[Diy_efi] 928/749

Tue Aug 21 02:55:50 UTC 2007

Bill, if you haven't already, check to make sure there are good 
connections at both ends of the engine to chasis ground cable (under 
car on passenger side), check the connections at the battery 
(especially those smaller wires on the positive terminal), and the 
ground connection near the coil.

Mike

At 09:22 PM 08/20/07, you wrote:
>Well,  a new coil helped some but didn't cure the problem.  It will 
>start and run now but is still dropping spark intermittently in a 
>couple of cylinders.  The pulses from the ignition module to fire 
>the coil are nice and steady,  don't miss a beat.  Using the timing 
>light on a couple of the plug wires shows erratic firing. It's 
>starting to look like something in the distributor now,  but I can't 
>imagine what.  The rotor and cap look find and check out OK with an ohmmeter.
>I was thinking about changing to a different ignition 
>controller,  maybe something like a MSD 6.  Multiple sparks might be 
>of some benefit,  but the MSD really won't give me a stronger spark 
>than using the GM 7 pin module,  will it?  The spark is generated by 
>the coil,  not the module,
>so I'll get the same voltage won't I?
>
>Bill
>Still not sparking on all 8
>>>
>>>
>>>----- Original Message ----
>>>From: Bill Shaw <b.shaw at comcast.net>
>>>To: diy_efi at diy-efi.org
>>>Sent: Thursday, August 16, 2007 10:12:12 AM
>>>Subject: Re: [Diy_efi] 928/749
>>>
>>>
>>>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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