OK, here's one that will probably show how little I know, but I'm going to ask it anyway. I recently took ownership of a 1977 F100 with a mid-80s 351W installed in it. I believe this truck/engine has been sitting for a number of years (at least one with no start; probably several since it's been used regularly) now but I was able to get it running fairly well without too much trouble. In any case, I ran a compression test on it a couple of weeks ago and found that 7 of 8 cylinders were somewhere between 120 and 130; most averaging around 125 or a little higher. One cylinder, however, demonstrated only 60. With the engine running as well as it did I thought this was funny so I ran the test again, and came up with the same result. I then added a capful of oil to through the plug hole and sure enough, it jumped up to be around the same as the others. So I was thinking I had some sort of ring problem.

Anyway, I ran the truck around the yard a little yesterday, maybe about a half hour or so's worth of run time. I thought, "this thing runs too good to have a dead cylinder," so I pulled it into the garage and ran the compression test again. This time, I came up with 130. I've tried it hot, cold, and every which way and the cylinder checks out fine now. Anybody know why this is, when I had a low reading initially? I thought about maybe something sticking a valve due to the length of time that the truck had been sitting around but then why did I get the high reading with the wet test? Thanks for your answers and your patience.

 

It's possible one of the rings was stuck in it's gap and not sealing properly and the oil in the plug hole helped free it up. There is not a lot of oil reaching the upper rings under normal operation, and it's very likely the compression would have come around eventually if you had not added the oil, so what you did just helped it along.

 

I agree. The rings were stuck. Drive it and don't worry any more about the initial low compression.