Compression Test on Mercury 225 Optimax

[lewis_is_bassin]

Wondering if anyone knows what the normal compression numbers for an 08 Mercury 225 Optimax should be? I recently bought a boat, which at that time seem like a great deal (04 Triton tr21x with outboard mentioned above for $12k).

At time of purchase, seller showed receipt from his mechanic with compression check on all 6 cylinders at 100 psi. After owning for two weeks, I decided to do preventative maintenance (spark plugs, wires, oil, fuel and air filters, etc.)  also decided to do my own compression check and purchased a Mercury VesselView. My compression check shows 80 lbs on 5 cylinders and on number 6 it reads 105. Mercury VesselView shows 820 hrs. Which such variables and hours, I think the outboard may have seen it's better days. Thoughts?

[Mike Cork]

80 is a little low but not unreasonable for a motor with that many hours. With the one being so much higher than the rest I have to wonder if it had been re-ringed not too long ago.

Good news is that with all of them about the same, the motor is wearing well. If there is no leakage, meaning it will hold the 80 on each cylinder for a couple seconds, I would be okay with it. Make sure to use your fuel additives and a quality oil to prevent build up and fuel line issues.

Has the motor seen better days? Sure. Does it need put to pasture? I don't think so. Continue to take care of it and there should be plenty of hours left. If the difference is an issue for you, you could re-ring all of them and put a new set reeds and crank bearings and it will be a new motor. I say driver it and enjoy for now.

[Princeton_Man]

80 seems pretty low. I would expect to see 110-120 on all of the cylinders.

[Princeton_Man]

80 is a little low but not unreasonable for a motor with that many hours. With the one being so much higher than the rest I have to wonder if it had been re-ringed not too long ago.

Good news is that with all of them about the same, the motor is wearing well. If there is no leakage, meaning it will hold the 80 on each cylinder for a couple seconds, I would be okay with it. Make sure to use your fuel additives and a quality oil to prevent build up and fuel line issues.

Has the motor seen better days? Sure. Does it need put to pasture? I don't think so. Continue to take care of it and there should be plenty of hours left. If the difference is an issue for you, you could re-ring all of them and put a new set reeds and crank bearings and it will be a new motor. I say driver it and enjoy for now.

Yup, number 6 might have been reworked. I guess with that many hours, one might expect to see lower compression.

[Bud Kennedy]

Lewis here is a guideline on how to properly test compression.  Did you follow these guidelines when testing?

When performing compression tests, it's imperitive that the tests be done with:

-A fully charged and PROPERLY RATED cranking battery
-Engine warmed up to Operating Temperature
-Kill switch OFF
-All Spark Plugs Removed
-Plug wires on Spark Tester (or grounded with jumper leads- an added precaution)
-Throttle plate at the WOT Position
-Water supplied to the engine while cranking!

Insert compression tester into #1 spark plug opening, and crank the engine until the reading no longer rises. Record reading. DO NOT spin/crank for longer than 10 seconds at a time.

Repeat above step for each cylinder.

[apenland01]

I was going to say the Optimax has special rules for doing a compression test and Bud pretty much covered it.

[Bud Kennedy]

I was going to say the Optimax has special rules for doing a compression test and Bud pretty much covered it.

I had these instructions in my files.  They originally came from European Marine in Greenville, Sc a well known high quality location for boat motor repairs.  Instructions are specifically for the Optimax

[lewis_is_bassin]

Thanks, all. Appreciate the feedback. Bud thank you for the information on proper testing. I did all, except check to assure the battery was fully charged and assure the throttle plate at the WOT position. I will try once again with these steps in mind. Additionally, I'll try using two different compression testers.

Wish me luck fellas. Hoping for 100 on all 6. Still the 820 has me feeling 😵‍💫.

[Mike Cork]

Don't sweat the 820, that just means she's a solid motor

[apenland01]

I was going to say the Optimax has special rules for doing a compression test and Bud pretty much covered it.

I had these instructions in my files.  They originally came from European Marine in Greenville, Sc a well known high quality location for boat motor repairs.  Instructions are specifically for the Optimax

Yep, I buy my parts for my Optimax from Don and Rhonda.  Our local shop at the lake went out of business (actually sold it).  I just replaced the oil pump on my 2004 Optimax 225 this past weekend.  Hope to get it out this weekend for a test run.....

[D.W. Verts]

You need to have a LEAK DOWN test done of you're concerned. It'll show you more than a compression test. Those numbers are as far apart as I like to see, but if the motor is running fine, i.e. idling, etc. then you should be alright- those are not extreme hours. Do a leak down test.

Dale

[lewis_is_bassin]

Update: Conducted a compression test again this evening-using Bud's recommendation and two different testers. Compression on all 6 cylinders ranged from 100-110 on both testers. Numbers on each cylinders were the same on both testers. Although these are still not ideal numbers, it make me feel a lot better. I'm going to take the advice of maintaining her well and running her till she blows (knock on wood).

[Bud Kennedy]

Good Deal Lewis.  Sounds like you are good to go for now.  Just be sure to keep up the maintenance and you should get continued good service from your Opti.   Dale is also right.  A leak down test would be appropriate for the age of your motor.  Just might save you some down the road problems and give you a better idea of each cylinder.