Compression test - feedback

Boater71

Boater71

Member
Mar 5, 2018
321
Boston, MA
Boat Info
2008 Sea Ray 310 Sundancer
Engines
MX6.2 MPI Bravo III DTS with Axius and SeaCore
Kohler Genset
Looking for some guidance, did a compression test and results were: two at 100, one at 115, and three at 120. The others are 140 and 135. I'm looking to buy this boat and this is my initial test - will do another compression test after the sea trial in a couple of weeks. Engine is Mercruiser horizons 6.2 with 224 hours - 2008. This was out of the water test. The port engine broke the sparkplug so we could not so the compression test at this time. Oil samples being tested. Looking for guidance on this.

I figured they needed to between 140 and 150.
 
if the boat appears to be well cared for and the engine runs and sounds smooth and develops expected power i would not care about compression test numbers . if the engine has running issues walk away .

224 hours should be nothing for a 6.2
 
was this a dry or wet compression test?

cliff
 
who did the test? doesnt sound like it was run first....how long has it sat? are the two low cylinders next to each other? center? did the tester squirt any oil in the cylinders? my bet is rust on the valves/seats due to not being used or fogged when layed up.....this may "go away" when it is run, but obviously some damage will have been done necessitating a valve job at some point....i would probably either adjust my offer accordingly or cut my losses and walk a this point...
 
According to Mercruiser on a cold compression test all cylinders should exceed 100psi and the lowest cylinder should be within 70% of the highest. In your case it is close the low cylinder is 71% of the high. I did a compression test on my engine last year (Merc 4.3 600hrs) cold all the cylinders were between 120-125psi warm they were all 145-150psi. If the boat has been winterized and not run, it needs to be run first. Then I would do a warm test. If you still have that much difference between the high and low I would be thinking something is up with the low cylinders.
 
thank you for the quick responses. This was a dry test - boat has sat through the winter. It's a seacroe engine and not sure if it made a difference. The test was done by the shop mechanic while surveyor observing. I asked the surveyor to do oil samples and observe compression test so if there are issues, I would have shop address before i buy. Sea trial and another compression test after sea trial. Make sense?
 
Also make sure when you do a compression test the throttle is wide open if not this can drastically change the numbers
 
Boater71 said:
thank you for the quick responses. This was a dry test - boat has sat through the winter. It's a seacroe engine and not sure if it made a difference. The test was done by the shop mechanic while surveyor observing. I asked the surveyor to do oil samples and observe compression test so if there are issues, I would have shop address before i buy. Sea trial and another compression test after sea trial. Make sense?
Click to expand...

Yep, that makes good sense. Also, make sure it reaches W.O.T. on sea trial. Good luck.
 
I'd pass, healthy vortec headed motor should be in the 180 range. 150 for non vortec engines. A leak down test would give you more insight as to where your losing it. Cold or hot the leakdown is way more accurate
 
Strecker25 said:
The numbers will come up after its been run, I'm sure. How did the spark plug break?
Click to expand...
the mechanic was trying to take it out to do the compression test of the port engine.
 
techmitch said:
Yep, that makes good sense. Also, make sure it reaches W.O.T. on sea trial. Good luck.
Click to expand...
Planning on that with load. how long typically for a WOT on sea trial?
 
Plus throttle wide open have all the spark plugs removed.
And leak-down test is a much better indicator.
 
Doesn't need to be to long but at least a minute or 2. What is most important is that it gets to the spec. for your boat and engine package. with normal load. Also, pay close attention to your gauges.
Look for oil press. to be within spec. and not drop, temp to be in spec. and not rise.
 
Here are the results.
  • Cylinder #1: 100
  • Cylinder #3: 100
  • Cylinder #5: 115
  • Cylinder #7: 120
  • Cylinder #2: 140
  • Cylinder #4: 135
  • Cylinder #6: 120
  • Cylinder #8: 120
 
techmitch said:
Doesn't need to be to long but at least a minute or 2. What is most important is that it gets to the spec. for your boat and engine package. with normal load. Also, pay close attention to your gauges.
Look for oil press. to be within spec. and not drop, temp to be in spec. and not rise.
Click to expand...
Thanks. Will do for sure. Any other tools that i can use to get or capture readings during the ride?
 
Boater71 said:
Here are the results.
  • Cylinder #1: 100
  • Cylinder #3: 100
  • Cylinder #5: 115
  • Cylinder #7: 120
  • Cylinder #2: 140
  • Cylinder #4: 135
  • Cylinder #6: 120
  • Cylinder #8: 120
Click to expand...

100-140 range is too much IMO. This boat needs/is going to need soon, a valve job. That's not cheap. If it were me, I'd pass.
 
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