Low compression

A couple of years ago, I wrote in about seeing – on both a compression gauge and an oscilloscope – an occasional, intermittent low compression on one cylinder. Until I subsequently saw a Scannerdannet video about an intermittent misfire, it never occurred to me that the problem might be due to a hydraulic valve lifter/tappet. I simply assumed the exhaust valve was occasionally sticking in its guide, even though there was no evidence of a lack of free motion. Maybe if I understood how hydraulic tappets work, I might not have entertained the thought that they either work or they don’t, intermittently or otherwise. I wonder how often you have come across a dodgy hydraulic tappet that occasionally stops a valve from closing fully and giving a low compression? Martin Rubenstein

Hydraulic lifter problems are more prevalent on vehicles that have been poorly serviced. I did come across this a few some years ago, but that was a lifter that was not pumping up correctly and allowing too great a gap, resulting in a noisy tappet. Very often hydraulic tappet problems can be solved using a lifter-free additive or, in some cases, simply changing the oil. This is one of the reasons that using the correct grade of oil is critical on a modern engine.

One extreme problem I came across on a Daewoo was a blocked oil-way preventing the oil from escaping from the hydraulic tappet blocks. This had the effect of pumping up the hydraulic lifters and preventing the engine from starting due to lack of compression. However, as a rule, the hydraulic lifters give very few problems.