It is a clearish liquid that is cleaning the piston, so you are right to assume either an injector or coolant. Maybe try to rig up something to swab the top of the piston and see what the liquid is? I wouldn't waste my time with a leak down or compression test, but you could test the coolant for exhaust fumes. If the engine misfires for the first ten seconds and smooths out, i would also try cycling the key on and off quite a few times and then starting it to see if it misfires longer than usual(this would indicate a leaking injector).
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