Blow-by is the expanding gases of combustion that make it past the piston rings. To measure blow-by, simply open the oil filler cap whilst the engine is idling and place it on the opening without latching it. If the cap just rattles around on the opening, you don't have to worry. If the cap is blown off, you've probably got low compression in one or more of your cylinders. As for smoke from the tailpipe, (which I assume is black- which is unburnt fuel- soot), try adjusting the valves and/or the injection timing.

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1983 240D 3.0T 4-speed manual, now sold

1989 Subaru GL Wagon 5-speed Touring Edition