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Old 04-20-2017, 10:32 AM   #1
Logan71
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Injector question (Not GTO / LS2 related)

This is not related to our cars, but since so little posting goes on here, I thought you might entertain a question I'm having trouble with. Mods, Please delete if this is frowned upon or not permitted.

Vehicle: 89 Silverado
Engine: 5.0L / 305 (formerly 5.7L 350)
Fuel pump: Stock
ECM: Stock, still programmed for 5.7
TBI: Stock from 5.7

Issue: After I replaced the engine with a used long block a couple of years ago, reusing all accessories including the TBI, it has never run perfectly. Super reliable, smooth idle but bogs at certain RPMs.

I've slowly done things to try and improve it. New plugs, wires, knock sensors, 02 sensors, EGR, trying variation on the timing, rebuilding the TBI and regulator.

In recent reading, I'm realizing the stock '350' injectors are likely 51lb/hr and the 305 injectors are likely 45lb/hr. My assumption is the ECM still 'thinks' I have a 350, causing too much fuel delivery and perhaps that's my problem.

I've ordered a pair of remanufactured TBI injectors rated @ 45lb/hr for a 1989 305 truck. The parts arrived yesterday.

Before I install them, I wanted some input. I have no logs, no AFR data and no way to gather it, so I'm shooting in the dark somewhat.

Your thoughts are appreciated.
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Old 04-20-2017, 10:42 AM   #2
cookmeup272
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Throw em in and see how it likes it. Check spark plugs. That's what I would do.
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Old 04-20-2017, 11:40 AM   #3
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Quote:
Originally Posted by cookmeup272...View Post
Throw em in and see how it likes it. Check spark plugs. That's what I would do.

That's my plan, except that a set of new injectors that reek of gasoline may not be refundable if my guess is wrong.
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Old 04-20-2017, 11:49 AM   #4
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Well a 305 with 350 injectors and a 350 computer automatically sounds wrong. Can you not just use a computer from a 305 truck with a 305 TBI? I don't see how you couldn't find them in a junkyard.
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Old 04-20-2017, 03:29 PM   #5
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Quote:
Originally Posted by cookmeup272...View Post
Well a 305 with 350 injectors and a 350 computer automatically sounds wrong. Can you not just use a computer from a 305 truck with a 305 TBI? I don't see how you couldn't find them in a junkyard.

Good idear here but not sure if trucks came with 305 tbis. Might have to look in the camaro/trans am or maybe even better astro van section.
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Old 04-20-2017, 03:44 PM   #6
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those old vehicle ECM's still run on the basic principles the newer ones do.

i'm sure it runs on alpha-n or speed density.

at idle, low throttle, and cruise, it has probably learned the trims for what will make in run halfway decent. at higher rpms or heavy throttle i would imagine it would revert to whatever open-loop power-enrich mode those computers used back then. so, just dump the preprogrammed amount of feul.

i think LTFT principles were similar under WOT back then. adds positive feul, but does not subtract at WOT.

i dunno, i say try the injector swap and see how it does. reset the computer and swap the TBI injectors.

make sure all your sensors are up to par, as well. those older ECM's that don't rely on MAF sensors are very reliant on the TPS, MAP, and ECT sensors to run right.
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Old 04-20-2017, 06:38 PM   #7
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Cookmeup - Swapping ECM would probably be more expensive. I only paid $46 total for replacement injectors. I'd also be worried about other things like RE ratio and shift points. At least right now, I have the factory ECM paired with the original chassis.

Humble - I was able to pick the engine I swapped in out of a wrecked '90 Silverado with TBI, and they pulled it for me and delivered it. Only after I had it almost ready to drop in, did notice it was stamped as a 305, but I was too far along.

NotHubert - All good points. I've checked the TPS before and have some faith in the rest of the sensors.

>> i dunno, i say try the injector swap and see how it does. reset the computer and swap the TBI injectors.

That's my gameplan tomorrow. Wish me luck. Any idea of the 'relearn' time on something this old?
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Old 04-21-2017, 04:47 AM   #8
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My point was to go to a junkyard and get an ecu out of a truck, possibly the one you got the motor out of? I mean how much is a junkyard ecu...?
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Old 04-21-2017, 07:49 AM   #9
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The injectors are different.

5.7 is pn 2172286
5.0 is pn 2172285

So, he'd have to swap them anyway, i would imagine.

The ECM for that year model c1500 should also be the same for each engine.

Not sure there are still places that will make a custom PROM for you, but you may want to go that route. Or find a PROM from a silverado in the yard with the 5.0 engine/same ECM... provided it isn't fried.
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Old 04-26-2017, 10:07 AM   #10
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Update:

I disconnected the battery cable and swapped the injectors on Friday. After several commutes to work and back, I can safely say its much better. More responsive down low, much better quality 1-2, 2-3 shifts (used to bog right after the shift/rpm drop), and all around a better experience.

I appreciate all the opinions that backed up what I was thinking.

Have a good one!
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