1994 jimmy 4.3 vortec. revs and idles perfect in park. start driving down road and engine surges almost cuts out. even tried putting under load with foot on brake gave it gas, no misses till driving down...
This sounds like a dirty fuel filter to me hence the load has no effect till driving as the fuel supply will be proportional to the revolutions of the engine whether it's fuel injection or not.When you are driving you are obviously using more fuel so the demand is greater for fuel and if the fuel line is blocked in any way it will do what it is doing.Although there might also be a blockage in another section of the fuel line and if yours is a carburettor model then the fuel jets or air jets in the carb might have dirt in them.Plus fuel pump might be on it's way out due to wear.If fuel injection model, then your fuel distributor may also have dirt or gunge build up, or fuel injection pump is not functioning as it shouild due to age or dirt.If none of these solutions are correct then you will have to start looking at the ignition line, Faulty module, distributor etc.
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