[Diy_efi] 928/749
Rick McLeod
dunvegan
Thu Aug 16 15:33:17 UTC 2007
A poor mans tester:
gap a plug at about 0.175 and test free air
a .025 gap under compression relates to about a .166 gap in free air IIRC from my engine technical training classes
this has worked reliably for me in the past
----- 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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