MerCruiser Compression Test Results

[Lightwin 3]

Hello All,

Looking for some opinions.

2003 MerCruiser, 4.3L (190-carb) Serial#: 0M047305.

This boat is a project and I don't want to go any further if the engine is toast.

I ran a compression test today and this is what I observed.

Starboard side:

1. 175 PSI (dry) 175 psi (wet)
2. 115 PSI (dry) 125 PSI (wet).
3. 165 PSI (dry) 165 PSI (wet)

Port Side:

Exactly the same.

Notes:

1. This engine has unknown hours.
2. The engine has the original "Bat Wing" exhaust manifolds.
3. I do not believe it was ever run in salt.
4. When I say "wet", I mean a healthy dose of fogging oil down the carb. throats.
5. All tests done with carb throats wide open.

What do you think?

 

[alldodge]

IMO your compression is fine and so long as there is nothing else wrong (cooling channels, oil passages) with the motor, its good to run.

Don't understand the bat wing phrase on a Merc, they were used on only OMC to my understanding.

 

[Lightwin 3]

IMO your compression is fine and so long as there is nothing else wrong (cooling channels, oil passages) with the motor, its good to run.

Don't understand the bat wing phrase on a Merc, they were used on only OMC to my understanding.

Great big one piece manifolds. Newer versions have the manifold, riser and down spout as separate parts.

I don't think M/C used them very long. Which, in itself, makes me wonder

 

[Scott Danforth]

Mercruiser use the prone to failure bat wings for about a year

 

[alldodge]

Mercruiser use the prone to failure bat wings for about a year

Today was a great day, never knew Merc used them.

Suggest use these as scrap iron or lawn art

 

[Maclin]

Lightwin 3 Did you have all the spark plugs out when testing? Seems like you know that drill. The 2 cylinders with the 125psi wet are a concern to me. May indicate a needed valve job in the near future. I think I would do a leakdown test on those 2 cylinders to discover where the leak is, valves(s) or rings or headgasket.

On the batwing things, I looked at a 1999 model boat with the 4.3LX that had those, they sure surprised me when I saw them. I disqualified it almost immediately.

 

[Lightwin 3]

Those are it! I think I'll scrap the boat. It is aluminum so I'll probably get my purchase price back. See thread link below.

https://forums.iboats.com/forum/boa...epair/10036078-2003-fisher-bow-rider-aluminum

 

[Lightwin 3]

I'll start parting it out this week.

 

[Maclin]

Ok, but it may just need a valve job, it would be good to know what a leakdown test shows. If you are not familiar with it, a leakdown test is not difficult to perform. There is a fitting you screw into the sparkplug hole that an air compressor connects to. You pressurize the cylinder it and observe see how long it holds, while listening for air escaping into the intake (carb) and exhaust manifold, and the oilpan.

Hissing at exhaust or intake probably means just needs the heads refreshed along with new gaskets of course. Hissing into the oil pan usually means worn rings or worse, and indicates an overhaul is needed.

Now if you are worried about the batwings, that may be a gamble regarding water leaking into the cylinders. Replacing those may be pricey added to the heads rebuild.

 

[Lightwin 3]

Ok, but it may just need a valve job, it would be good to know what a leakdown test shows. If you are not familiar with it, a leakdown test is not difficult to perform. There is a fitting you screw into the sparkplug hole that an air compressor connects to. You pressurize the cylinder it and observe see how long it holds, while listening for air escaping into the intake (carb) and exhaust manifold, and the oilpan.

Hissing at exhaust or intake probably means just needs the heads refreshed along with new gaskets of course. Hissing into the oil pan usually means worn rings or worse, and indicates an overhaul is needed.

Now if you are worried about the batwings, that may be a gamble regarding water leaking into the cylinders. Replacing those may be pricey added to the heads rebuild.

Thank you. I know what a leak down test is..

To be honest, I've just lost my interest in this project and boating altogether. Too many idiots on the water.

I have more fun taking things apart than rebuilding them.

I figure $500.00 worth of paint and carpet (plus purchase price of entire rig) will be recovered at the scrap yard and a nice trailer to sell.

Thanks all for your help! Much appreciated.

 

[fishrdan]

Bummer, I wouldn't get too discouraged with the low numbers, it's fixable, but depends on how much you want to sink into it. Just getting rid of those batwing manifolds = BOAT.

If you need to rely on a mechanic to fix the engine (most of us here don't) it could get expensive real quick. I wouldn't let that stop me on the project, just another bump in the road. Do some further diagnostics to determine what the issue is, people here are willing to help...

 

[fishrdan]

Too many idiots on the water.

I deal with the same thing and avoid the masses, pick and choose the times I go out and avoid crowded days like the plague.

 

[Maclin]

Thank you. I know what a leak down test is..

To be honest, I've just lost my interest in this project and boating altogether. Too many idiots on the water.

I have more fun taking things apart than rebuilding them.

I figure $500.00 worth of paint and carpet (plus purchase price of entire rig) will be recovered at the scrap yard and a nice trailer to sell.

Thanks all for your help! Much appreciated.

Gotcha, gotta know when to hold or fold

 

[Lou C]

I might not scrap it that fast. For one thng how is the rest of the structure? How is the transom, decking and overall aluminum construction? If these factors are all good, it bears looking into the engine further. Also, how did you do the comp test? Did you warm the engine up and keep the throttle wide open? If not those results are not really accurate. Also a wet comp test involves putting motor oil in the plug hole not fogging oil in a carb/intake. Lastly I'd remove the manifolds and look in the exhaust ports, see what the exhaust valves look like. If you see nice ligtt grey carbon that's good. If you see rusty valve stems, then you may need a valve job. Fresh water or salt water use? Merc made a conversion kit to get rid of those manifolds. I had the OMC version of the one piece V6 manifold on mine for many years (almost 30) and never had a problem though.