'747 top end fuel leanout

[NickG]

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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