Outboards, compression and cold temps

[Thunderyacht]

I had a survey done on an 01 Osprey 250 with twin Evinrudes 150s today. The temps were below freezing, the engines fired right up, but the compression was low.  Compression was consistent across all cylinders, but was lower than expected.  I haven't gotten final report so I don't know actual numbers. Could the cold affect compression numbers?  Surveyor did say adding oil to cylinders brought compression up do I'm thinking rings, but want to eliminate cold too. He said they ran great for the little bit they ran them.
TIA

[wingtime]

Without a doubt a cold engine will have lower compression than a warm one.  A true compression test should be done at operating temperature.  The key you found  out here is not the number of the compression but the fact that it was consistent across all the cylinders.

[fitz73222]

First off Thunder,
What were the numbers? I've never checked compression in sub freezing temps on cold engine but it would stand to reason that numbers would be lower. A warm engine test would make sense to get a more accurate numbers. What are the chances of both engines having identical low numbers? Also, don't think for a minute that compression gages don't go bad either. I once lent a compression gage to a buddy that I found out later had been dropped on the ground and it never read above 110 psi again and that little O-ring where is screws into the cylinder is the only seal you have between a good reading or a bad reading depending on the condition. The proper method to test cylinder health is a leak down test where each cylinder is brought to TDC, the flywheel is locked into position to keep it from moving and the cylinder is charged with say 120 psi air pressure and then the leakdown rate is measured over one minute and a 5%-7% pressure loss is considered normal and healthy. Thats what your surveyor should be doing for his money...

[wingtime]

For aircraft piston engines we do a differential compression test.  This is similar to a leak down test but is different.  The cylinder is brought up to TDC (this is easy just turn the prop most of them are direct drive!).  Then a compression tester is attached to a spark plug hole.  The tester has two gauges and a regulator.  The first gauge reads the pressure of the compressed air you are supplying to the cylinder which is typically 80 psi. The other gauge is the one that is read for the test resluts.  It reads the amount of pressure the cylinder is holding.  This is not a leak down test since there is a constant supply of compressed air being supplied to the cylinder.  In a perfect world you would get a reading of 80 PSI.  But this is impossible.   Oh and you need at least two or more people to do the test.   One guy to run the tester... and one or two guys to hold the prop still.  Let me tell you 80 PSI in a 80 cubic cylinder will put one heck of a load of the prop if your just a tad past TDC!