Mercruiser 140: Water in oil, Low Compression (1 Viewer)

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Jun 9, 2003
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Mesa, Arizona
We have a 1984 Caravelle with a Mercruiser 140 engine. This is the 4 cylinder GM engine, with an Alpha 1 outdrive. The engine serial number is 6045155. This boat has been in the “family” since 1986. It was owned by a close friend until 95 when I bought it. I sold it back to the friend in 2005, and last month he sold it to my brother. When he sold it to my brother he said it wasn’t running well, and he had paid for a tune up that did not cure the problem.

We took the boat out to the lake. It was hard to start, but it started eventually. After running it for a few minutes we noticed that the vent hose from the valve cover to the carburetor had slipped off the carb, and there appeared to be a lot of steam coming from the hose. After a few more minutes these steam started looking like a brownish oily mix, and we went back to the dock. We went home and changed the oil. The oil was a brown watery mess.

I suspected the problem was the exhaust/intake manifold. I know that I replaced the riser some time in the early 2000’s, but the exhaust manifold has never been changed. But I also thought it could be cracked block or something else, so we ran a compression test. We warmed up the engine, and did the test. We got dry compression values of (starting from the front of the engine) 55, 125, 105 and 150. We then added a bit of oil to the first cylinder and the compression value jumped up to about 65.

One more thing—when we were doing the compression test we noticed that there was a decent amount of what looked like water squirting out spark plug hole of the back cylinder.

At this point we are not sure what to do. My best guess is that we have a leak in the exhaust manifold somewhere near the back cylinder. I am inclined just to replace the exhaust manifold and see what happens. I have another boat that had low compression in one cylinder and that boat has ran just fine for 10 years.

The boat does not have a lot of value. I am not sure what it would cost to fix the low compression, but it might exceed the value of the boat. OTOH, it is a fun little boat when it is running well, and given how much we use it, the engine might last a long time just like it is.

So what should we do? Should we just replace the exhaust manifold and see what happens? Or is it foolish to not deal with the compression issue at the same time? And should I replace the riser again?

Jared
 
Joined
Jun 13, 2010
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It likely has a cracked block if the oil is mixing with water and milk shaking it out. If you run it hard it will eventually blow that mess right out the carb, it is quite the spectacle. You can visually check the block for external bulges, cracks, or popped freeze plugs to confirm it got a bad winterization job.
 
Joined
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Messages
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Mesa, Arizona
Thanks for reply. We are in Arizona, and it does not get cold enough here to do that. It may get below freezing at night, but I understand that it takes longer than that to crack a block.

Wouldn't a bad exhaust manifold also let water into the oil? Or am I wrong about that?
 
Joined
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Messages
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Interesting, I only have experience with places that it freezes! From my experience in the small boat business it would take a couple nights of below freezing temps to crack a block (in the 20's). Did you check the freeze plugs or block for visible damage? It's a dead giveaway for freeze damage. With the low compression on the one cylinder I would do a leakdown test and see where the air goes as a cracked manifold will not mess with your compression numbers. I would tend to think that if the manifold is filling up a cylinder with water it would hydrolock before the water gets past the rings and into oil, but I can't say that with 100% certainty.

You could always check Weather Underground's history for past local temps for where the boat was stored, maybe one winter it dropped for a couple nights in a row, if it is stored in the shade of a barn that often doesn't help either as it will stay cold in the boat during the day too. Just throwing ideas out there.
 
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Jun 9, 2003
Messages
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Mesa, Arizona
New Update (almost a year later):

First, we pulled the exhaust manifold. We found signs of rust in the exhaust manifold at the 4th cylinder, but I couldn't find evidence of leaks in the manifold itself.

Next we pulled the head and took it a machine shop. They found a crack in the head-- it was near one of the valves on the third cylinder. There was also sign of a blown head gasket between the 3rd and 4th cylinder.

So what is the best way to go about finding a new head? Are they still making new heads? What about a source for redone heads?

Again, this is a 1984 Caravelle with a Mercruiser 140 engine. This is the 4 cylinder GM engine, with an Alpha 1 outdrive. The engine serial number is 6045155. I think it is the 2.5 liter engine, but I guess it could be a 3.0. I don't know how to tell the difference.

Thanks

Jared
 
Joined
Jan 19, 2017
Messages
174
Location
Apache Junction, AZ
I am a marine tech. Get a hold of S&J motors. They will have a head for you. I think can get a merc long block from them for about 1800. I have all the books at work for that motor. If you need anything, feel free to contact me. Also the GM 4 bangers tend to crack on the outside of the block before they crack internally. Im guessing something else gave up the ghost. Ive seen cracked blocks produce textbook spec compression.
 

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