2.8L s-10 motor with 4.3L throttle body

Hello I have been running a 4.3L throttle body and bored out intake manifold. My question is that I am getting somewhat poor performance at take off and anything below 2200rpm.....its like the engine is leaning out. When I take off I can rev it a little bit, let out the clutch and step on the gas and it will fall flat on its face for a split second then somewhat take off. Even while I am cruising I can feel a slight hesitation like its trying to go in the lower gears. Can a worn out throttle body shaft cause this??Not sure how much slop is acceptable. I know I can wiggle the shave up and down and feel play. How bad do these things have to be before they cause problems. I have changed fuel filter and taken injectors out, hooked them up and cleaned them out. so that must not be it. also I do have a fuel pressure guage. doesnt seem to drop. mods are electric cooling fan, msd ignition and coil, k and n air filter. This thing just doesnt seem to have the response it should at low rpms and speed. when I first put the 4.3L throttle body on and bored out the intake, it really had power, now it noticebly has less


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is this a recent thing? or has it been doing it since you put the bigger TB on there? how long has it been since a tune up (i.e. plugs, wires, fuel filter, air filter...)? Is it throwing any codes? Have you checked the fuel pressure? could be a bad regulator.

From how you explain it, it kind of sounds like you have a vacuum leak.

The throttle body probably wont make alot of difference either way. If yuor EFI is working properly the computer is only going to inject the the required amount of fuel. First thing I would do is check fuel pressure. approx 9 to 13 psi for TBI injection. If its ok idling you need to drive it with a gauge and see if pressure falls off. You engine will run on 7 psi or less but it will be a dog. If thats ok check for codes, and repost.

Since this is a wet system (TBI), intake velocity still has an effect. Not as much as it would with a carb, but some nonetheless. In other words, the hole in the 4.3L TB is to big. At low speeds the velocity in the intake is to slow, which means the cylinders don't fill as fast as they would with a smaller opening and higher velocity. That's why you get a bog at higher speeds too -- it takes just a split second to get the air moving when you open the throttle. Bigger isn't always better! The 2.8L might handle a bigger TB easy enough, but obviously the 4.3L TB is to big. I'd check the size on a 4.1L (4100) Caddy V-8 and a 3.4L V-6.

If you put that on a stock 2.8 you now have entirely TOO of a TBI.

If you did, try putting the 2.8 injectors in and see if things improve.

The bored intake and 4.3 TBI is a great mod when you put the 3.4 crate motor in. If you still have the 2.8 and put a performance cam in, it still may be too big. Do you have any exhaust upgrades?

well that is the first time I have heard of only putting a 4.3L on a 3.4 or bigger or the 4100 throttle body on a 2.8. I have only heard of positive things about putting 4.3L throttle body on a 2.8.
For the first few months it ran absolutely great. maybe as much as 5 months. Yes I have the walker advantage or is it flowmaster muffler and I made the tail pipe 2 and 1/4 also. I just cant figure out the slight chugging at cruising speeds below 45 or so with rpm below 2000. As far as the bog on take off..........I may have reduced that a lot.......by putting on the 4.3L air cleaner(and hooking up the exaust heat riser hose and vaccuum lines that operate the hot air valve in the air cleaner.......that helped a lot.
All the stuff I have read suggest better throttle response....and I think they are right. I just dont understand performance issues expecially fuel injection........why would this thing peel the tires when I first put on the 4.3L throttle body with an automatic(now I have the 5 speed manual.) did the computer figure out what I did and compensate........seems like every time I screw with it and it runs better,.......the computer figures it out

how did you retune the computer ?

if its the stock computer still ... theres your problem ....

these engines dont have a mass air flow sensor, so theres nothing to tell the computer that the engine is getting more air, except for manifold air pressure, and the o2 sensor

the "simple" way to compensate for the larger throttle body is an adjustable fuel pressure reg

the slightly more involved way is to see if an "safc" unit can be adapted to that car, and then use that to retune the fuel curve ... but im not sure how well that will work for ya ..

i know safc's work great on MAF cars ... because maf's are a direct measurement of the incoming air

well I do have the 2.8L injectors in the throttle body because it started throwing a rich code. didnt do it right away, took a while, but dont remember how many days weeks or months, but anyway once I put in the 2.8L injectors back in, the code stopped. Now if I let it warm up a little with the 4.3L injectors in it the code light would not come on. 2.8L inj in it now. Now I have 2.8L injectors in it. I did notice that even before I did the 4.3 throttle mod, the cruise control would surge at 35 on up, also the slight chugging at 35ish .....so maybe we do have a vaccum leak somewhere. Lately its even started doing the idle up and down just after a cold start too. As far as plugs, wires, they are new. Cap and rotor are somewhat new also. Fuel pressure is bumped up the cheating way......I clamped off the return hose a little with washers to see if raising fuel pressure would help with 2.8L injectors........and it did.

There is another way -- an adjustable MAP sensor. Try a google search. Basically it's a finely adjustable power supply inserted into the MAP power line. The MAP usually takes around 5V, but by SLIGHTLY increasing the voltage to the MAP, it will increase fuel supply a slight amount. This isn't a substitute for bigger injectors, but a fine tuning tool.

For example, on the Jeep 4.0L MPI engine, bored 0.030" over and using a 258 (4.2L, 0.44" longer stroke) crank, you determine that you need 24.6# injectors. No one makes them -- it's 24# or 26#. Most of us opt for the 24# then increase the voltage to the MAP to around 5.15V. That changes the reference voltage of the MAP and increases fuel delivery slightly -- across the entire fuel map in the computer. 26# would run slightly rich.

I don't think I'd go over 6.0V, and I don't know anyone who's running at more than 5.5V. I'm also not sure the engine will run if the voltage is below 4.5V -- the Jeep computer seems to sense a low voltage as a faulty MAP and shuts the engine off, but doesn't care about a higher voltage. Or maybe it's just turning it down that low is decreasing fuel to the point it won't run, but I doubt it.

You did mention the five speed in place of the auto, and chugging "under 2000 rpm". Try down shifting. The little engine isn't making enough power to cruise in fifth at 2000 rpm. If you get a vacuum gauge, you can see as you drive where the engine makes power. Basically you want to hold speed while producing the most vacuum for the best cruising speed and economy. With the auto trans it was turning a bit more rpm.

My 4.6L six in a 3700-3800 pound car doesn't make good power under 2000 rpm with an OD auto trans (0.70:1 OD ratio, 3.55 rear axle). According to my vac gauge, the engine will pull up to 25" of vacuum. Cruising at higher speed it pulls about 20" in OD, around 65-65 mph. If I increase to 70-75 mph, it will drop to 15". Accelerate or hit a hill, and it drops down to 10". At least in theory, the best economy is achieved at the highest vacuum reading. That means the throttle plate is closed more, or I'm using less throttle to maintain speed. If I drop below 1800 rpm on a level in OD, the vacuum drops considerably just maintaining speed or at the slightest hill. I can only maintain 15" or so of vacuum and have to drop to 10" on the slightest hill. That's around 55 mph. Out of OD I can maintain 50-55 mph without going over 2000 rpm and maintaining 20" of vacuum, and without falling under 15" just to maintain speed on an incline. With this in mind, I can safely determine that my torque peak is very close to 2000 rpm. You need to keep the engine near the torque peak for best power and efficiency, running to low rpm will actually decrease power and mileage.

I first ran 3.08 gears with the same car/engine/trans combo. The big in-line six had plenty of power to pull that ratio in OD, even down to 1500 rpm (it does now, of course), but vacuum was way down. The engine wasn't straining, but would "chug" a little on takeoff at that low rpm. So I installed the vacuum gauge, and then changed to the lower rear gear ratio (3.08 to 3.55), and stopped trying to run a steady speed under 2000 rpm. I used to shift around 1500-1800 rpm, but now wait until 2000-2200. Fuel mileage has improved noticeably, and the engine is much more responsive. Your little 2.8L V-6 probably peaks closer to 2500-3000 rpm. So try increasing rpm at cruise, and shifting at a higher rpm. With the OD five speed you may need a lower geared rear end. Most old cars with manual OD transmissions used as low as 4.11 rear axle ratios. This car originaly had a three speed OD trans that I ran with the 4.0L and a 3.31 rear axle. That combo worked great -- though it feels a lot better with the 3.55. I was looking for 3.31 gears when I ran across a good used set of 3.55 gears... close enough!

yes I see you have multiple points to consider..........bottom line this thing was slightly chugging and cruise was surging at 35 on up to 40..or even with cruise off I could tell a little bit of chugging with automatic.....its like someone is ever so slightly manipulating throttle and I think it happens above 2000 rpm also as well as below..........so something is causing this because it didnt used to do this...........gee maybe I wonder if timing has anything to do with it.........timing too far advanced in combination with msd ignition box and coil triggering knock sensor..........lol.........dont know jack squat about tuning an engine........wish I did.............no I dont think there are any codes........will have to put fuel pressue guage back on and see about fuel pressure

I had the auto with a 3.42 rear end in it so I swapped out the 5 speed first and then a while later I put in the 3.08 because I wanted better gas mileage and I hated the low first gear and 35 mph meant high 2's or 4th below 2 grand and I hated that, now I can go 35 and the engine speed I like......now I shift into 4th at 40 to 45. Yes I think the engine is leaning out at low rpms because it idles up and down after the rpm dropps from high cold start idle. The heat riser and hose put back on the air cleaner really helps the bog, its not nearly as bad................dont know if thats because of the heat or because it makes the engine run richer because it has to suck all the air throught the small heatriser hose...............I think the map idea is a good one. I have bought some of the necessary parts for the map....but cant find the terminal strip at the radio shacks.......so I will either have to solder it all together or come up with something else.......but not a big deal since I know electronics...........I still have to wonder what a loose throttle shaft will do. I have bearings that will replace the bushings and will work well because they are sealed. could the loose shaft cause the idle up and down thing? or maybe when you let off the gas and push the clutch and come to a stop sign, the idle will dip and come back up

The loose shaft can cause idle problems due to air coming in at the shaft. The IAC shold compensate for it, but pressure on the shaft and varying vacuum inside the TB can be causing the amount of air "leaked" in at the saft to vary.

If you're cruising under 2000 rpm I still think that's your main problem. Less rpm doesn't always mean greater mileage. I get better highway mileage with the 3.55 gears than I did with 3.08 gears in my torquey big six, though I think 3.31 would be the ideal compromise. I'm going to go up a tire size in the rear, that will drop me 200 more rpm at cruising speed.

This has already been covered in little bits in several of these posts, so forgive my repeating some of it; I just wanted to consolidate it into one answer for him.

The problem is this: the stock computer uses mostly MAP and TPS to determine fuel trim based on a stock fuel plot. On a stock TB/stock motor, a certain amount of throttle position ingests a certain amount of air based on its 4.3L of displacement, cam, heads, etc. Deviations from that are detected by the MAP and other sensors and adjusted to compensate. Since you have now put that on a 2.8 with 2.8 sized injectors the computer thinks that its injecting 19 lbs/hr of fuel when its actually only injecting 13 lbs/hr. And for the same amount of throttle opening, its not injesting nearly as much air. The PCM must be completely remapped from scratch. Not just a little tune here and there; a complete re-do. I'm sure with as common as this swap is, several people already sell custom chips for this swap.

Basically how it works is a stock PCM is given a road map that plots out one highway. Small changes like altitude, temperature, quality of fuel, etc are like getting off the highway at an exit for lunch. You can still see the highway and you're not lost. In that situation, the sensors allow for a little deviation from the highway itself. The problem is you have now gotten off I-10 and now you're on I-55. You're map can't find your trim. You need a new map.

Often times we LT1 guys switch from our stock 48mm TBs to 52 or 58mm and we have to reflash our computers. The problem is at 10% throttle opening, we're letting a LOT more air through than before at 10%, The result is the MAP sensor and MAF sensor read tons of air at only a tiny TPS signal and its off the map.

May does that last post REALLY explain it all.......makes sense........now since I need to re do this computer............and I am sure I couldnt do the reflashing or remapping myself........then what or who would I ask to do this?
Is this an inexpensive mod?.....I already have the parts for the map sensor power supply mod.............but if the computer needs done anyway then I would thing that should be done first....I will fix the throttle body this weekend, there wont be any vaccum leaks after I am done