"Compression check" by Pulling Spark Plug Wires?

Car: 1997 Mercury Sable V6 Vulcan Engine

My engine went bad, so I had an engine from a junkyard put in the car. This garage said I was to buy the engine, and they would install. I did so. The junkyard assured me that he knew the son of the guy who had the car the engine was in, and it was in good shape. Now that's it's in, it seems to me running nicely, but I just wanted to check one thing, because sometimes engines can break a piston ring if oil is not squirted into the engine through the spark plug hole and let to soak in if they have been left for many months.

Instead of buying a compression gauge, I decided to try a little trick I heard about way back. That is, as the engine is running, take off each spark plug wire and then reattach, one after the other.. If the engine seems to run rough visibly rougher when you take the spark plug wire off the cylinder, that means you are getting output from that cylinder and it is relatively alright. I did this for all six cylinders, and each time I took off the spark plug wire it ran visibly rougher and then ran noticeably better when reattached.

Am I safe in assuming that this quick-and-dirty test means that the cylinders of this used engine are in good shape?

The compression could get pretty darn low and still affect how it runs if the spark is taken away to completely take it out of the running for some engines. But I would think with a Vulcan, what you did was a pretty good indicator that engine should get by for a little while. I'd check with some auto parts stores and see if a compression tester is one of their loan a tools that you can do a real test with.

Much thanks, I appreciate the answer.

I have to second the motion from 65comet. I'd do a real compression test and get real numbers. that's the only way to be absolutely certain that you have a healthy engine.

Also... forgive me for going off topic, but I noticed that you're from Bayside, New York. I lived there on Bell Blvd for 5 years when I was a kid. Went to school at PS 169, which was basically down the Street and around the corner, then for middle school I went to IS25 on Francis Lewis Blvd. They didn't use yellow school buses so we all were issued special metro cards and rode the Q28 to school.
I miss that city, the last time I was there they had just started building what is now the freedom tower. Nice looking building, but I still miss the twins.

I'm with these guys. If compression was bad enough on a cylinder that you would notice a difference doing that test then you would notice a difference before you even did the test.

Compression could be lower across the board on all cylinders and the car might run fine enough, and pulling spark plug wires would still give you the "chug" result on each. I had a Talon TSI years ago that I fried the compression rings on when it leaned out hard. The car was down to 60 psi on each cylinder, yet it idled no different than it did before the melt down. Hell it still boosted and pulled hard as hell being down 90 psi on compression.

The only way you knew something was wrong without testing it was that it turned over faster than normal for starting.