Source of No Compression?

[Viva Stuttgart]

Hello fellow W113 enthusiasts; It's been determined that Cylinder #3 has NO compression. I'd like to understand why there is no compression. I've already tested the spark plugs (cleaned each spark plug then pressure checked all cylinders individually, then swapped a "good" spark plug at the underperforming cylinder #3 which resulted in no pressure BUT there was a spark);

  The Pagodas and Beer guys have been extremely supportive in the past and I've learned much from attending the bi-annual get togethers in Rancho Santa Fe.

   But a non-P&B person recommended I "squirt" oil into the cylinder then follow up with a compression test because how the engine responds would reveal whether the issue is in the gasket or the valve? Does that sound correct?

     As always I'm ready to get my hands dirty. But before doing so, I'd like to have a clean brainstorm and understand what direction or action I need to take to address this underperforming cylinder.

     Thank you,

    Marisa

[ja17]

Check the valve adjustment and camshaft on that cylinder next.

[Cees Klumper]

No compression has nothing to do with ignition/spark/spark plug. Unless you had no, or a very loose, plug in there. No compression at all means there is a permanent opening somewhere: a valve that is not closing (hence Joe's suggestion) or, for example, a hole in the piston. Low compression is more common than no compression.

[Pawel66]

I am not a mechanic, so I will throw in a word like amateur to amateur: the compression may escape from cylinder via bottom of it (piston/cylinder walls) or top of it (valves, spark plug opening, head gasket).

ja17 advises to check the valve timing and camshaft alignment - to confirm or eliminate some of the possible issues on the top.

What you were advised about pouring a bit of oil down the cylinder and check compression was to confirm/eliminate bottom - oil would seal the piston for a moment of measurement, so if the compression appears, it means the leak is through the bottom most likely (it will not help if there is a hole in the piston).

Follow the advise you are receiving here, it will be professional and systematic.

[Vander]

Check the valve adjustment and camshaft on that cylinder next.

I agree, do this first. I still have low compression on 2 cylinders on my 280 after doing the above.  I will be removing the cylinder head this winter for a rebuild. At 104k miles I expect to find the valve guides to be the main culprit.

[Kevkeller]

I’m not sure how much worn valve guides would affect your compression unless they are completely loose and the valve isn’t seating properly.

Before I would takeoff the head I would put some oil in the cylinder, recheck the compression and see how much it goes up. If it goes up a lot, your rings aren’t sealing and a head rebuild might not do much good.