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I'm replacing my valve cover gaskets because they are leaking. I noticed a significant difference between the valves in the front and rear banks. The front ones look quite dirty while the rear ones are much cleaner. Is this something I should be concerned about, or should I not worry about it?

This is a Honda J series engine in a 2011 Odyssey.

The rear bank: enter image description here

The front bank: enter image description here

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  • I'm pretty sure this is a byproduct of VCM, which is Honda's version of cylinder deactivation on the J-series engine. It turns off one bank of cylinders to achieve an inline 3-cylinder engine. I don't know why it would do this, but seems logical.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jul 30, 2023 at 14:07
  • You say you replaced the gaskets because they were leaking, was this dirty side leaking worse?
    – GdD
    Commented Jul 31, 2023 at 7:50
  • @GdD: Yes, the leak on this bank was much worse than the other one.
    – Phil Anderson
    Commented Jul 31, 2023 at 12:27
  • A leaky gasket lets stuff in as well as out, it could be that's the cause, but that's speculation on my part.
    – GdD
    Commented Jul 31, 2023 at 12:40
  • Have you had the vehicle long enough to know whether one of the heads has previously been replaced?
    – HandyHowie
    Commented Aug 1, 2023 at 7:56

2 Answers 2

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This is caused by VCM, the front bank works all the time.

VCM stands for Variable Cylinder Management. It is a technology that allows a V6 engine to run on 3, 4, or 6 cylinders depending on the load and driving conditions. This can improve fuel economy by up to 5%.

On J series engines, VCM is controlled by the Engine Control Unit (ECU). The ECU monitors the engine speed, throttle position, and other factors to determine when to deactivate cylinders. When the ECU determines that 3 cylinders can be deactivated, it will close the valves on those cylinders and stop the spark plugs from firing. This effectively turns the V6 engine into a 3-cylinder engine.

VCM is not active all the time. It only activates when the engine is under light load, such as when cruising at a steady speed. When the engine is under heavy load, such as when accelerating or climbing a hill, VCM will deactivate.

VCM has been criticized for causing problems with engine oil consumption and emissions. However, Honda has made improvements to the VCM system over the years, and the problems have been reduced.

Here are some of the benefits of VCM on J series engines:

Improved fuel economy: VCM can improve fuel economy by up to 5%. Reduced emissions: VCM can help to reduce emissions by running the engine on fewer cylinders when they are not needed. Smoother ride: VCM can help to provide a smoother ride by eliminating the vibrations that can occur when a V6 engine is running on only 3 cylinders. Here are some of the drawbacks of VCM on J series engines:

Increased oil consumption: VCM can cause increased oil consumption, especially in older engines. Increased emissions: VCM can slightly increase emissions, especially in older engines. Problems with the VCM system: The VCM system can malfunction, which can cause problems with the engine, such as misfires and rough idle.

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I believe it is (my mechanic said) that it is shellac. I think he meant that it is a heavy petroleum distallate that gets burned off by the hotter cylinder bank. He said it often (always) ruins engines over 350K miles...

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