4.3 TBI to 350 carb swap/ecm?

Hamrtym2

Not-quite-so-new-guy
Jun 29, 2022
6
3
Iowa City, IA
i have a 1987 Monte Carlo LS with the 4.3 TBI in it that my uncle bought in 93. He has since passed of cancer and my father obtained the car and is now also diagnosed with cancer so I’m trying to finally get this car going. The 4.3 that is in it is the 2nd engine this car has had and is knocking. Again. Go figure 🙄. I have decided to 350 swap this car and go carbureted because I’m sick of the TBI extra crap. So, I’ve been doing research for days and nobody seems to have answers specifically for what I’m doing. Some things yes, some no. I do not have wiring diagrams like other guys and I’m not exactly a wiring guru like some guys. I’m going to be running a 350 with a edelbrock 600cfm carb and edelbrock performer intake. I already know that the engine mounts might need moved and I know I need to do all new exhaust. So that’s out of the way. I know the fuel pressure is different however I’m keeping the in tank pump so I’m thinking a fuel pressure regulator at the carb with a return option. The car already being fuel injected has a return line for the tank I believe. The transmission is a th350. As of this minute it’s unknown if it has a lock up converter or not. I’d have to find out somehow. (Yes I know the common trans is a 200r4. I got under and looked and mine has the 350 trans.) so I know that will work with my 350 just fine. I want the speedo, and ac and heat and cruise etc to all work. The car only has a fuel gauge and a speedo gauge being as it’s an LS not an SS. So my questions mainly pertain to the ecm of the car. Do I simply leave the ecm in place and everything will work normal but engine light would always be on? Or do I remove that ecm from the right side kick panel? What do I need to do to make sure the fuel pump keeps pumping, cruise works, and of course gauges and ac etc. i need some guys that can give me answers. And dumb it down a little for me too please 😂 Too many people keep saying “Google it” or say that there’s tons of forums already on this topic yet I can’t find any that are answering my questions. So I am posting my own. Any help is appreciated!! Thank you guys.
 
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Hamrtym2

Not-quite-so-new-guy
Thread starter
Jun 29, 2022
6
3
Iowa City, IA
i have a 1987 Monte Carlo LS with the 4.3 TBI in it that my uncle bought in 93. He has since passed of cancer and my father obtained the car and is now also diagnosed with cancer so I’m trying to finally get this car going. The 4.3 that is in it is the 2nd engine this car has had and is knocking. Again. Go figure 🙄. I have decided to 350 swap this car and go carbureted because I’m sick of the TBI extra crap. So, I’ve been doing research for days and nobody seems to have answers specifically for what I’m doing. Some things yes, some no. I do not have wiring diagrams like other guys and I’m not exactly a wiring guru like some guys. I’m going to be running a 350 with a edelbrock 600cfm carb and edelbrock performer intake. I already know that the engine mounts might need moved and I know I need to do all new exhaust. So that’s out of the way. I know the fuel pressure is different however I’m keeping the in tank pump so I’m thinking a fuel pressure regulator at the carb with a return option. The car already being fuel injected has a return line for the tank I believe. The transmission is a th350. As of this minute it’s unknown if it has a lock up converter or not. I’d have to find out somehow. (Yes I know the common trans is a 200r4. I got under and looked and mine has the 350 trans.) so I know that will work with my 350 just fine. I want the speedo, and ac and heat and cruise etc to all work. The car only has a fuel gauge and a speedo gauge being as it’s an LS not an SS. So my questions mainly pertain to the ecm of the car. Do I simply leave the ecm in place and everything will work normal but engine light would always be on? Or do I remove that ecm from the right side kick panel? What do I need to do to make sure the fuel pump keeps pumping, cruise works, and of course gauges and ac etc. i need some guys that can give me answers. And dumb it down a little for me too please 😂 Too many people keep saying “Google it” or say that there’s tons of forums already on this topic yet I can’t find any that are answering my questions. So I am posting my own. Any help is appreciated!! Thank you guys.
I also forgot to mention the car is getting an HEI distributor as well.
 

57 Handyman

Master Mechanic
Feb 6, 2017
253
63
Here are a couple of suggestions:

You are not the first and only person to make the switch from a 4.3L TBI to carbureted V8. Your first task is to decide whether you intend to keep the ECM that controls both the spark/ignition via sensors and fuel delivery system OR get rid of the computer and wiring that controls the TBI, distributor, and, if installed, the smog system.

Not everyone is going to layout each of the steps you need to do in order to make this swap work. Therefore, this is what I do when I approach these kinds of projects. Set up a directory on the desktop of your computer. Do your online research/google searches and cut 'n' paste the information that you find that applies to your project and each task. Initially, don't try to make sense of and understand it all; there is just too much information to digest. Keep track of the statements or terms you use to search for information at the top of each page. That way you don't make redundant searches. Later on, come back and familiarize yourself with what needs to be done and become knowledgeable in the scope of this project.

Set up a budget and form an understanding of how long this will take. Assess if you have the necessary space and tools to carryout the job. Be honest with yourself and adjust your goals.

Begin the search for the necessary parts.

To answer some of your questions:
If you switch from TBI to a carb, you won't need to have a fuel return line. Depending on the SBC you use, there maybe a mechanical fuel pump location on the passenger side of the motor. So, Just plug it up the return fuel line at both ends and leave it in place.

Once you decide you're going with a carb, you can get rid of the ECM in the kick panel. But do not just rip out all the wires and harness because you can use some of the wires for circuits that you need or want later. A big help in doing this conversion is to get you hands on a GM/Chevy Service Repair Manual (forget the Chiltons and any other books that you find at Autozone or Oriellys). Bag and tag parts as you remove them; also mark and tag the connectors on the harnesses as you remove them. DO NOT CUT any wires/connectors until you are absolutely sure your will not be needing them.

I don't believe a TH350 has lock up torque converter capability.

You will find in your searches that others have made the switch from idiot lights to gauges...just take notes and learn from their mistakes/experiences. Lastly, if you are willing to learn and ask the right questions, there are a lot of great guys who can help you get solutions and answers.

Good luck!
 

truckertim

n00b
Jun 29, 2022
1
1
You do not need to use the fuel pump if you have a mechanical it will draw thru the in tank pump and if yo take out the ecm you will have to hook up aftermarket oil and temp gauges. Your engine mounts will be different the ecm will cone out with all wiring on that side you will not need but have to drop the inner fender to get them out. And with the hei dist it will be one wire comming from ignition its a hot wire it is fairly easy to to if you have the motor you can swap it out in a day. also your ac will bolt right to the 350 using the brackets from the 4.3.
 

Hamrtym2

Not-quite-so-new-guy
Thread starter
Jun 29, 2022
6
3
Iowa City, IA
You do not need to use the fuel pump if you have a mechanical it will draw thru the in tank pump and if yo take out the ecm you will have to hook up aftermarket oil and temp gauges. Your engine mounts will be different the ecm will cone out with all wiring on that side you will not need but have to drop the inner fender to get them out. And with the hei dist it will be one wire comming from ignition its a hot wire it is fairly easy to to if you have the motor you can swap it out in a day. also your ac will bolt right to the 350 using the brackets from the 4.3.
Sorry for the late response here guys, I didn’t get the emails for notifications on my thread for some reason. So, as for what you’re saying it sounds fairly easy and good, now my question I suppose is what all does that ecm control? Because I do plan to swap out with aftermarket gauges instead of using the dummy lights that my gauges currently have. Ignition with be the HEI. As for fuel I wanted to keep my in tank fuel pump and run it with a regulator at the carb. Does that ecm control my fuel pump at all? Will my ac controls and my cruise control etc all still work without that ecm? I didn’t think a 350 trans had a lock up either but Google is telling me different. I do have a electrical plug on the left side of the bell housing of this transmission next to the shift linkage sort of. It looks like a 3 or 4 wire plug. Square plug. Some guys on Google were saying there were a multitude of ways to bypass a lock up. But said it grounded in the ecm? So I guess I’m not sure what to do with the ecm or the fuel. I know what I want to do, but I need to make sure I know exactly what would happen if I pull that ecm out before I do so.
 

jiho

G-Body Guru
Jul 26, 2013
994
93
I don't believe a TH350 has lock up torque converter capability.
Some did, some didn't. The TH350 was used with an ECM, where lockup was ECM-controlled. It was also used with the Olds diesel, where lockup was controlled through some other screwball means.

It was mainly the TH400 that never had a lockup converter.
 

57 Handyman

Master Mechanic
Feb 6, 2017
253
63
The ECM controls the fuel pump and cruise control via a speed sensor. I converted my 87 El Camino from lights to gauges and don't believe there is direct connection between the ECM and the workings of the gauges. Depending on the ignition for your 4.3L, you may be able to use some of the wiring to the distributor and coil. With regard to the 3- or 4-wire square plug, that may be the connector going to the 2004r transmission from the ECM. As stated before, don't cut any wires until you are absolutely sure you don't need it for something.
 
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78Delta88

Master Mechanic
Supporting Member
May 23, 2022
417
43
SW Arizona
I've done several, and they are not too difficult.
Yours is an 87 so it will be a lock up trans. Lock up basically a standard starting 1981. Yours probably started out as the metric 200 or the 250. Could be that the th350 is what came with it but more likely was replacement after the other failed.

If your shifter display has a circle around the "D" then it came with 2004R. If you only have "D" 2 & 1 then it came from factory with a 200, 250 or 350. Keep what you have for now, but suggest look at getting a 700R4 for future.

ECM controls FP as a limited item. Key turned to "ON" & ECM energizes pump for about 8 seconds. Once the engine starts the FP is basically ignored by ECM. FP stays running by oil pressure switch that is on the back of the block on drivers side.

There are 2 sometimes 3 temp sensors. 1 is only a switch. One is for the gauge, the other for ECM logic... I.e.: engine temp above pre-programmed threshold? Yes/No?

Same for oil pressure, one is for gauge, the" other for the ECM. Engine running... Yes/No.

You can't run HEI with the other emission "stuff" unhooked. Engine will hunt and will not idle properly.

You could actually get a TBI intake manifold for the 350 and run it as TBI if you wanted. It wouldn't well until you switch out to the 350 TBI unit. But you will be better off with the carb.

On the carb ... Get a 650 cfm unit.
 

78Delta88

Master Mechanic
Supporting Member
May 23, 2022
417
43
SW Arizona
On the TBI the injectors are hot at all times and grounded by the ECM as the engine runs. The ECM also looks for detonation through use of a knock sensor. If detonation occurs the ECM backs off the advance. The ECM is also making adjustments based on RPM and O2 sensor by adjusting Lean/Rich based on O2 sensor readings.

The TH250/350 shift is based on vacuum and governor. On the 2004R and 700R4 shift is based on line pressure controlled by the TV cable. If the vehicle had the TH250/350 you have to add the conversion kit to the carb if using the 2004R or 700R4. If you run with 4L60E/4L80E you need to purchase the TCM unit or it will not shift properly.

The TBI distributor has no advance, so you need the stand alone HEI. You can not reuse the TBI distributor without the ECM, as it will not run properly.
 
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64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
5,124
113
Upstate NY
KISS - keep it simple ______, this truly applies here.

You don’t need the ECM at all. You need motor mounts, exhaust, FPR (fuel pressure regulator) and some ingenuity with hose and pipe plumbing.

The trans, whether lockup or not, will work with the planned motor. If it has a lockup, then that can be addressed after the power plant is in and running and driving.

The engine bay wiring is darn easy, 80% of it will not be used. You need to trigger the starter, power the fuse block off from the starter, excite the alternator and power the ignition coil. After that all the rest is unneeded to get it running and moving.

From reading your responses I feel your getting to worried unnecessarily. If I’m wrong, then great. If the 4.3 is still in it, then identify every harness connector and label them for future use if needed.

Other than that, swap away with the motor.

Best of luck and look forward to seeing your progress - Jim
 
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