I have a 94 GMC Z71 with a 350 in it. keeps bogging down?

Question

I have replaced the fuel filter, the throttle positioning sensor, cleaned the injectors, and also replaced the oxygen sensor. I dont know where to look next.

Usually It idles ok, but when you accelerate it hesitates before speeding up. When you get about 45 or 50 it runs pretty well, but between 30 and 45 its a little rough.

Answer 1

I own a shop, and I would think you need to take a long hard look at your EGR valve. These are bad to stick, and cause all kinds of running problems but never set a code in the computer. You can't clean them to work either! I would replace it, and above all; go over the entire engine, and look for vacuum leaks such as split where they plug on, too loose & sucks air around the hose, gummy hose that is collapsing, a hose that is off or full of cracks, and replace any/all hoses that are bad. Check the igniton timing, and suspect you may be in need of a timing chain if it is out, or moves around. Most of these I have worked on had a bad EGR valve that was sticking just enough to wreak havoc on the performance of the engine. Clean out the carbon inside where the valve goes too. Also; Clean the throttle body where the IAC goes, and the channel around the butterfly as this too can cause the problem. Don't allow this gummy mess to go down inside the engine, so stuff a rag behind the butterfly to catch this or it will do harm to the EGR and/or the O-2 sensor. I use a long rag that won't be hard to get back out.
Part 2:
I have had several that did this exact thing, and it turned out to be a bad throttle positon sensor. It works like a volume control on a radio, and many times it is worn in the area that sees the most action, and these are the areas that your complaint takes place. As you push the gas pedal to take off, it hits a bad spot, hesitates, then goes on, and finally when you get up to speed, it works in the area that is most used, thus hits an misses across the spot. The throttle position sensor will test good 75% of the time, and will never set a code. The bog would come from air introduced into the engine, and not enough fuel to maintain proper mixture, thus you get a bog until the fuel catches up. The thottle position sensor is nothing but a spring, with a arm that travels across the spring as you move the gas pedal. The more you press the pedal, the less resistance the circuit has due to you are making the spring shorter as you push more on the gas pedal. The arm that moves back an forth on the spring will wear the spring out in the areas that are used the most, and the result is; bogging, and rough or jumpy power from the engine. I wouln't rule out bad wires, plugs, cap & rotor, but the best way to rule this out is; Get the engine up to temp, and just floor it from stop to about 70 mph. Any break down in the igniton system, or valve train will rear its ugly head by popping & cracking either/or out the exhaust or the intake.
Glad to help out, Good Luck!!!

Answer 2

I take it you don't have any codes stored. As you can see, replacing parts because you suspect does not work with cars and trucks anymore. The first thing to do is determine if you have a basic motor problem. Remove air cleaner and rev motor up quickly and listen and look for any backfiring through the TBI as this would indicate a valve or camshaft problem. While the motor is running, check for vacuum leaks at all the hoses and from between the intake and the TBE base gasket. Then perform a compression test on all eight and compare results, should all be about 10psi within each others readings. If that is good, carefully inspect the tune condition of the motor by looking at the plugs for wear and fouling and replace as nec. Then start looking at all the secondary ignition wires for splits in the insulation or corrosion. Inspect every one of these including the coil as these were really bad for corroding. Also inspect the dist. cap and coil tower for the same corrosion. Lift the cap off and make sure the rotor is okay and then grab a hold of the dist and wiggle it to see if the bushing is worn out. Have a good look at the pick-up coil and module connections inside the dist as well as they had a tendency to corrode with a green stuff on the connectors. Replace anything that you determine is no good. If you find nothing make sure the coil can produce strong and consistent spark. You might now need to actually check fuel pressure, need around 12-15 psi. I would then move on to checking exhaust backpressure as it is possible the convertor or muffler is starting to plug. If all that is good and you are sure the trans. is not causing you grief, you do have something weird going on and now is the time to look at aftermarket add ons or connections to them and the factory stuff. Good luck.