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Chevy V6 4.3 with 281k miles. It recently threw a misfire code isolated to cylinder #3. The misfire seems systematic and not random, though, as it idles rough. I've got an open question on it already, but I have more info and hopefully a better question.

For a baseline I compared the #1 cylinder to the misfiring #3. The spark plug from #1 looked great given its age, and with all the other plugs remaining in place, that cylinder registered 124 psi of compression. Next I pulled the plug from the #3, which also looked fine (not black, not oily, but somewhere between "normal" and "overheated" compared to the chart here) and its compression registered at 70 psi, and which is definitely a problem. I disconnected and reconnected the equipment and got the same measurement on a second test.

Unfortunately these plugs install horizontally so I'm not sure how to confidently perform a "wet compression test" since the oil won't necessarily flow down or evenly coat the piston..

But there is no smoke in the exhaust. None. And I feel like that's a key detail.

Based on the above, is this most likely an issue with valves controlling cylinder #3? If so ..would this be more characteristic of a bent or a stuck valve? ......if the latter (stuck), is there anything I can try to do to "un-stick" the valve without necessarily disassembling the head??

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Pull valve cover for #3, remove both rocker arms for #3, pull the spark plug and apply compressed air (100lbs) into the cylinder #3, listen for a air leak sound in the exhaust tail pipe, Intake manifold or crankcase.

Exhaust= Exhaust valve problem

Intake= Intake valve problem

Crankcase= Piston ring failure

Two more things can cause low compression, camshaft lobes are damaged and not opening the valves enough to build good cranking compression, or bent pushrods if the engine has them.

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  • You describe exactly the test I did... without the safety tip... – Solar Mike Nov 6 '19 at 18:03
  • @Moab, some questions: 1) how to "lock the engine" as SolarMike mentioned. and 2) does the cylinder need to be in any particular position of the crank sequence (top, bottom, other) prior to this test? – elrobis Nov 6 '19 at 18:11
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    No need for either if you remove the rocker arms, air pressure will push the piston to the bottom and valves will be closed. – Moab Nov 6 '19 at 19:15
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Why would you expect smoke in the exhaust? Any smoke from that cylinder can be combusted by the other hot exhaust gasses in the exhaust manifold.

You have a clear indication of a problem in one particular cylinder.

Your next step is to consider applying compressed air to the cylinder through the plug hole and then listening for where it comes out. Do make sure you lock the engine so it does not rotate suddenly...

Once you hear where the compressed air is escaping you can decide to remove heads and/or take out pistons...

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  • Doesn't no smoke = 1) no oil blow-by? 2) no coolant in the combustion? I would think these are useful details. Anyway, is this the sort of experiment you're describing? I started it at the useful part, 5:17.. – elrobis Nov 6 '19 at 16:34
  • @elrobis smoke can be from poor combustion or water vapor, not only from oil... – Solar Mike Nov 7 '19 at 12:47

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