misfire problems

[Gregs]

the other week i started loosing my top end around 5000 r[m or so and now i'm having the same problems at 3000-4000 rpm. I get a flashing engine light that reads p0300 random misfire. I've replaced wires, plugs, fuel filter and still the same. I've done some logging and i see that for the most part its cylinder 4 and 6 that are misfiring. I tried switching the coil packs around and it still is cylinder 4 and 6. anyone have any ideas. I'm starting to be at a loss as what else it could be. fuel pressure regulator is good. the only thing i can think is that it might be the fuel injectors. Can anyone help me out?

[joecar]

Check for spark using a spark tester like KD2756.

Yes, try swapping injectors; also check that injectors are firing; are injectors dirty...?

Pressure test the cooling system; test for internal leaks using a block test kit.

Check for leaks at the intake manifold.

Leakdown/compression test.

Inspect rockers/springs.

[Gregs]

thanks joecar,
injectors are fine we checked tonight
spark is also fine
two guys i know who are techs both said they think its a leaking intake manifold gasket, so i'll be sticking that in tomorrow, hopefully that does the trick!

[Gregs]

new intake gasket is on and still misfire on 4 and 6. I guess i'll check compression...i don't know what else to do

[joecar]

Are you losing any coolant:
when cold, is radiator full...?
when cold is overflow bottle at the same cold level every time.
when hot is overflow bottle at the same hot level every time.

[joecar]

Has any part of the rotating assembly and/or the CKP sensor and/or PCM been changed...?

[joecar]

I don't know if this comes with the fluid shown in the pic: Lisle75500.

[Gregs]

to my knowledge it isn't using any coolant. the resivour remains the same. just did a compression test:

cylinder-psi:
2-165
4-150
6-155
8-170

1-170
3-165
5-175
7-130

i tested number 7 about 5 times and it tested out at 130. thats odd since i'm not having any problems at all with that cylinder. My problems are with 4 and 6.. confused. I'll do a coolant pressure test tomorrow and i think i'm going to try switching the fuel injectors around and see if the trouble follows...

[Gregs]

joecar
Has any part of the rotating assembly and/or the CKP sensor and/or PCM been changed...?

nothing has changed

i'm wondering if that mouse i had a while ago might have nicked a coil wire or something..i really don't know i'm almost at the end of my wits

[joecar]

Check each coil and plug wire using something like this: KD2756.