Should i fix the TBI or go to carb? And what's this wire?

[AyWoSch Motors]

I recently got an 87 K20 (v20) with a TBI 350. TBI does not want to run, ive gone threw all the systems i could think of and replaced a bunch of parts, still wont work.
So i intended to just swap to a carburetor, which i know way better. Got an 4bbl intake, thats modified to accept TBI bolt pattern, got a rottencheser, HEI dizzy, a mechanical fuel pump, push rod and plate. Was all ready to go onto the swap, but relized that this block does not have provision for a mechanical pump. That throws a wrench in the plans.

So now my question is, do i proceed with my carb swap, and try to figured out an electric fuel pump and regulator and such. What would that entail, how would i do it?
Or, do i dig extra deep, and try to fix the TBI and keep it stock. How do i fix it?

After looking over the engine, i did find a bunch of wires that dont see to go anywhere.
If fxing the TBI is as easy as a few wires, ill be really happy. So i figured i should start there.
Here are the ones i found...

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Thanks in advance for any help you guys can give.

 

[bucket]

The first one... what color are the wires?

The second one... fuel pump test wire

The third one... tach signal hookup

 

[fast 99]

TBI systems are pretty basic and for the most part bulletproof. They run better than a carb and I am a carb person. Really need to get some sort of scanner to see what the data shows. Otherwise, you're just poking in the dark. Which sounds like what you have already done. Also need to check fuel pressure.

Changing back to a carb would require a pressure regulator.

 

[gmbellew]

throw a vac gauge on it. the TBI computer doesn't update fast enough to show vacuum fluctuations that could indicate a mechanical problem with the engine. i have been chasing a rough idle on mine for 6 months without solving it. all the diagnostics came back ok, rebuilt TBI, smoked for vac leaks, etc. I even checked compression and it was good. I finally threw an actual vacuum gauge on it and have obvious valvetrain issues based on the fluctuations, likely worn down lifters or cam lobes. i have the parts on havd and i am having a mechanic tear into it later this week to replace lifters and/or camshaft.

 

[Ontheboulder]

I just rebuilt my TBI, first one I ever did,very simple and it fired right up! got a sensor or something giving me fits but overall it was a simple job.

 

[AyWoSch Motors]

The first one... what color are the wires?

The second one... fuel pump test wire

The third one... tach signal hookup

I think the first is orange, yellow/dirty white, and black with white stripe.

Wha does the test wire do?

Tha makes sense, wierd it would have the wire, but not have a tach. Probabaly leftovers from another TBI model.

 

[AyWoSch Motors]

TBI systems are pretty basic and for the most part bulletproof. They run better than a carb and I am a carb person. Really need to get some sort of scanner to see what the data shows. Otherwise, you're just poking in the dark. Which sounds like what you have already done. Also need to check fuel pressure.

Changing back to a carb would require a pressure regulator.

Im familiar with TBI. My yukon, 3 of my suburbans, and my 1500 are tbi. They all run well, no complaints, ive just never had one in a square before, and never had one thats given me this much issue.

Truck hasnt had a battery in months, so all the trouble codes would be gone, and it wont run, so it wouldt reset them

 

[AuroraGirl]

I recently got an 87 K20 (v20) with a TBI 350. TBI does not want to run, ive gone threw all the systems i could think of and replaced a bunch of parts, still wont work.
So i intended to just swap to a carburetor, which i know way better. Got an 4bbl intake, thats modified to accept TBI bolt pattern, got a rottencheser, HEI dizzy, a mechanical fuel pump, push rod and plate. Was all ready to go onto the swap, but relized that this block does not have provision for a mechanical pump. That throws a wrench in the plans.

So now my question is, do i proceed with my carb swap, and try to figured out an electric fuel pump and regulator and such. What would that entail, how would i do it?
Or, do i dig extra deep, and try to fix the TBI and keep it stock. How do i fix it?

After looking over the engine, i did find a bunch of wires that dont see to go anywhere.
If fxing the TBI is as easy as a few wires, ill be really happy. So i figured i should start there.
Here are the ones i found...

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

Thanks in advance for any help you guys can give.

whatever you do, dont feel bad when you get it running because you will not think its running right by virtue of your carb exposure.
Gm put out a TSB on that even because of attitudes/expectations related to the 2 barrels in back opening up can lead to concerns and then dismissing outright.

 

[AyWoSch Motors]

throw a vac gauge on it. the TBI computer doesn't update fast enough to show vacuum fluctuations that could indicate a mechanical problem with the engine. i have been chasing a rough idle on mine for 6 months without solving it. all the diagnostics came back ok, rebuilt TBI, smoked for vac leaks, etc. I even checked compression and it was good. I finally threw an actual vacuum gauge on it and have obvious valvetrain issues based on the fluctuations, likely worn down lifters or cam lobes. i have the parts on havd and i am having a mechanic tear into it later this week to replace lifters and/or camshaft.

It wont run at all except if i bottle feed it, so i really can't check vacuum.


I had a thread on trying to fix this truck, back when it was still my boss's truck. Didnt have much luck.
Quick run down:
I replaced the ignition module, cap, rotor, plug wires and plugs, fuel pump and sender unit, fuel pump relay, fuel pump fuse, fuel filter, fuel pressure regulator, fuel pressure sensor, swapped injectors from a TBI that ran, swaped the whole top half of the TBI from one that ran... nuttin. Same exact thing that it was doing.
It would crank and crank no start. Throw a little gas down the throat and itd fire right up. If you bottle feed it, itll run as long as your feeding it. Every once and a while, itd start right up and run great on it's own, and then itd randomly shut off like a switch and wouldn't start up again.
As far as i could diagnose, its getting a good clean consistent spark. Its getting power to the pump, pump runs and shuts off like it should, and its getting fuel up to and threw the regulator and returning.
The issue is it has no injector pluse, so they're not allowing fuel to run threw them as they should. I put a test light on the connector, and no flashing with the injector pulse.

So thats why it wont run, but the question, is why is that happening. My last thought was the computer shittin the bed, or something i just don't know about.

 

[AuroraGirl]

It wont run at all except if i bottle feed it, so i really can't check vacuum.


I had a thread on trying to fix this truck, back when it was still my boss's truck. Didnt have much luck.
Quick run down:
I replaced the ignition module, cap, rotor, plug wires and plugs, fuel pump and sender unit, fuel pump relay, fuel pump fuse, fuel filter, fuel pressure regulator, fuel pressure sensor, swapped injectors from a TBI that ran, swaped the whole top half of the TBI from one that ran... nuttin. Same exact thing that it was doing.
It would crank and crank no start. Throw a little gas down the throat and itd fire right up. If you bottle feed it, itll run as long as your feeding it. Every once and a while, itd start right up and run great on it's own, and then itd randomly shut off like a switch and wouldn't start up again.
As far as i could diagnose, its getting a good clean consistent spark. Its getting power to the pump, pump runs and shuts off like it should, and its getting fuel up to and threw the regulator and returning.
The issue is it has no injector pluse, so they're not allowing fuel to run threw them as they should. I put a test light on the connector, and no flashing with the injector pulse.

So thats why it wont run, but the question, is why is that happening. My last thought was the computer shittin the bed, or something i just don't know about.

your PCM is probably failed, I sound like im just lazy sometimes when i say that to people with 86-90 vehicles but they , i cant emphasize enough, had a severe quality issue with GM pcms through 1990
1988 it was just pure horse manure and they would try a new PCM first line diagnostics in a lot of cases because of it.
The part numbers darted around a lot and they got the issues ironed out later.
Your injector pulse can be from other things but im guessing the PCM controls the ground side (id think) and if the wire is not broken and the connector on pcm is good, try loosely fitting the pcm where you can lightly tap it and listen closely for any life

 

[Soundmound]

By no means am I qualified to give any advice at this point in my life......That said, your list of went through or replaced makes me suspect the computer. Seeing that @AuroraGirl suspects the same, I'm almost inclined to trust myself lol.

 

[AuroraGirl]

as someone who has seen first hand just how unaware these early PCMs are when they are knocking on heavens door (I was 18 and in high school and trying to figure out distinct symptoms that fit no pattern and left codes but not related and the fact it works, and then just doesnt, but WILL go back to working really confused me then
Then some are really fricken fragile like literally lightly tapping with out much force was enough to ruin em
Then they had brute force amounts of pressure to insert a prom. i dont care what gm said you cant just lightly insert a PROM chip into one of those. even with connection lube that would still need large painful blackhead removal amounts of pressure. so that would break boards and solder

 

[gmbellew]

my guess is chart A3 is going to lead you to chart A4 and A4 will lead you to the promised land

 

[AyWoSch Motors]

my guess is chart A3 is going to lead you to chart A4 and A4 will lead you to the promised land

That looks like it will be super helpful. Im going to save those and review again this weekend. Thank you.

 

[gmbellew]

That looks like it will be super helpful. Im going to save those and review again this weekend. Thank you.

the driveability manual is super helpful for troubleshooting TBI. they are in the reference library on this site. glad to help!