[Diy_efi] 928/749

[Steven P. Donegan]

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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