4.3 TBI to 350 carb swap/ecm?

69hurstolds

Geezer
Supporting Member
Jan 2, 2006
7,047
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If you don't know how you're going to do the engine swap yet, and have to "google it", then no way in heck you're going to convert the existing ECM to work with your 350 carb engine without some serious help. I agree with 64nailhead on this. Keep it simple. Go get your 350, an HEI distributor- a non-computer controlled one with vacuum advance. A TH350 would work if you have a 3 speed 200 trans already. If you have the 200-4R and want to keep it, they make stand alone lockup kits for these I believe but not sure. You can use a regulator to the carb and let the fuel circulate with the original pump. Not sure on that, but that's probably how I'd approach it.

Regardless, the swap would be much easier to do without an ECM. Still a PITA as far as I'm concerned, but that's just me. Also, get you a good, non-CCC Quadrajet carb, either 750 or 800 cfm (not that you'd ever use all that anyway) for it if you're going to leave it stock and not use the ECM. Buying a garbage E-brock AVS or Holley for a stock setup is a big waste of money, IMO. Unless you're going to use the Holley as intended as a performance carb. Those are not uber-friendly street carbs. Tuneability of the Quadrajet will do everything you want on the street and you can get them cheap.
 
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Hamrtym2

Not-quite-so-new-guy
Thread starter
Jun 29, 2022
6
3
Iowa City, IA
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.
So what do I do then to eliminate that ecm? I have no clue exactly what should be done for the trans to work right and of course ac and cruise and speedo etc. i know I want carb. But I wondered if leaving the ecm in there but run carb would be fine but from your description it sounds like the ecm is going to always cause issues and maybe not run my fuel puMp like it should. The car doesn’t show an overdrive circle around D either btw. So then it didn’t have the 200r4. I still have the r code sticker in the trunk if I wanted to see if the 350 was factory or not. But of course I’m more concerned with trying to find out what I’m needing to do to eliminate that Ecm. Smog pump and EGR are already bypassed/not hooked up.
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
5,124
113
Upstate NY
I should've mentioned this previously, if you ever intend to LS swap this, then now would be the perfect time. The issue I have from your responses, is that it's somewhat of a steeper learning curve and it could be more money upfront. But if you have a known good running 350, those are not as easy to find as they used to be, and are planning to buy a crate SBC or have one rebuilt, then the money your using for that could very easily offset the expense of an LS swap.

Where are you located?
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
5,124
113
Upstate NY
So what do I do then to eliminate that ecm? A set of dykes and some electrical tape. I have no clue exactly what should be done for the trans to work right and of course ac and cruise and speedo etc. Cruise control will be difficult and possibly quite expensive i know I want carb. But I wondered if leaving the ecm in there but run carb would be fine but from your description it sounds like the ecm is going to always cause issues and maybe not run my fuel puMp like it should. The fuel pump can easily be ran from a standard 4 pin relay that is triggered by the power to the ignition coil or from an oil pressure sensor/switch.The car doesn’t show an overdrive circle around D either btw. So then it didn’t have the 200r4. I still have the r code sticker in the trunk if I wanted to see if the 350 was factory or not. You should be able to use your current flex plate, torque converter and trans - if it's a TH350 with lockup or without, then you only need to transfer the hard parts I just mentioned. If you get the correct motor mounts, then the driveshaft length will not need to be adjusted. But of course I’m more concerned with trying to find out what I’m needing to do to eliminate that Ecm. Smog pump and EGR are already bypassed/not hooked up.
I was typing the same time you were.
 

Hamrtym2

Not-quite-so-new-guy
Thread starter
Jun 29, 2022
6
3
Iowa City, IA
I should've mentioned this previously, if you ever intend to LS swap this, then now would be the perfect time. The issue I have from your responses, is that it's somewhat of a steeper learning curve and it could be more money upfront. But if you have a known good running 350, those are not as easy to find as they used to be, and are planning to buy a crate SBC or have one rebuilt, then the money your using for that could very easily offset the expense of an LS swap.

Where are you located?
I’m in Iowa city iowa. I’m not doing the swap alone I see a lot of guys mentioning that I sound green with the wiring which is true. However, it’s not me putting it in. I’ve done engine replacements before but I haven’t swapped to a v8 before so my buddy that does a lot of it is going to do it. He’s usually more into the square body’s. I also see a lot of guys saying if I “plan to swap to a 350 trans” when it’s got one in it. I was told years ago from a shop it had a 350 trans. They tried telling me it was too big for the v6. I crawled under it the other day and based off the pan shape and size and the amount of bolts in the pan it matches up with a 350. So I’m thinking it already has a th350 in it. But yes basically all my questions have to do with the ecm. Running carb I won’t need it to control my engine that much I know. But of course I don’t want the fuel pump to stop or anything from the ecm not reading right or not being there at all. Gauges I’m going aftermarket for the temp, oil pressure, and voltage. The only fsctory gauges this car has is fuel and speedo which I want those 2 to still work. Some guys are saying the trans will shift normal just fine even if the lock up isn’t hooked up. Which would be great cuz that tells me it is not an issue to address, (correct me if I’m wrong). Cruise and ac I want to work. I’m not ignoring anyones responses when they say the ecm is not a BCM I’m just trying to gather all the information I can and learn more about an early ECM set up like this so I know what to do with that ecm in the kick panel and still have things work. As for getting a GM service manual I’m not sure where to buy one. I tried GM directly and they say I have to get it at the first dealer the car sold at. Which was what? 35 years ago? So good luck with that.
 

Hamrtym2

Not-quite-so-new-guy
Thread starter
Jun 29, 2022
6
3
Iowa City, IA
I’m in Iowa city iowa. I’m not doing the swap alone I see a lot of guys mentioning that I sound green with the wiring which is true. However, it’s not me putting it in. I’ve done engine replacements before but I haven’t swapped to a v8 before so my buddy that does a lot of it is going to do it. He’s usually more into the square body’s. I also see a lot of guys saying if I “plan to swap to a 350 trans” when it’s got one in it. I was told years ago from a shop it had a 350 trans. They tried telling me it was too big for the v6. I crawled under it the other day and based off the pan shape and size and the amount of bolts in the pan it matches up with a 350. So I’m thinking it already has a th350 in it. But yes basically all my questions have to do with the ecm. Running carb I won’t need it to control my engine that much I know. But of course I don’t want the fuel pump to stop or anything from the ecm not reading right or not being there at all. Gauges I’m going aftermarket for the temp, oil pressure, and voltage. The only fsctory gauges this car has is fuel and speedo which I want those 2 to still work. Some guys are saying the trans will shift normal just fine even if the lock up isn’t hooked up. Which would be great cuz that tells me it is not an issue to address, (correct me if I’m wrong). Cruise and ac I want to work. I’m not ignoring anyones responses when they say the ecm is not a BCM I’m just trying to gather all the information I can and learn more about an early ECM set up like this so I know what to do with that ecm in the kick panel and still have things work. As for getting a GM service manual I’m not sure where to buy one. I tried GM directly and they say I have to get it at the first dealer the car sold at. Which was what? 35 years ago? So good luck with that.
I see you answered a few things when I was typing. For the FP relay, the car is equipped with one on the passenger fender well
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
5,124
113
Upstate NY
I’m in Iowa city iowa. I’m not doing the swap alone I see a lot of guys mentioning that I sound green with the wiring which is true. However, it’s not me putting it in. I’ve done engine replacements before but I haven’t swapped to a v8 before so my buddy that does a lot of it is going to do it. He’s usually more into the square body’s. I also see a lot of guys saying if I “plan to swap to a 350 trans” when it’s got one in it. I was told years ago from a shop it had a 350 trans. They tried telling me it was too big for the v6. I crawled under it the other day and based off the pan shape and size and the amount of bolts in the pan it matches up with a 350. So I’m thinking it already has a th350 in it. But yes basically all my questions have to do with the ecm. Running carb I won’t need it to control my engine that much I know. But of course I don’t want the fuel pump to stop or anything from the ecm not reading right or not being there at all. Gauges I’m going aftermarket for the temp, oil pressure, and voltage. The only fsctory gauges this car has is fuel and speedo which I want those 2 to still work. Some guys are saying the trans will shift normal just fine even if the lock up isn’t hooked up. Which would be great cuz that tells me it is not an issue to address, (correct me if I’m wrong). Cruise and ac I want to work. I’m not ignoring anyones responses when they say the ecm is not a BCM I’m just trying to gather all the information I can and learn more about an early ECM set up like this so I know what to do with that ecm in the kick panel and still have things work. As for getting a GM service manual I’m not sure where to buy one. I tried GM directly and they say I have to get it at the first dealer the car sold at. Which was what? 35 years ago? So good luck with that.
Sounds good. The AC is very simple to have work. I can't speak from experience with an '87, but an '84 and '85 is very easy to have the AC working. I believe you're going to need different hoses due to the different location in the engine bay of the AC compressor. But that is not hard to overcome.

Regarding the cruise, I've never kept the cruise control and I have no idea if how to make this work with a conversion. So I will defer to others on that one. But it won't be as simple as the remainder of the wirng/conversion.
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
5,124
113
Upstate NY
I see you answered a few things when I was typing. For the FP relay, the car is equipped with one on the passenger fender well
You can use that relay, but you're going to have to trigger it differently than how it was originally. Very very easy to overcome because a carb doesn't need to prime the injection system to start. It only need to run when the motor is running and maybe cranking.
 
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69hurstolds

Geezer
Supporting Member
Jan 2, 2006
7,047
113
Did you bother to look for a service manual? If you don't want the big, fat paper copy which is pricey IMO, you can get the CD version for 30 bucks. Worth its weight in gold.

 
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78Delta88

Master Mechanic
Supporting Member
May 23, 2022
417
43
SW Arizona
Pic of 350 going into 87 Caprice. Replaced the TBI 305 with 1970 350 4 bolt main , EBrock Performer & 650 carb, HEI from pre-81. Trans is th350 lock up, but not hooked up. 2300 stall TQ Converter. Header plugs, 9mm under header wires. Flat tappet hyd cam Summit 1108 grind. 305 HO heads 58CC 1.94 intake / 1.50 exhaust.
 

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