But backing up for a moment, a 4x4 Suburban with 350 is going to be working the engine pretty hard, especially when towing. Such a machine would have been better with at least a 383 on a small block, or better yet a 454, if you really already have a 4L80E gear box, that could be a good solution.
10 to 1 compression is too high for anything besides aluminum heads. Combine that with a Pete Jackson gear dive on the cam leaves very surprised the detonation sensor doesn't shut the ignition off, which makes me wonder if the sensor is there. If it's gone, the engine can easily be operating into the detonation zone which costs a lot of power before parts actually start coming out.
You can't change the timing because the computer is heading your efforts off at the pass.
You need to find out for sure what cam is in there, TBI isn't too keen on much cam timing and certainly doesn't like short LSAs. TBI sensors need good manifold vacuum, cams that diminish vacuum result in a situation where large changes in engine fuel requirements occur in a narrow vacuum band. however, TBI sensors are designed around a situation where small changes in engine fuel requirements occur around large changes in manifold vacuum. So changing the cam timing quickly gets the relationships of throttle position to manifold vacuum (what's called Manifold Absolute Pressure or MAP) out of coordination with the fuel and ignition advance maps. The only solution is a custom chip with programming specific to the engine modifications. The computer issues are also exacerbated by other changes that modify the engine's volumetric efficiency such as changes to the head and the exhaust system. These alone most TBI systems can accommodate in closed loop thru the Learn function, but this function at its absolute best can only accommodate about a 10% change to mass flow thru the engine; when these changes are combined with a different cam, the computer just can't deal with the changes using its Learn Function. On top of that Learn only can occur with an active O2 sensor when the injection is in Closed Loop. If it's prevented from entering Closed Loop because of a low temp thermostat or a disconnected O2 sensor, it will never even get into the Learn Function.
If you don't need to smog going back to a carb with HEI and a 700R4 tranny would be simpler, but the instruments on the panel are electronic and run from sensors in the tranny thru the computer. There are black boxes available to convert electronic signals from one type sensor to what the speedo needs, but this stuff gets pricey, but its doable.
If the knock sensor is bad or inoperative for whatever reason, the ECM should have gone to a retarded ignition default setting. This sure would kill the bottom end.
Water injection will not help w/a rich condition (helps suppress detonation), which also leads me to believe the ECM has gone to a default setting AFA the fuel/air mixture goes, as well. Maybe not in the "limp home" mode, but not a normal map, either.
The camshaft is likely the culprit here. You'd probably see better performance from a bone-stock OEM TBI cam than what you have now, IMO.
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