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My 6.4L (150K miles) started idling rough, especially when in gear with foot on the brake. Seems to smooth out fine when accelerating, except the other day when it would miss pretty badly when accelerating (it had been rainy/humid for several days). I'd get a severe miss/stumble and a big puff of white smoke and then it would seem to be normal until slowing down and accelerating again.

1st MIL code was P0306 (misfire detected in cylinder 6). Cleared that code and when the check engine light came on again (about 50 miles and several days later) it came up as P0305 (misfire in cylinder 5). I cleared that code and about 20 miles later code came up again a P0300 (random/multiple cylinder misfire).

Just replaced all fuel filters (Motorcraft) and ran a tank with HSS fuel additive and am now running a tank with Rev-x additive. Rough idling got worse after filter change.

Also should note that I just pulled my CAC/intercooler and found a pin-sized hole in one of the tubes, so I've got a replacement ordered and on the way (should be here Tuesday). Could this cause a rough idle and the P0300 codes?

 

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2008 6.4l rough idle & P0305 code

I replaced the Intercooler / CAC and the EGR valve, which I took apart just to inspect and clean but was unable to remove it without destroying it...and not for my lack of being gentle with it.

Here's what up now: no smoke but still have rough idle, especially in gear with foot on brake. Accelerates fine, no stumbling or hesitation. Cleared codes and drove it about 40 miles before code set with P0305, Cylinder #5 misfire.

Coolant level hasn't budged and no other symptoms except the rough idle...which gets rougher if I idle more than a minute or so.
 

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There is NEVER ANYTHING IN THE CAC EXCEPT positive pressure. There is NEVER a vacuum. A hole like that would have NO impact on drivability. However, it's never a good thing to have a hole in the charge air system.

More than likely, you've got a couple of injectors going. When we see that here in our shop, we'll pull them all and send them to our fuel shop to be cleaned and tested. If they pass the tests, the get resealed with new O-rings. IF they don't pass the test, the get fixed or replaced.
 
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