Valve cover oil leaks causing cylinder misfire OBD II codes

  • 4.6L
  • V8
  • 2WD
  • 117,000 MILES
On 4.6L 32V DOHC V8, getting misfires on 3 to 5 cylinders and sometimes random misfire. At Idle engine is slightly rough with Idle sometimes slightly high around 1,000 rpm. Later might be low with Idle around 750 rpm with more shaking and almost shutting off, but never does.
Probably some bad COP's and spark plugs from oil saturation? At least 2 spark plug tubes had some oil inside, spark plugs were covered in oil and coil boots were saturated. Is this oil coming from bad/leaking spark plug tube seals and would replacing them fix my problem? Are the seals just at the top of the tubes and attached to the valve cover, or are there o-ring/seals at the bottom of the tubes also?

How do you replace these seals, and are they part of valve cover gasket replacement?
How do you replace valve cover gasket?

There might be some indication of coolant leakage in this area in the past, but does not seem to be leaking now. [I replaced the radiator previously for other leaking.]

Before the misfires started, engine seemed to run really well around town about 45 mph or less; and a few times on the highway about 75 mph engine ran fine also.
But gas mileage was abnormal - only about 10 miles per gallon, instead of normal 15+.

I am planning on replacing all COP's and spark plugs; but I have read of people doing this and problem wasn't fixed? Are there other (bad) sensors and/or vacuum leaks etc, that could cause this problem?

I really like this engine/vehicle and hope to salvage it. I previously owned a 1996 Mark VIII with same engine minus COP's and it ran like a dream for almost 200,000 miles, with many 120 mph sprints.

Thanks for you help/insight.
Do you
have the same problem?
Monday, May 25th, 2020 AT 11:13 AM

1 Reply

Hello, I'm Danny.

Yes, you pretty much answered your own question.The spark plug tubes have an o-ring to be sealed off from the valve cover.Yours are leaking and allowing oil into the tubes. I've attached a picture below of the gasket set available from AutoZone. Here is a tutorial showing what is involved for a valve cover gasket replacement:

I've attached picture steps below for valve cover gasket replacement for your vehicle.Hope this helps and thanks for using 2CarPros.
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Monday, May 25th, 2020 AT 4:24 PM

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