the 3 below. Compression seems to be the only thing 
> > left.
> > 
> > On Wed, 2007-08-15 at 23:23 -0400, Bill Shaw wrote:
> > > 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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