I have a 1989 W150 318 TBI. It starts and idles fine in park. When you drop it in gear it still idles fine but has no power at all. If you give it gas up to even full throttle it doesn't want to move. It will stall or back fire through the throttle body. TPS, hall effect pickup, MAP, fuel filter, EGR and transducer, H2O sensor, catalytic converter, both injectors have all been changed. Fuel pressure is at specs when running. SMEC was changed 2000 miles ago. Timing is set at factory specs. I even disconnected the exhaust after the manifolds and still the same thing. I am about to pull my hair out. The self diagnostic feature shows codes (12) battery disconnected lately, (37) part throttle unlock circuit malfunction (which is not on my truck), (55) fault code completion. I have run out of ideas so any new ideas would be appreciated.

 

Two ideas come to mind. You say that it is timed to factory specs, this is according to the marks on the harmonic balancer and the timing cover? I have had the outer ring on the harmonic balancer and cause the marks to be useless, you may try toying with the timing and see if the problem continues, also how many miles do you have on it? It could be you need new timeing gears and chain. The fact that it is backfiring through the intake tells me it is off time.

 

Thanks 83. It guess my balancer has moved. I set the timing by ear and the truck runs better then it has in years. My timing light is adjustable so I can tell I am now set at 23 BTDC. 10 is factory spec. I will continue to play with it until I get peak performance. Thanks again.

 

My guess is timing chain.