I have a 307 5.0 i rebuil the motor it start up run good when i put it in rear it slow and no power it feel like it get too much air or gas what it can been. everthing esle in it is new.

what does wrong mean,. put in backwards? or too low CCA spec.? or DID HORROR# 1 CONNECTED IT BACK WARDS.LUGS. !!! AND BLEW UP LOTS OF FUSES. OR FAR WORSE.$$$$ ELECTRONICS###
not told does starter motor crank the engine fast or not?. if dead starter there are 6 electrical causes.
no engine start is 50 causes(up to or more) but cranks fast.

P code power train gross errors all.

222- tbw dead. throttle by wire, not good that.
1624 is TBW fails
1417 same caUse , low air supply at TB

cars that fails TBW can start and only drive VERY slow
or can be dead engine, due to unsafe readings from TBW,
the bad parts are at the throttle pedal POTS.
or TB or both' (TB MEANS THROTTLE BODY))
TBW runs 4 pots aka TPS sensors 2 on TB and 2 on pedal (gas as folks say) if the 4 TPS anY disagree engne EFI goes to limphome, mode if the error is extreme
NO START
TPS MEANS THROTTLE POSITION SENSORS.
OR THE TB MOTOR FAILS, OR JUST JUST JAMMED UP A BIT, i CLEAN IT GENTLY
1417 hint that the motor is stuck closed in TB.

check the compression if its low mite miss at slow rpm is the valve set to tight can you feel a little space between the valve stem end and rocker or tapet if not adjust or file down to get .006 thousths
next if carbon crumb is squashed onto the valve seat you may have to put some carb spray into the port exhaust take muffler off spray inside to we tvalve then crank cylinder several time try this is the fuel pump working enough put gas in tank then prim the carb with a spoonful of gas try start the motor repeat if nessesary running now? no spray oil on the governer springs and linkages reve motor while its running by moving governor linkage dose this restore to normal it did my motor its weird

Ok ...is the air filter been changed ...start there first.at idle the engine isn't in need of any big air and will use the surface air around it to run.if there is any air restrictions for air to flow it won't .then that's when engine dies.make sure it's getting enough air
Number one thing of all thing that run on gas at fault is the neglected air filter.a engine that runs on gas needs 3 thing to make it go fuel.spark.and you guest it air!

Possibly a bad or dirty Idle Air Control motor. The engine computer controls the operation of this motor to control idle speed. A pintle on the motor extends and retracts, according to signal from the computer. The pintle controls an air passage below it to give or take away air during idle -when the throttle shaft is closed, no air will get into the engine through the throttle body except through the idle air passage . The IAC motor or valve is on the throttle body, with a 4 wire connector. Take it off and look for carbon or coking deposits around the valve's pintle, and in the air passage beneath it. With the idle valve off, plug the connector back on it, then watch the pintle as the key is turned to on. If there is no movement whatsoever, the motor is probably bad.

I have also heard that a bad coolant temperature sensor can cause such a hard start. If the computer does not know the engine is cold, it won't put enough gas into the cylinders for easy starts. The engine needs more gas to start and run when cold.

Is the check engine light on ?
Does it feel like you have a miss on one cylinder ?
Or could it be a lack of fuel or air at idle ?

You may need to replace the reostat in the heater box. Do you have the hi speed only? Only have off and I, then it is definitely that switch in your box. You can easily replace it.

Air in the fuel line. Take one small wire off the starter relay and turn and hold the key in the start position for a while. Listen for the fuel pump. It will run fast for a second then slow down. When it does, put the wire back on the starter relay and crank the engine.

Your diff could be going out or the transmission is having issues. Does your burb have a Locker in the rear? My 02 had the Locking Differential and i blew two of them up; a mile from home it busted so i put it in 4wheel and drove back on the front axle, the rear was spinning freely then it locked up and i came to a very quick stop- any sounds while taking tight turns?

Their is a blower motor resistor and a blower motor relay under your hood one of these are out causeing the problem!

The transmission is this truck is a 4L60E. It is electronicly controled through several sensors. It will sense the load on the motor mainly through the TPS (throttle position sensor).It is not uncommon for you to have to really press the gas to make it down shift. Alot of the feeling depends on gear ratio in the rear end. The 4.3 is a very good engine, but it combined with a high gear rear end and overdrive may really seem under powered. You can put a programmer on it to change all the shift and power feel.If you are not having a hesitation issue then it will most likley not be the tps. You may just need a fuel and air filter, not knowing when it was last changed. The programmer is the best way to get the feeling you may be used to. Good Luck