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Discussion Starter · #21 ·
Another update.
And no I didn't buy another truck :D but yes there is another trooper at the house.
Not mine though. There was a fella up the road from me that wants the 2.8 I got from Choppy to replace his 2.8.
I tested his first and had no compression on two cylinders so at least Choppy's heart now has a home.
I'm half way done with the swap and good thing too, I'll treat this as a practice run on the black LS.
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Someone used stop-leak in the white trooper so I took both my black LS radiator and his to a radiator guy to get cleaned out and repaired.
Both are original copper rads, not the slightly thinner new plastic aluminum rads that you get aftermarket.

I did also find a 3.1 computer! @Ed Mc. didn't you say that I needed the 3.1 ? Hope those numbers match and are legit!
One more question Ed. How many teeth and how large is the 3.1 flywheel? I remember you saying to run the 3.1 or can I run the 3.4 flywheel?
Rockauto is showing the same flywheels for the 2.8 as the 3.1 so I'm stumped here.
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@geoffinbc I sent you a msg on getting the intake bored out. Do you need just the upper intake or the tbi bottom half? Help me out and I'll get that knocked out.

I also grabbed some whole knuckle outer hub assemblies from a Rodeo for the Big Brake conversion. What's just as important as going? Stopping!!
Not positive I will run those junkyard rotors though. For the money, I think a complete front end rotor and caliper package from rockauto is going to look real good and stop real good too for the coin.
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As for the white and gold/bronze 88 Trooper. It is almost done. I sent the pwr steering pump off to rebuilt only to get denied and now need to find a replacement for it, pulley and all.
It seems the 88 is a cutover year where Isuzu switched to a spline drive pulley and cardone won't rebuild mine for whatever reason. So I'm hoping to switch to the new style with a spline pulley.
Last tidbit on the 88 I did was upgrade the front disc and caliper to the dynamicFriction special rockauto has right now. Ceramic pads too. Repacked the bearings and am going to put gold painted Aisins on it. Need to put the new clutch line in place with a new master and slave. Seems she bled out when I was on vacation. Once that is done, then she is for sale. I'll post a pic of the beauty when I'm done.
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last little teaser.... I have an awesome surprise coming out of Atlanta for wheels. Just you wait!
I will get on the Black LS here soon!
 

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I used a 1991 pickup 3.1 flywheel (same part no as 2.8, so either/or is fine) and 3.1 clutch kit on my 3.4 (just checked my rockauto receipt from 2015).

Regarding ECUs and injectors in a 3.4:

2.8 ECU + 2.8 injectors = fine at idle, lean at WOT
3.1 ECU + 2.8 injectors = fine at idle, may need fuel pressure turnt up to keep up at WOT (heavier spring for an additional psi is available)

2.8 ECU + 4.3 injectors = can be rich at idle, fine at WOT
3.1 ECU + 4.3 injectors = likely to overfuel at idle and WOT

It will likely depend on how hopped up the motor is. An untouched essentially stock 3.4 may do fine with just the 3.1 ECU and the 2.8 injectors. Cam upgrade and some other stuff may require adjustment or jump to the 4.3 injectors + 2.8 ECU combo plus additional fine tuning.
 

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What Paul said, if you order the flywheel and clutch kit for a 1991 3.1 4WD pickup, you can't go wrong. The 3.4 should run better with the 3.1 ECU. I've been running my 3.4 LS for years with the OE 2.8 ECU and a Holley Big-Bore TBI, which does pretty much the same as a GM 4.3 TBI. Haven't had any big issues with it, although I do have a spare 3.1 ECU ready to go if need be.

Each swap is gonna be a bit different on tune, depends on what you do for exhaust, how you build the engine. As Paul was saying a heavier spring (14 psi spring) in the fuel pressure regulator will fix most ills with the swap. If you use 2.8 injectors you're definitely gonna have to increase fuel pressure; a 3.4 on 2.8 injectors will "lean surge" when cold. Having more fuel pressure/fuel injected will cure the lean surge, and when the ECU goes into Closed Loop it's regulating with input from the O2 sensor and should be OK except as Paul mentioned might be lean at WOT.

Conversely, you may run rich with the 4.3 TBI and bored-out intake, depending on the build. In that case you might need to reduce fuel pressure. Just a tuning thing. The stock regulator can be made adjustable within a few-psi range, and that could be enough to make the engine happy. I tweaked on the FPR when I did a "3.2 Stroker" build back in 2005 and it was just right, killed the lean surge and had good fueling all-around.

Use the stock 2.8 intake manifold, that's what you'd need to ship off to have bored to fit a larger TBI. The best mod (besides exhaust) that you can do to these engines. They respond well to increased airflow.

You've been busy, have fun!..........edc
 

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One other thing that will likely help in any condition is swapping to a heated o2 sensor. Geoff had a writeup on his site. I know mine will not go into closed loop at idle no matter how long I let it sit, even with the o2 sensor sitting in the cat directly where the 2 exhaust downpipes enter. I have to actually drive it for that thing to heat up enough to go into closed loop. If I end up keeping my motor longer term it will definitely be on the to do list.
 

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Discussion Starter · #25 ·
Both you guys are super immensely helpful. Thank you.
So after your message @Ed Mc. I looked around some yards and there are tons of Astro 4.3 TBs.
I got one for $50 complete.
I think what you were saying too is the throttle plate on that ebay one looks identical or similar to the 2.8.
Well check this out. yeah its dirty but little elbow grease and she's new again.
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Duder where's another Astro TB?? I am thinking of swapping back to the GM TB and huckin' the Holley and it's sticky injectors.
 

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Paul, maybe that's the problem with mine. On rare occasion it gets cranky, won't idle well, seems like it's running on half power. If I rev it up, the problem clears. I can't think of anything else but what you describe as "sticky injector" would do that. I drove it this week and it ran perfectly. But the time before, after the engine had already been fully warmed, it did it once. Revved up to get going, problem went away, didn't come back. Very strange indeed.

I think we may have covered this a while back, but you can modify the Holley TBI base by drilling a hole for an offset air cleaner lid hold-down stud. Then install a GM injector pod. Plus rewire the injector plugs. You'll lose the adjustable fuel pressure regulator, but you can always modify the GM F.P.R. to be adjustable.

Here are some informative links that I bookmarked a while back:


 

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Paul, maybe that's the problem with mine. On rare occasion it gets cranky, won't idle well, seems like it's running on half power. If I rev it up, the problem clears. I can't think of anything else but what you describe as "sticky injector" would do that. I drove it this week and it ran perfectly. But the time before, after the engine had already been fully warmed, it did it once. Revved up to get going, problem went away, didn't come back. Very strange indeed.

I think we may have covered this a while back, but you can modify the Holley TBI base by drilling a hole for an offset air cleaner lid hold-down stud. Then install a GM injector pod. Plus rewire the injector plugs. You'll lose the adjustable fuel pressure regulator, but you can always modify the GM F.P.R. to be adjustable.

Here are some informative links that I bookmarked a while back:


Yeah Ed, it may be the injectors. My holley was new old stock and sitting for probably 20 years, so the injectors were gummed slam shut when I first got it, and it took 2 trips to a nearby injector shop to get them cleaned up (they did ultrasonic cleaning). And I don't drive it super often, so it's entirely possible that they got gunky again. My problem, I think, is that it overfuels, so I think they are sticking open. Could be my tune, could be something else out of order.
 
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