compression check?

[1fife]

merc 2001 200 efi

want to do a compression check+leakdown

heard all kinds of opinions

warm, cold-plugs in out. squirt oil in cylinders and do again

what all the real scope and how do you actually do it

leakdown?

[kroppe]

I have done compression checks on many automotive, marine outboard and small engines. It's easiest to run the test cold (won't burn your hands) and the results aren't that different than hot. If you have poor compression you will know it from the results, regardless of if you run it cold or hot. The "hot" result might be more accurate but only if you're tuning the engine for ultimate performance. Maybe this is what you are doing.

Adding oil in the cylinders is theoretically supposed to seal the gaps in the rings for a brief moment while you run the test. The theory goes that if you have higher compression after a teaspoon of oil is put in the cylinder, then you have bad rings because the oil sealed the rings momentarily. I would take this "theory" with a pinch of salt.

My recommendation is pull all the plug leads and secure them where they won't arc or shock you (been there, done that, it ain't fun). Then pull all the plugs (less current drain on the battery), install the gage (make sure you have a good seal) and hit the starter for about a good few seconds. The needle will stagger up to a final value. When it stops going higher, kill the starter. This is your reading in PSI. Look at the manufacturer's rating for compression ratio and multiply this by 14.7. This should match your reading, in psi, on your gage. If your reading is too low, repeat the test 2-3 more times to be sure you are getting consistent results. If it's still low, by a large amount, you may have a compression concern.

When you are convinced you have a good reading on one cylinder, move to the next. It's not a difficult test, just follow a repeatable procedure and you'll get correct results.

p.s. I have never done a leakdown test, but my opinion is it is a fine tuning test. Compression will tell you good or bad. If you are fine tuning your engine, leakdown may be useful to you.

[TONGA]

YEP!