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I just measured cylinder leakdown rates on my 2007 Tacoma V6. 5 out of the 6 cylinders had retention rates of 75-85% (leaking 15-25%) but my cylinder 4 (left engine bank in the middle) was leaking about 75%, retaining only 25%. The easiest of the several places to check for air movement coming out was its intake manifold shaft (circled yellow) and I could sense air coming out. Of course, it could too be leaking on the exhaust valves, piston rings and the head gasket but those were not accessible to check easily. At least one possible outlet has been diagnosed positively.

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My guess is that, when I did my head gasket job some 6 years ago, and lapped all the valves, I may have not done it right on one of those valves or both (2 valves each side). What are some possible effects of a cylinder (one out of 6) that leaks that much through one or both intake valves? Can I drive with so much leak or do I need to lift up that cylinder head?

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You need to fix the issue. The biggest problem is you'll have an engine imbalance. Power output on the No 4 cyl is going to be less than what the other five are doing. This can cause excess wear on the engine, as well as (probably) a noticeable vibration. I mean, you could drive it, but you're going to be causing other issues as time goes on.

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  • do you think this could have been causing loss of oil?
    – amphibient
    Dec 23, 2021 at 1:06
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    If you put new valve stem seals in when you did the head, probably not. Other than down past the valve stem seals, there's pretty much no other place for the oil to get into the cylinders from the head.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Dec 23, 2021 at 1:08
  • Also, you would think that, with that kinda leak, the intake manifold shaft would have a lot of carbon blowback and other grime but it didn't. Maybe it doesn't leak that much when the engine is running
    – amphibient
    Dec 23, 2021 at 1:08

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