[Steve Tinker]

barks...

1) The chances of damaging he ECU / sensors by "arcing" the HT side of the coil pack to the block is remote, as that is exactly what the spark plugs do - the 100,000v (or whatever voltage the coils produce) jumps the air gap of the plug to the block. Many times per second.

2) If your coilpacks are cracked / damaged, then they will arc to the block and cause a misfire when wet - again this does not normally cause damage to the ECU.

I'm assuming the mechanic did the compression test by removing all the plugs and left the coil packs "hanging" and not connected while he turned the engine over on the starter motor to get the compression of each cylinder. At least that's how I have done it in the past although I tend to ground the coil packs to avoid voltage buildup & potential damage to the coils.