Merc 496 Mag Compression and Leakdown Results

[AShipShow]

Hi all,
I'm tearing into my new-to-me 496 mag (0W358880) and did a compression test and leak down test. The motor has been sitting winterized for about 3 years so wasn't sure what to expect. The motor is currently in a cradle in my garage, not in the boat

The compression results were from 165psi to 185psi so I'm happy with that, however the leak down test has me a little concerned.. all cylinders tested with right around 5-8% leakage but cylinder 6 was up near 15%.. I know that's not horrible but it's more the lack of consistency that worries me.
From what I can tell, the sound seems to be coming from the intake so at this point I'm assuming the intake valve is leaking. I tried rotating the motor a few times to see if maybe some debris or carbon was lodged in there but it still tested the same...

If the motor was used regularly and I did the test, I would assume there is a valve issue (bent/bad seat/etc) but part of me is wondering that since it sat so long, if that valve was in the open position, could it have gotten some surface rust or something?

What would you guys recommend at this point? Try to run it a little and redo leakdown to see if valves reseat? Or should I assume at this point I'm tearing into the heads?

 

[Scott06]

Hi all,
I'm tearing into my new-to-me 496 mag (0W358880) and did a compression test and leak down test. The motor has been sitting winterized for about 3 years so wasn't sure what to expect. The motor is currently in a cradle in my garage, not in the boat

The compression results were from 165psi to 185psi so I'm happy with that, however the leak down test has me a little concerned.. all cylinders tested with right around 5-8% leakage but cylinder 6 was up near 15%.. I know that's not horrible but it's more the lack of consistency that worries me.
From what I can tell, the sound seems to be coming from the intake so at this point I'm assuming the intake valve is leaking. I tried rotating the motor a few times to see if maybe some debris or carbon was lodged in there but it still tested the same...

If the motor was used regularly and I did the test, I would assume there is a valve issue (bent/bad seat/etc) but part of me is wondering that since it sat so long, if that valve was in the open position, could it have gotten some surface rust or something?

What would you guys recommend at this point? Try to run it a little and redo leakdown to see if valves reseat? Or should I assume at this point I'm tearing into the heads?

I would not tear into it at all based on those numbers. Is it possible the 15 % cylinder was not at tdc when taken and valve was not quite closed? Would definitely run it then retest if it makes you feel better but I think you are fine with what you have

 

[Scott Danforth]

I wouldnt tear into it.

 

[AShipShow]

Glad to hear others on the same page as me... especially looking and seeing what a new gasket set alone costs

Once I get done with a few other checks, I will try running it and retest the leakdown on cylinder 6... Hoping it gets a little better but even if it doesn't, I'll probably run it as is.

I know everything "marine" means its 5x more expensive, but I feel like these 496 motors tack another 5x on lol

 

[AShipShow]

I would not tear into it at all based on those numbers. Is it possible the 15 % cylinder was not at tdc when taken and valve was not quite closed? Would definitely run it then retest if it makes you feel better but I think you are fine with what you have

Yea, I was looking through the spark plug hole with a light as my wife was turning the crank and when it stopped moving I had her stop... This worked perfectly fine on all other 7 cylinders and I even tested cylinder 6 twice so I'm pretty confident the crank was in the right position. Funny enough though, I didn't have one quite at TDC the first time through and when I hooked up the leak down test, it made the wrench on the crank whip around because the pressure pushed the piston down lol

 

[Scott06]

Yea, I was looking through the spark plug hole with a light as my wife was turning the crank and when it stopped moving I had her stop... This worked perfectly fine on all other 7 cylinders and I even tested cylinder 6 twice so I'm pretty confident the crank was in the right position. Funny enough though, I didn't have one quite at TDC the first time through and when I hooked up the leak down test, it made the wrench on the crank whip around because the pressure pushed the piston down lol

to quote Jackie Gleason- that's an attention getter ...

I think you would be more lucky than right just bumping the starter. I suspect if the engine is out you have bigger issues than a little higher than ideal leak down, especially if the engine sat.

 

[AShipShow]

to quote Jackie Gleason- that's an attention getter ...

I think you would be more lucky than right just bumping the starter. I suspect if the engine is out you have bigger issues than a little higher than ideal leak down, especially if the engine sat.

LOL..

There are some definite issues that I'm starting to uncover..
The quick backstory.. bought this boat (28' Campion cabin cruiser) knowing full well it was a project... I pulled the motor mostly because I wanted to have better access to evaluate everything on the engine and it will make accessing everything in the engine compartment significantly easier... I also plant to remove the Kohler genset for the same reason, and the engine had to come out in order to remove the genset. I also want to give the structural members a good look while I have it out.

Anywho... I've already found the old style cool fuel 3 module was not drained and the housing is cracked, plus the whole potential paint delam issues still need dug into... The bilge must have had some water in it for a bit because the starter is super rusty (took some serious percussive maintenance to even get it to turn over for the compression test). The oil pan also has some pretty serious corrosion on it and the sea water pump is probably going to need a full rebuild...

So thats really the reason I'm starting with a good health check of the motor itself because if I found out there was a major mechanical issue, I would probably be rethinking the whole project, but at this point, I feel pretty good about the compression and leakdown, so I will probably start pulling the trigger on replacement parts ordering.

 

[Scott06]

LOL..

There are some definite issues that I'm starting to uncover..
The quick backstory.. bought this boat (28' Campion cabin cruiser) knowing full well it was a project... I pulled the motor mostly because I wanted to have better access to evaluate everything on the engine and it will make accessing everything in the engine compartment significantly easier... I also plant to remove the Kohler genset for the same reason, and the engine had to come out in order to remove the genset. I also want to give the structural members a good look while I have it out.

Anywho... I've already found the old style cool fuel 3 module was not drained and the housing is cracked, plus the whole potential paint delam issues still need dug into... The bilge must have had some water in it for a bit because the starter is super rusty (took some serious percussive maintenance to even get it to turn over for the compression test). The oil pan also has some pretty serious corrosion on it and the sea water pump is probably going to need a full rebuild...

So thats really the reason I'm starting with a good health check of the motor itself because if I found out there was a major mechanical issue, I would probably be rethinking the whole project, but at this point, I feel pretty good about the compression and leakdown, so I will probably start pulling the trigger on replacement parts ordering.

Was the bilge full over the engine ? If you have corrosion on the pan I would replace it while out. I think what my buddy who is in salt water did was replace the pan prime and coat with rhino liner or some bed liner material.

Get a new starter while its out. While the pan is off you can see inside the engine and if water was in there .

Are the stringers and transom sound ?

 

[AShipShow]

Was the bilge full over the engine ? If you have corrosion on the pan I would replace it while out. I think what my buddy who is in salt water did was replace the pan prime and coat with rhino liner or some bed liner material.

Get a new starter while its out. While the pan is off you can see inside the engine and if water was in there .

Are the stringers and transom sound ?

I don't know how full it was as it was empty when I got it but the starter is the only thing that is really rusty so I think that's as high as it got... But honestly, it could just be from normal corrosion too.. but it seems more than coincidence that most of the rust is on the seawater pump (lowest pulley) and starter..

Heres a couple pics of them removed.. FYI, seawater pump has lots of scoring



Before I bought the boat I did the best I could to inspect stringers and the transom... Without drilling holes, everything seems very solid and Campion has a good build quality reputation so I'm feeling pretty good, but I will be drilling a few holes in the stringers near the engine mounts and the transom.. Though the transom is a Penske (same thing as Coosa).

Anyways... Opened up the wallet last night and dropped another $800 on a starter and hardin marine stainless sea water pump