Low compression in cylinders 4 and 6

If compression is low in one or more cylinders, you can isolate the problem to the valves or rings by squirting a little 30 weight motor oil into the cylinder through the spark plug hole and repeating the compression test. The oil temporarily seals the rings.

If the compression readings are higher the second time around, it means the rings and/or cylinder is worn. No change in the compression readings would tell you the cylinder has a bad valve.

You can also do a leakdown test to really get at it-cause it will only come out on 3 different location-tail pipe-dipstick tube-out of the I/manifold.