[Diy_efi] Re: Diy_efi Digest, Vol 30, Issue 2

[Bill Washington]

Bill,
    I had a similar problem about 25 years ago, everything appeared to 
be there but it would not go - it turned out that the coil had 
'partially' failed - a shorted turn - it would spark at atmospheric 
pressure but not in the cylinder under compression - changed the coil 
and away she went ...
Good Luck
Bill
>
>
>
> ------------------------------------------------------------------------
>
> Subject:
> [Diy_efi] 928/749
> From:
> Bill Shaw <b.shaw at comcast.net>
> Date:
> Wed, 15 Aug 2007 23:23:25 -0400
> To:
> diy_efi at diy-efi.org
>
> To:
> diy_efi at diy-efi.org
>
>
> 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
>