[Diy_efi] 928/749

Thu Aug 16 15:12:12 UTC 2007

Yep,  swapped the coil with a spare I had in inventory (forgot that 
one,  and a few more I'm sure!).  I need to go back there and double 
check,  the spare was used.   No ballast resistors,  I took them out 
when I did the 749 conversion.  I'm off to buy a spark tester today.

Thanks for all the input guys.

Bill

Steve Ravet wrote:
> 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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>>>>         
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>>>       
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>
>   