Well, we all eat a little more during the holidays...
Have you tried replacing the fuel filter? May not play a part, but probably a good idea with old gas that was in there. Test the oxygen sensor, but I'm leaning to the old fuel. A friend once got a car that sat 10 years. It started right up on the old gas in the tank. The gums and resins that had formed in the fuel worked there way into the engine where they eventually caused a bent push rod.
HEET is just ethanol, and already in most gasoline. It won't help any more in dissolving any resins in the fuel than the fresh gas. What fuel injector cleaner was it? Perhaps it contained solvents that had little to no effect on the age degraded solids. If there was an improvement with the premium fuel, it could be because they contained a more effective cleaner for whatever was in the tank.
At this point I don't think draining the tank will help. The old fuel has gone through the engine. Try a different injector cleaner, preferably one labeled a fuel system cleaner. Also add acetone. you should be able to find a 16oz bottle of pure stuff at Walmart or a beauty supply store. Use a funnel with it and have a towel ready in case of drips. It will remove car paint, but is the thinner used for varnish, which may have formed in the old fuel. Isopronal in the form of iso-HEET or 91% rubbing alcohol won't hurt.
replaced fuel filter, thermostat and just yesterday it started running rough, like it was missing on a cyclinder. I replaced cap and rotor and sparkplugs, did not help anything, still runs rough. Feels like a dead cyclinder. Could it be injectors? I am stumped as of now.
Do not assume the new plugs or wires are not defective as well as a bad cap too! Check by pulling a plug wire out of the cap to see which plug may not be firing or a safer way is to pull the plugs and check them for gas fouling. Double check the firing order of the wires to the plugs and does it use points? Check plug gap as well!
I just removed the EGR valve and noticed the diaphram was really easy to push. is this normal? I also noticed the car ran smooth when the EGR was pulled off the intake manifold, but back to the same thing with EGR back on the intake manifold.
Certainly sounds EGR-related. Manual says EGR connects to ported vacuum, not manifold...having it connected to manifold vacuum would definitely explain running bad at idle.
It doesn't say anything about how it should feel when you push it but the test procedure is:
Start engine and allow it to idle.
Disconnect the vacuum hose from the EGR valve and connect a hand vacuum pump to EGR valve.
Apply approximately 150 mm Hg (6 in. Hg) vacuum to EGR valve. Vacuum should remain steady and engine should die. If this occurs the EGR valve is working properly. Remove hand vacuum pump and reconnect the hose, test is complete.
If vacuum does not remain steady and the engine does not die, then replace EGR valve and retest.
Should vacuum remain steady but the engine not die, then remove EGR valve and check for blockage, also check manifold for blockage. Clean and replace as necessary and then repeat test.
Pull and read the plugs is one fouled out? Do a compression test. Start at step 0 don't just try this that and the other. Make sure you are running on all 4 cylinders. If one is low ie 3 are 175 and one is 125 it will run and it will run crappy. No amount of parts will fix that. Always start at the beginning. Test vacuum. Check timing then. You can look at egr and so on. If you have a vacuum leak it will show op with a vacuum test. If one of the plugs is all fouled out or looks diffrent then possibly injector on that cylinder. Just take a step back and find out where the motor is. What condition it's in.