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Re: '747 top end fuel leanout



Are you sure the injector constant wasn't 148 instead of 108 ?? Here's my
list:
4.3L TBI is 181 BPW#
5.0L TBI is 148 BPW#
5.7L TBI is 135 BPW#
7.4L TBI is 134 BPW#
I noticed your mods are with a Holley TBI ?? Do you have the injector #'s
handy ? Instead of measuring duty cycle--can you measure pulse width with
your Fluke ??

Lyndon.
-----Original Message-----
From: NickG <nikog@mediaone.net>
To: gmecm@diy-efi.org <gmecm@diy-efi.org>
Date: March 11, 2000 4:55 PM
Subject: Re: '747 top end fuel leanout


>Thanks for the suggestions. Unfortunately, all the items you've mentioned
>have already been checked. The in-tank pump and filter have been replaced.
>The fuel pressure remains at a constant 12psi at all RPMs when at WOT. The
>problem doesn't seem to be from the fuel system, but rather, the injection
>system. The injectors won't go higher than 57% duty cycle.
>
>Nick
>
>>Lots if not all fuel pumps lose there ability to supply volume, before
>>completely dieing.   Try rigging a Fuel Pressure guage, and see if it
>drops,
>>at extended WOT.  If that shows bad try a new fuel filter before replacing
>>the pump.   I've seen the gm TBI pumps die with as little as 25K miles on
>>em.  Also, be advised fuel contaninated with water can immediately screw
up
>>a paper filter element.
>>Grumpy
>>
>>
>>> Vehicle in question:
>>> '93 GMC pickup, 5-speed
>>> 1989 '747 TBI ECM installed
>>> 350cu in, ported iron heads, mild cam
>>> ported alum intake, Holley TBI unit, headers, dual exhaust
>>>
>>> The part throttle driveability is very good, with a smooth idle and good
>>tip
>>> in acceleration. WOT acceleration is also good, but only up to 4600rpm
or
>>> so. Using a Diacom, we've seen the O2 put out 950mv up to around
4600rpm,
>>> and then it drops to 500mv or so (and the power dies out). Obviously,
>it's
>>> running out of fuel.
>>>
>>> So I installed a Fluke 88 today to see what the injector duty cycle was
>>> (maybe they're going static?). The DC peaks at 57% right around 4600rpm.

>I
>>> figured that there's plenty of DC time left, so I increased the injector
>>> constant (BPW) from 108 to 130 as a test. I go for another drive, and
the
>>> max DC didn't change one bit.
>>>
>>> Any ideas? Is there some set limit in the control program that limits
the
>>DC
>>> to the injectors? How do I get more fuel in the upper RPMs?
>>>
>>> Thanks,
>>> Nick
>
>
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