snoman1214

PROBLEM: Motor cuts off at 4200 rpm - fuel cuts off like a rev limiter would at higher rpm. As soon as the rpm drops below this it picks right back up (instantly).

Ok here is the deal we have a 94 S-10, Had a 4.3L V6, installed a rebuilt 305 TBI motor bought from a machine shop - used the original 4.3L harness and computer. Truck had over 300,000 miles on it before motor swap but has rebuilt tranny (700R4E).

Parts replaced or bought new so far: Fuel pump, fuel filter, distributor and coil, plugs and wires, changed computer, rebuilt TBI with 4.3l fuel injectors, checked vacuum lines.

Motor runs GOOD just cuts off at 4200 rpm - it does this with either computer installed.

Has anyone ran into this before? Any idea's are appreciated. Thanks

ZONES89RS

iTrader: (25)

Well, if it is the factory computer, maybe it is registering more RPMs since it is a V8 and activating a rev limiter?

9000th01ss

iTrader: (10)

Whats your fuel pressure? It's very important on TBI trucks.

I don't see how a V6 computer could even run a V8, and why the 4.3 injectors?

I have Moates APU1 tuning/chip burning hardware and TunerPro software, I've seen the programs and burnt chips. You need some tuning equipment and a wideband or you're going to be running in circles getting nowhere.

snoman1214

We have the factory 305 computer as well but it runs the truck worse than the 4.3 one does. (The motor runs very rough with the 305 comp and the transmission shifts weird too) The guy who rebuilt the TBI told us that the 4.3 injectors are better for a 305 and are actually almost the same. He also said that putting 4.3 injectors in 305 is very common swap when rebuilding a 305 TBI due to the better spray pattern of the 4.3 injectors. As for the pressure I dont know the exact # but it has a new V8 fuel pump, filter, and regulator. We believe we have eliminated a pressure issue at this point. Again the truck runs great until you romp on it and it shuts off at 4,200 RPM. The combo of the V6 stuff and 305 stuff seems to be great if we could just fix the shut off problem.

You are correct about the software, it would be nice to have but is not in the budget at this time. If the computer needs flashed I will have to find someone to send it to.

I dont know if this is computer related though because the 4.3 and the 305 computer both shut the engine off at the same RPM. It has to be something small that we have overlooked since both computers do the same thing. Someone on another forum suggested that it may be getting a signal from the trans or gear selector that it is in nuetral and setting the rev limiter at 4,200 thinking that it is in nuetral. He said that GM set the rev limiter way down for nuetral as apposed to drive. We do have an aftermarket B&M shifter since the 305 did not allow use of the factory column shifter. Anyone here ever heard this before?

9000th01ss

iTrader: (10)

It has nothing to do with the shifter.
Theres nothing I can think of to do without at the minimum a wideband.

And the fuel pressure, you can replace stock parts with stock parts and end up with the same pressure. All stock TBI pumps are the same except for the '95 454 pump.
You can put in a high pressure pump and have the same pressure because of the regulator.
You can put in an adjustable regulator and stock pump and it'll be a waste of time because the the stock pump only goes up to 15 psi if you're lucky.

You need to start with a fuel pressure check, then get an adjustable regulator. Stock regulators hardly put out enough pressure for stock engines.
Start here: http://www.cfm-tech.com/gm_tbi_fuel_pressure.htm

What about ignition timing?
Cam?
Exhaust?
Speedo reading right? Not sure on the S10 but the 1/2 ton trucks have a seperate speed sensor buffer that is specific to gear ratio and tire size, the wrong one could read higher mph and signal the ECM to cut fuel.

snoman1214

You are correct the shifter had nothing to do with it. We just checked that a few minutes ago and nothing. Honestley I dont think that the fuel pressure is the problem I know alot of people keep going back to that but something electronic is shutting the fuel off completely at 4,200 RPM. It doesent matter if it is in park or driving down the road. We have watched the fuel from the TBI and the fuel flow doesent slow down it STOP'S at 4,200 RPM. We are starting to think that the ECM is going to have to be flashed. It seems like something is giving it a false signal that it is running more RPM's than it actually is. Is it the computer? We dont know because two of them are doing the same thing. But if you turn the REV limiter off it might take care of it.

9000th01ss

iTrader: (10)

You need a chip, there is nothing to be flashed. Heres what I use on my TBI truck. http://www.moates.net/index.php?cPath=25_36