What could cause MISFIRE CYLINDER 8. I already

Compression test.

Injector?

Have you already run any tests? I would suggest a compression test (wet and dry) to get your base line. If you have a low cylinder or bad valve then you will have a misfire. Then I would check the fuel pressure. Take a look at the vehicles repair/maintenance history. Usually a dirty distributor cap will cause a misfire on that engine (but you replaced yours). Did both distributor cap screws go in well or did they strip out? You may want to pull the cap back off and check the shaft (that the rotor slides on to) for side to side play. Double check the spark plug wires. The way they push into place, on to the cap can be tricky. Inspect the terminal inside the boot ( of the spark plug wire). Make sure that every terminal has a black metal spring clip on it. Without that spring clip, the wire will not make good contact with the cap. Is the Engine Light on? If so, can you have the codes read? I hope this will help and give you a head start or a good direction. let me know if I can be anymore help. Good luck.

Gas or compression. Check the injector circuit on 1 and 3. Are they pulsing, to put gas in the cylinders? Do a compression test to rule out loss of compression as cause for the misfire.
Kind of odd it would be 1 and 3, but that is one right after the other in the firing order of 1-3-4-2...so, you have a single coil? So you have a distributor-examine closely the distributor cap and rotor if those were not changed too, with the plugs and wires. Look for damage, little black tracks that indicate shorting of the spark, maybe damage at the electrode tips on the inside of cap on 1 and 3 plug wire towers. Most people change the cap and rotor when doing the plugs and plug wires.
Just a possibility also, that the injector ground wires for 1 and 3 might share the same ground. That is to say, they might splice together before going to the computer where the computer internally grounds injector circuits to cause pulsing. Well, those are my thoughts, good luck. And let me know what you find, always interested.

Check your coil pack. Sounds like one or two need replacing.

Cylinder #2 is the the second spark plug counting from the right to left.
EX.Below are spark plug on engine
engine
-------------------------------
' (oil cap) '
' (4) (3) (2) (1) '
'_________________'

Your misfire can be cause by couple of things but the cheapest way that you can try to fix your car is to do a tune up by replacing:
1)all spark plugs
2)do a oil change
3)spark plug wires
3)distributor cap and rotor

note misfire can be cause by:
1.bad gas
2.spark plug and/or wire
3.distributor cap and/or rotor
4.valve adjustment
5.injector
6.off timing
7.ECM
8.bad wiring(power, ground or feed line)

Probably you have a cracked or otherwise bad Distributor Cap. I would have changed the dist. cap and rotor first, and probably the wires too. You said you changed the boots, did that include the wires?

Pay attention to the order of wires before removing from cap. Draw yourself a diagram.
Are you sure you returned the plug wires to correct plug?

The codes mean as follows
PO300 multiple cylinder misfires
PO301 #1 cylinder misfire
PO308 #8 cylinder misfire
Some of the more common causes for these codes popping at once are, if you went through a big
puddle and the engine and wires are wet or the distributor cap and/or wires are bad or the camshaft
sensor(located inside dist cap) is bad and sending false signals. Hope this helps.

If you already replaced the wires, I would suggest replacing the plugs, distributor cap and rotor. They are also very important parts of the spark. Fuel injector problems can also cause misfires.

cylinder 1 misfire and low compression. Plugs and wires changed plus new fuel injectors. The engines still misses and no one can find the cause. When put on the computer it read misfire cylinder 1.