I have a 617 engine shake at idle (84 300SD) and intend to do some diagnosing. I thought there was a compression gauge in the shop but can't (haven't) find it.

How effective is loosening the oil cap as a method of testing bow by and by inference, compression?

Also, I remember someone rigging an electronic method of checking IP timing but haven't found it. Anyone have it book marked?

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.