misfire problems

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?

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.

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!

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

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.

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

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

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...

Quote:

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

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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

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

The misfire algorithm may or may not pinpoint the right cylinders... :)

Quote:

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

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will do, i got one in the shop

final update for the day:

tested and i have good spark. I pressure tested the coolant system and it checks out. Went on a test drive and it was chugging hard. couldn't get over 40 mph and it smelled like bad catalytic converter smell. Now that i think back about all the problems, a bad cat makes a lot of sence. we'll know for sure once they ummm...fall off accidentally :)

You can remove the front O2 sensor and see if that frees up the engine...

If you have an exhaust pressure gauge you could measure the pressure due to the cat, should be no more than 1-2 psi.

edit: for example http://www.tooltopia.com/star-tu-29pb.aspx

removed the o2 before the cat and it ran so much better. i'm glad i finally found the culprit

:cheers:

How many miles on the cat...?

about 79000