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UWSkier

Compression Testing How-to

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UWSkier

So I know in a car you need to remove fuel pump fuses, relays, coils, etc before performing a compression test.  However, cars don't have tether switches.  Is testing compression on a boat as simple as:

  1. Run up to operating temp
  2. Pull lanyard
  3. Pull plugs
  4. Crank
  5. Open throttle to WOT (manually hold butterfly open)

Or is there more required?

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23LSVOwner

Pull fuel pump relay.

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Woodski

I like to pull the coil wire to prevent arcing from any of the plug wires.

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Ryan1776

Running a compression check has a lot of variables that will effect your results. Some of them you've covered, engine at operating temp, but by the time you get all the plugs pulled, things disconnected you've lost some heat and by the time you get to number 4 lets say, you've lost a good amount. 

Also, how strong is the battery(s). Good idea to connect a charger on fairly high current to keep the battery at a peak point. Slow cranking speed will effect the results as well. 

Don't get me wrong this can be a good tool for catching big discrepancies in an engine, but if you find a bad cylinder it won't tell you where to look. 

A leak down test, in my opinion is paramount. It will tell you not only if the cylinder is "bad" but you can determine where the offending component lies, valves, or rings. 
The downside is you need a compressor. If you don't have one, obviously the compression test is what you have to use! 

A compression test is like getting your pulse taken, a leak down is an EKG. 
 

Edited by Ryan1776

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