I have a 2004 chevy z71 with the 5.3 and if u lay it on the floor on take off it lugs so bad you can out run it on foot for about 30 yards! i checked the fuel pressure and its about 52 so whats wrong?

Is your check engine light on? I would think it would be... When was the last time you changed your fuel filter? When at idle the fuel pressure will most times be at the range you described but when under a load and a dirty fuel filter it could cause the problems you have described.

Symptoms of a dirty fuel filter won't disappear at higher speeds. What's it like on the highway? What speed does the engine spin when going 55?

Fuel pressure key on engine off is supposed to be 55-60. And with the key off its supposed to hold above 50psi for 10 min. If timing is adjustable check that but i dont think yours is.

Also does it only take off sluggish if you floor it? If that is the case I would put everything I have on it being an excessive amount of back pressure in your exhaust. bent pipe or a clogged catalytic converter.... doubtful but also check the air filter. Either way it would be exactly like trying to sprint but only breathing through a straw while doing it. A normal walk would be fine with a straw, but a sprint demands more air and without air you will fall on your face.

You need to check your High Air Flow Mass Sensor. If engine stays running when you unplug sensor. Then it's bad. This sensor controls how your throttle responds to your air flow from air box. If not reading good air flow at W.O.T. engine wont respond to Quick acceleration. NO POWER is what you'll get until engine begins to throttle up then it's now running on pure engine power but no throttle response.

I have a 2004, 4 door z71 and experienced the exact problem!! To whoever said the mass air flow is right except for that don't buy a new one take off the mass air flow sensor clean the sensor with brake clean or carb clean hold can 8-10 inches away do not blast the crap out of it clean the whole surrounding area around the air flow sensor. Take off your throttle body, clean it also there is carbon build up hold the throttle body with all the electrical end up because u do not want all the nasty crap in ur map sensor or any other sensors. Clean very good with carb or brake clean. Put it all back together it and test it. Do not I repeat do not blow off any of the parts with a air compressor as u may destroy the sensor let it sun dry. I had this exact problem with my 2004, 5.3 and this fixed the problem!! Its a crappy designed air box and flow system. This problem will occur frequently because of mudding it will get dirty because of the p.o.s. design of the air intake do not just buy a new air flow sensor its 250 they very very very rarely go out!!!! Good luck repost with results!!!

What causes the throttle to stop working when you put a new motor in?

I have the same problem. I changed my mass air flow sensor and am still having the same problem. I found if i put pressure on the air filter box it will start