Did tune up on 2001 chevy blazer v6 4.3 ltr

If it runs good in park or at idle but is slugish when you try to accelerate it is likely a fuel issue. I would start with a new fuel filter. When you press on the gas you need more fuel pressure and if the pump cant get more through the clogged filter you will get that. If it runs bad even at idle it is likely a spark issue and needs new plugs and wires. I would give it a good tune up - oil change, spark plugs (platinum or iridium), plug wires, air filter, fuel filter, PCV Valve and see how she runs after that.

Having replaced the computer, best thing you could do now is a complete tune up-new spark plugs, wires, distributor cap, rotor, air and fuel filter. Change the oil and check all fluids. Have the computer scanned for trouble codes. Check if your catalytic converter is blocked or clogged-if bad, they will plug up and stop the car.

Could be the EGR valve or the idle adjust valve. Check Engine light on? Any codes?

I suggest a full tune up.

1. Air filter.
2. Fuel filter, carbureted engine filter is located inside the line fitting where it attaches to the carburetor. Fuel injected engine filter is located in-line under the truck near the tank.
3. Oil and filter.
4. Spark plugs and wires.
5. Distributor cap and rotor if applicable.
6. PCV valve.

New spark plugs, oil and oil filter, transmission flush and fill, fuel filter, air filter, coolant flush and fill, clean throttle body plate and bore, clean the MAF sensor, replace any loose or cracked vacuum lines, PCV valve, new air filter, new battery.

99% of the time all you should ever need for a tune up is new spark plugs, and a clean air filter and throttle body. But since your inquiry indicated "major" I gave it more critical thought.

My initial thought is that you have no spark in one cylinder. This could be from a bad spark plug or other ignition component failure. Have the spark plugs ever been changed? I would rule this out first, as they are inexpensive, and also under more stress than any other part of the ignition system. If the problem persists, then it's time to look at the spark plug wires or ignition coil.

Start the car and open the hood when it's dark. Do you see/hear sparks coming from the plug wires? Another inexpensive place to start looking.

Chevy usually puts the spark plugs down below the exhaust manifold.
You usually have to do them by feel, or use a mirror.

Ck your exhaust pipe at rear of car noting any obnoxious smell like rottrn eggs. Hold a piece of paper, I use a dollar bill ,in in front of the exhaust. If it blows the paper gently, you have a catalyctic converter going or is bad. Will need replace if so.

for a normal tune up, the only thing you can change are, fuel filter, oil filter and oil, cabin filters, transmission oil filter and trans oil, and spark plugs. The other (tune items) such as spark plug wires and points, and timing settings are either eliminated or non adjustable. If you have the equipment you can do a computer check, but if the truck is running well it probably won't tell you much.

Usually when it comes to bad gas mileage, an air filter change, fresh oil and filter, fuel filter and making sure you have no vacuum leaks....

Also, believe it or not, improper tire pressure will affect your gas mileage.

New spark plugs and wires are a "nicety" but if there is not damage to them, then they will be fine.