Some days ago I did a compression test on my XJ12 H.E. 1983, engine at operating temperature, all spark plugs out. Due to me and my helper being daydreamers, we forgot to step on the gas pedal. Anyway, the results are interesting despite this error, and they are somewhat worrying:
Dry test gave about 120 PSI on all twelve cylinders. Pouring a small amount of engine oil into the cylinders, had the readings jump up to around 190/200 PSI. I guess that the values would have been higher with the throttle wide open, but surely the huge difference between dry and wet test would still remain.
The variance between the cylinders is well within the range (< 15% of variance).
As far as I know, the values of the wet test are OK for the H.E. engine, especially if I take into account that the values would have been a little higher with open throttle. But the values of the dry test are really low, even with the throttle closed. All in all, this points to worn piston rings. A closer look at the cylinders with an endoscope reveilled no visible damage, the cylinder walls are in good shape, no scrapes, and the same applies to the pistons and the valves.
So far, so good – or rather, bad. But what are the consequences? Does the engine need an urgent overhaul yet, or would you advise me to just keep on driving? What do you think? The engine runs smoothly, no strange noises, no lack of power – no sign of a problem at all, if it were not for those low readings at the dry compression test. Better shouldn’t have done it.