Compression Question

[bwilliams90]

Good morning,

New boat owner, so I'm going to try and explain this as best I can.

I recently bought a Wellcraft 170 classic(1989ish?) with a 3.0 MerCruiser engine.

Today I did a compression test with the following results.

Cylinder 1 - 180psi
Cylinder 2,3,4 - 150psi
(note, I just numbered these from first cylinder most forward moving backwards)

The reason I checked it. Yesterday the boat was working well, then after a final rip down the lake it wouldn't rev past 3200ish rpm even with WOT, when I started to slow down the engine finally quit on me and took about 20 min or so of sitting for it to fire back up. It did look like it blew what looked like white pudding out of the vent into the carb area when checked on the lake.

Last night I had it on muffs and it fired right up without issue and sat an idled fine.

I checked the oil and its slightly milky, its not white its still brown in color, but hazy is how it would be best described.

Any thoughts?

Appreciate any and all input.

 

[Scott Danforth]

re-check cylinder 1. you may have water or oil in it, or your gauge is reading 30 psi high and the following 3 cylinders are a bit low.

spec is 150 psi for cranking compression numbers.

however since you mention your oil is hazy, I would change it.

 

[bwilliams90]

re-check cylinder 1. you may have water or oil in it, or your gauge is reading 30 psi high and the following 3 cylinders are a bit low.

spec is 150 psi for cranking compression numbers.

however since you mention your oil is hazy, I would change it.

Thank you for the reply, I did re-check cylinder 1 again after I got the other 3 readings and it still came up with 180. After a little research I have read about sea foaming that cylinder to maybe decarb it? However I don't want to do this if its not a safe or recommended thing to do.

 

[alldodge]

With cylinder 1 staying at 180, I would guess the head gasket blew or there is a crack

 

[bwilliams90]

With cylinder 1 staying at 180, I would guess the head gasket blew or there is a crack

That's disappointing to hear.... Do you have any suggestions how I may be able to confirm this without having to tear the head off?

 

[alldodge]

Pull plug 1 and see if its wet. Can also spin the motor over without the plug to see if anything spits out

 

[bwilliams90]

Pull plug 1 and see if its wet. Can also spin the motor over without the plug to see if anything spits out

As suggested, I pulled plug one and it is dry, also turned engine over without plug and nothing is spitting out. Also just pulled all the plugs and through the compression tester on them again. This time giving them all a solid 6-7 turns.

cylinder 1 - 180psi
Cylinder 2 - 170psi
Cylinder 3 - 175psi
Cylinder 4 - 170psi

this makes me feel a little better that the head gasket is fine and maybe the engine just had a little episode yesterday and coughed up some old junk that was sitting in it. I’m going to change the oil today and try it in the lake and hope for the best!

 

[alldodge]

With them all reading higher then 150, then it could be the gauge, so all good

 

[Scott Danforth]

I would go with the gauge reading high.

agree, it would be good.

 

[Kubaat3lover]

I have a question regarding wet test , how much oil should we put to the cylinder on V6 Merc? take care

 

[Rick Stephens]

Do the oil change, and then recommend you do all the regular maintenance items. Those tend to be skipped when an boat gets old to a previous owner. Skipped items tend to rear up and cost thousands to fix if left until something breaks. Get yourself the right Mercruiser manuals if you are going to keep the boat longer than a couple weeks. You don't say what year or serial number, so can't venture which manual is the correct one.

Your starting issue sounds like a fuel problem. Probably needs a carb rebuild. Do you know how long the boat sat before you bought it? Could the fuel in the tank be cruddy? Have you checked fuel filters, poured the contents out into clear container and looked?