I did a tune up. replaced spark plug, wires, and cylinders and now it is only firing on two spark plugs and running sluggish. It was not doing this before the tune up. What is the cause?

Are plug wires connected in the right firing order to the plugs? Also are the spark plugs gap correct? Was vehicle running rough before tune up or only after?

Hello, this engine has 3 ignition coils on the passenger side cylinder head, each coil fires two spark plugs, the spark plugs on thew other side have plug wires connected to one of the coils on the other side, the #6 cylinder wire connects to the #3 coil, try replacing the spark plug wires

do a compression test on all 4 cylinders...sounds like a burnt valve or blown gasket to me..

you need to get a compression test done to determine if the cylinder has compression and the fuel injector needs to be tested

From your description...recheck ALL of the vacumn connections...Did your tune-up consist of plugs and wires? Also, it is common to accidently crack (white porcelin) the spark plug when re-tightening...this usually causes a mis-fire and rough idle too...Last check would be to recheck the plug wire firing order...The #1 cylinder on your 3.1 V-6 is on the front left...your parts store can help you with the correct firing order to trace...These have been very good cars and motors...Hope this helps.

you need to have your engine tuned up, unless it's been done recently. misfire means it's not getting fire or fuel to #7 plug. you may have a plug wire bad or a bad spark plug on #7 if you go to effort of sending it to garage just to replace one plug or wire,you'll pay labor for whole tune up. wires,plugs, and maybe a bad coil pack. suggest you have tuned up with new plugs and wires. also while they are there, have them check fire to #7 plug on coil first before you do a complete tune up. thank you for choosing fixya.com

Could be several possibilities none of which I may be able to eliminate:
1. Tune-up!  You are not getting enough spark or Fuel!  Plug-wires may be cross-firing or bad ground from body to the engine - run a new ground wire to engine from battery or body-ground.
2. Replace fuel filter or do fuel pressure test to determine if f-pump is weak or lodged with corrosion or dirt particles.
3. replace plugs and wires and clean coil-pack ignition module (where plug wires originate from) use a brake-cleaner or throttle-body cleaner but be sure fumes are evaporated and dry before starting or spark could cause explosion from fumes. Oil on module could cause spark to jump.  Do not run plug wires along each other or will jump spark, wires must be crossed once or twice for prevention of this.\
4. check all vacuum lines around engine - a cracked vacuum line can cause a lot of problems.
5. injectors may be dirty or bad - easier to replace complete intake from scrap-yard then to pay $100.00 for new injectors each!
6. Coil-pack on ignition module could be cracked and may need to be replaced.
These are only possibilities of course with the little info you provided.
Hope this helps.

It sounds like a misfire. Check spark plugs 5 and 6. It could be bad fuel, bad plug wire, or weak coil. Check the fuel injectors as well. Other possible causes could be an egr valve stuck open, vacuum leaks, low fuel pressure.

change the mass air flow meter

It MAY be your plug wires. But i would think your mechanic should have checked that already.