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Old 10-22-2014, 10:20 PM   #1
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Default Oil consumption after head rebuild merc 5.7 2002 (maxum 2500 scr)

Hi All,

After I had an issue with my heads (2 of the exhaust valves got tuliped 370 hours) i had a mechanic remachine the heads and install stainless steel valves. After I picked up the boat the engine ran great but very shortly I've noticed that it consumes an oil. After doing something about 20 hours I got to conclusion that I'm burning about 1 qt every 5 hours which is a lot. I brought the boat back to the mechanic and left it with him. He called me back saying that after doing compression test it looks like one of the cylinders got lower compression (110 rest of the cylinders was fine) and thats why I'm using so much oil. He said that when engine failed originally its possible that besides head block also could get some damage. I didn't burned any oil before. I'm finding hard to believe that the engine burns 1 qt every 5 hours because one of the cylinders. I don't get any smoke while running the boat which in this oil burning ratio it should be smoking like crazy. No oil in the bilge. It smokes a little bit at the start but it goes away after a minute. Might be carburetor. I'm about to perform dry and wet compression test and just ordered leak down tester to investigate the problem. Do you have any advice as where to look? I'm thinking that maybe one of the valve seals is leaking badly and maybe instead getting inside the cylinder it gets directly into the exhaust and thats why I don't get any smoke. Not sure how to argue with the mechanic which obviously is not anxious to investigate the problem. Thanks in advance.
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Old 10-23-2014, 02:34 AM   #2
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A quart an hour would leave a cloud behind the boat all the time.

A leaking valve seal or worn guide would typically blow oil only when accelerating and not use near that much oil.

Worn piston rings blow oil all the time but having only one cylinder low and still above 100 psi just does not account for that much oil loss.

Try laying some oil asbor mats under the engine to verify no leaks.

I am assuming you are not getting any water in the oil, correct?

May want to pull the exhaust elbows to inspect for signs of oil.
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Old 10-23-2014, 02:51 AM   #3
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No signs of water in the oil.

I'll remove the water from the bilge and lay some mats but my bilge is clean and by this time I added about 5 quarts which in case of leak oil will be all over the place.

I was thinking about removing portion of the exhaust to see if I'll find any oil.

So far I can't find any reasonable explanation since the oil consumption is significant and I don't have any smoke.
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Old 10-23-2014, 02:58 AM   #4
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Do you trailer the boat? If so try running it on the hose at home and look for signs of oil the the exhaust water.

If kept in the water run the boat in your slip and look for an oil sheen in the water.

Possible leaking head gasket allowing oil to enter water passages, however I would think water would also get into oil if this was the case.

Were the heads checked for cracks when you had them reworked?
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Old 10-23-2014, 03:07 AM   #5
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I keep my boat in the water- no trailer.

I do get the sheen in the water but only after the start up and thought that it might be gasoline when engine running on choke. After some time sheen goes away. I might be wrong on that.

I did over 20 hours since the head repair and didn't see any sign of water in the oil.

Supposedly they did whole nine yards with the head including all new stainless steel valves etc. Not sure if I can trust the mechanic.
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Old 10-23-2014, 03:28 AM   #6
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Well if leak was in the heads or head gasket I would expect water in the oil pan. The reason is oil is delivered to the heads under pressure through the push rods once it squirts onto the rockers it is no longer under pressure and just runs into collector passages by gravity, from there the oil is channeled into passages in the block that run back into the crankcase.

If oil is leaking into the water yet no water into oil it must be when the oil is under pressure which is only in the block. Another possible leak could be an oil cooler but Merc did not one on it standard engines. There is a PS cooler but that would only cause PS fluid to leak.

So I guess I am thinking a crack in the block between a oil passage into a water passage. Not sure how to prove that nor do I think I was caused by the same issue that damaged the heads. Was the block drained and back filled with antifreeze last winter?
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Old 10-23-2014, 03:53 AM   #7
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The engine block was winterized by running antifreeze through it.

Is there any way to determine if the block is cracked? The engine runs fine besides oil consumption. I would like to take care of it but replacing an engine at this point sounds excessive. Is it possible that the possible oil which I see in the water comes from the exhaust?
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Old 10-23-2014, 01:04 PM   #8
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pull the spark plugs and see if any are oil fouled
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Old 10-24-2014, 10:06 PM   #9
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I did the dry compression test. Didn't have enough time to do the wet test. Cylinder #1 had 125 psi all the other between 170 - 195 psi. Im not an expert as far of judging the spark plugs but all of them looked bad to me. They had something which looked like burned oil on the tread. Is this some kind of anti seize lubricant or simply oil getting through the treads? Here's few images of the random spark plugs. All of them looked similar to me and had a build up. They were replaced when mechanic performed head job (30 hours ago). I'm leaning more towards the fact that he missed something during the repair. I just need to get enough of the arguments to prove ti and argue with him. At this point after spending close to 4k on the repair I'm getting really annoyed. I do get smoke at the start up. It goes away after a minute. I thought it might be because of the carburetor but I might be wrong. Next I'll perform wet compression test and leak down test. Not sure how to check valve guides and seals.
1.jpg3.jpg7.jpg8.jpg10.jpg
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Old 10-24-2014, 10:08 PM   #10
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Spurk plug #8 had a weird black deposit on ceramic part of the spark plug. I was wondering how it could happen.
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