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TOPIC: blow by - is this normal for a rebuilt mercruiser?

blow by - is this normal for a rebuilt mercruiser? 30 Sep 2017 16:22 #1

  • hookemdano
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I recently had a rebuilt merc 5.7(non vortec) installed in my boat...

is this normal blow by for a rebuilt? this was between hours 65.7 and 76.4 on this engine...yesterdays fishing trip...see pic

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1997 Bayliner Trophy 2352
5.7 Mercruiser/Alpha1/Full Closed Cooling
San Diego, CA

HookEmDanO out ......

blow by - is this normal for a rebuilt mercruiser? 30 Sep 2017 16:44 #2

  • Centerline2
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a newly rebuilt engine will have more blow-by that what an engine with a few hundred hours on it will have. the rings arent seated yet and they are the single determining factor on how much blow-by an engine has...

but also, under the valve covers where the blow-by vent is, there is a separator pod that must be intact and not damaged, as this is suppose to let most of the oil separate out of the steam and gasses that are venting. if this pod is damaged it cant do its job properly and so the gasses will carry more oil out of the engine.

I wouldnt even begin worry about the blow-by until after 35-50 hours, but even then if it seems like more than normal, it could clean up as the engine breaks in more... but after 150 hours if its still has a lot, there is a ring problem.
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1989 Bayliner 2556, 5.7 OMC Cobra

blow by - is this normal for a rebuilt mercruiser? 30 Sep 2017 17:12 #3

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Centerline2 wrote: I wouldnt even begin worry about the blow-by until after 35-50 hours,


I stated in my post that this was hours 65.7 to 76.4....so well past the 35-50 hours

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1997 Bayliner Trophy 2352
5.7 Mercruiser/Alpha1/Full Closed Cooling
San Diego, CA

HookEmDanO out ......

blow by - is this normal for a rebuilt mercruiser? 30 Sep 2017 17:36 #4

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hookemdano wrote:

Centerline2 wrote: I wouldnt even begin worry about the blow-by until after 35-50 hours,


I stated in my post that this was hours 65.7 to 76.4....so well past the 35-50 hours

yep, seen it but forgot.

if you could get a filter of some type on the tubes, you could tell if if one side is producing more oil that the other side...

when running on the muffs (or in the water), at a fast idle of 2500rpm, holding you finger over one of the vent tubes, is there a lot of pressure or "exhaust" from the other tube?
is the crankcase overfull by any chance?
its possible that just the way the oil is being splashed around by the rockers that an abnormal amount of it is getting into the separator box.... this is not common but not unheard of either.

I would look at all possibilities, as its rare that anyone who has the smarts to put an engine back together would install the wrong rings or incorrectly... but again, its not unheard of..
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1989 Bayliner 2556, 5.7 OMC Cobra

UPDATE: blow by - is this normal for a rebuilt mercruiser? 03 Oct 2017 22:11 #5

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UPDATE

I took the boat into the shop today...my mechanic said that this should not have happened...he is going to perform a leak down test and do a compression test....more info later

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1997 Bayliner Trophy 2352
5.7 Mercruiser/Alpha1/Full Closed Cooling
San Diego, CA

HookEmDanO out ......
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