I have a 1997 Mitsubishi Galant with rough idle and no power when accelerating. Replaced spark plugs, distributor cap, rotor, spark plug wires, MAF, both oxygen sensors, air filter and the engine still idles rough and has no power when accelerating. What is bad? ECU, fuel injectors, head gasket?

Have you changed your spark plug wires and distributor cap? You might have a spark jumping from a bad wire or distributor cap. Under acceleration, the amount of air/fuel in the cylinder increases and this may make it harder for a spark to jump across the plug gap. It may be finding a different path, and that is what's causing your engine to run rough. Spray water over your spark plug wires in a dark place and listen to see if your engine runs rough. Look for where the spark is jumping from a wire or distributor cap.

I don't know what testing you've done? How long since a tune-up? Any applicable trouble codes?
Have you checked for a spark misfire? Sometimes you can use a digital multimeter, engine idling, touch spark plug wires, one at a time, notice the readings. This is an inductive reading, you don't take the spark plug wire loose and you don't penetrate the wire insulation, you just touch it. I've seen readings of .6 or .7 volts, not even a full volt. If you get reading bouncing off zero, that's where you start checking. With distributor ignition ignition, the spark plugs-- wires--cap and rotor are tune-up parts. I know some fords have had trouble with tfi modules.
Just to add, the basics for almost any vehicle with a rough idle, check for a leak of some kind, fuel--egr--vacuum, something of that nature.

1 IT SEAMS LIKE HOOD LEAK OR DRAIN PUGED SPRAY DRYWIRE ON ALL WIRES CLEAN CONTACTS

scan for codes

Check that the O2 sensor is actually connected, and functioning.

check your motor mounts

Sounds like your Timing Chain or Belt Are Damaged and Caused your Ignition Timing to Jump out of Time, Also check your Distributor Cap and rotor NOTE: if you have a Distributor ,Some are Distributorless. Pull your Spark Plugs out BEFORE you Do Locate #1 Plug its in Front of your Engine.And Trace the Wire to the Distributor Cap. If you have a 4 Cylinder #1 is in the First Plug in front of vehicle, if its a V-6 #1 Plug is on Left side first plug in Front. Depending on which engine you have will determine if your wires go into the Distributor cap Clockwise or Counter Clockwise. So BEST to Simply Mark them to get them back in same order.You'll need to Get the Enging to crank over and have a Buddy Put a Compression Gauge in #1 Cylinder and Bring it up to TDC (Top Dead Center) thats when the piston is up and gets the Gauge to Move up then your on the Compression Stroke, Then check distributor rotor to see if its pointed to where your #1 Spark Plug wire came off or at least close to it.Just hit the Ignition Switch off and on Until you have it showing compression on Gauge. JUST ON and OFF that Fast. If the Rotor is pointing To a different Spark Plug Wire in the Cap, Sounds like timing Jumped. Hope this will help you solve your problem or at least find it. If you don't have or not sure on How to do this i recommend you have a Qualified Mechanic Shop run the Tests. Thanks and Good Luck

I Have a 1999 ford f150 4.6 that back fires and is just running really ruff when idling alot of nack fire a sord of like a pulse going on what is my problem?

You should have your TPS and EGR looked at as those cause rough idles...Other causes are bad idle air control valve, bad plugs, bad distributor cap & rotor, a bad wire, bad O2 sensor, etc...

vacuum advance working ? do manuel check with cap and rotor button off and see if it's moving. chris