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Comments
the risers remain cool, but the manifold is cracking?
can you confirm whether you have dry or wet joint exhaust? the wet joints fill the manifold, and then water is pushed into the base of the riser through internal ports, to be pushed further into the exhaust stream after the bend. Dry joints are readily identifiable by having little 90* passage 'elbows' from the top of the manifold to the spacer/riser external to the riser (but sometimes formed into the manifold).
dry joint:
wet joint:
the exhaust cooling function can be seen in this cutaway:
this^ demonstrates a wet joint manifolds interior, and you can see the exhaust collector tower, the bosses the bolts that merge the riser to the manifold (adjacent the collector tower) and then the water passages outside those. A 'dry' works the same, it just has the ports on either side of the 'tower' as opposed to escaping through the tower casting.
a 'crack' in the internal exhaust runners will push water (in form of steam) out the exhaust as the pressure is greater at the head's port- UNTIL you drop that pressure. (in the form of RPM's)... then the 'steam' pressure is greater than the exhaust pulse's, and it will attempt to keep the pulse from escaping. This is something i suggested sometime back as a possible issue you were facing- as this will cause tremendous back pressure and will absolutely limit the RPM's achievable ESPECIALLY after you've been up on plane, back down, and attempt to re-throttle.
now when you come off plane, that 'steam' collects in the form of water this time and drains back to the lowest place, which is in the head resting on the exhaust valve seat, or, on the piston if the exhaust valve is even slightly open. hello, hydro-lock. that juice can't be compressed, and the cylinder winding up for the compression stroke in't going to be able to complete the rotation- pressing down on wrist joints, bearing caps, rings... trying to escape through gasket'ed surfaces, valve seats, even plug bosses. something has to give.
IF the breakage is close to the head's exhaust port, i would wager a guess that one cylinder will suffer more than others. this would be evidenced by being able to 'massage' the engine to turn while you're on the water... basically, you hope to catch the engine in exhaust or intake stroke- where intake won't allow but the lessor volume of air to be ingested (because the remainder of that space is occupied with water) or the piston simply pushes the majority of the water out on the exhaust stroke.
if the breakage is north of the center cylinders (4/6, or 3/5) the water can go where ever it wants, or equally dispense to all of them. understand the water is actually steam and makes collecting in the head/atop the exhaust valve more likely on a cooling engine. this is what sounds like is happening to you.
_________________
if I were you...
I'd remove the manifolds and scrape the surface clean with a plastic putty knife. i'd rub it out with acetone until i could clearly see the metal mating surface, and i'd inspect it for scorching..... water will scorch that polished mating surface and tell you precisely where it's leaking IF it's due to bad mating surface and NOT a cracked manifold. this could be due to not having a flat surface on either the heads (less likely) or the manifold (far more likely). IF the surfaces appear uniform, free of burs or nicks, and doesn't indicate scorching (or even if it does and you've cleaned it up), I'd do this, in this order:
carefully fold blue paper towels (shop towels; they're tougher) into squares and put them in the exhaust ports. get a can of spray gasket glue and spray the surface with a single even coat. while you're waiting for it to tack up, cut the heads off your old manifold bolts and thread them in their bosses 3/4 of the way... place the gasket on the extruding bolts and press it against the now tacky mating surface, and walk away for ten minutes..
upon return, remove the towels carefully as not to disturb the gasket. now turn attention to the manifold mating surface- again clean and acetone the mating surface. you don't have to spray this surface. you do have to be careful as you're sliding it on the extruding bolts you threaded in, though, and as you press it against the head... then remove a single center bolt and thread in a new one in its place... hand tight + 1/2 revolution with wrench. then the other center bolt, same thing... then outside ones always replacing the cut bolts with the new ones finger tight + 1/2 revolution with wrench... when they're all out/replaced, begin your torquing sequence.
do the same basic concept on the riser. make sure mating surface is clean+tacked+cut bolts+properly oriented gasket.. then seat riser on cut bolts, pull+replace one by one same way as above... four of them... properly torqued...
make sure your 'blue plugs' on the bottom of the exhaust manifold are in and are seated to the little o ring gasket... make certain all torque specs are right.. walk away...
come back after a day and make sure the torque specs are holding.... fire her up. run her at moderate pace... let her cool... check specs again. If they're still right, chances are you're good. if you had to tighten them, keep an eye on it- even consider gong to torque spec and adding a 1/4~1/2 a revolution beyond the spec as you might have a bolt that isn't stretching as expected (so, make it stretch, but don't go bonkers).
if you follow this procedure, and you feel good about all the mating surfaces, you're 99% likely to be able to walk away from the manifolds for 5~7 years (with proper maintenance)... if you don't feel good about a joint or a bolt, you know precisely where the issue will likely exist because you're familiar with it intimately at this point.
it's time, my friend, you take this thing in your own hands. if you twirl the wrenches yourself, you know more about that engine and where the issues will arise than if you don't. it's not that i don't trust techs to do the work, it's that until i'm the one threading in a bolt and feeling grits or resistance that shouldn't be that i know and 'map' the engine's 'weak' points mentally, and go straight to them when encountering an issue.
also.... think about this, please:
are those marine heads? have you checked the depth of the manifold bosses? are you bottoming out the bolts without achieving proper clamp? is there a chance something is occupying the bottom of those bosses? can you shave the length of the thread a mm or two and achieve better clamp?
Check depth of those bolt bosses. Make sure your manifold bolts arent bottoming out. You'd achieve torque, but not the way you'd want to... Steaming your exhaust makes all of it add up, from different tone at idle (fighting the backpressure) to decent midrange as the pulse overcomes the backpressure, just to encounter pressure the pulse can't overcome at higher rpms.
PC BYC, Holland, MI
PC BYC, Holland, MI
if there is a crack, and water has gotten in there somehow, it would most likely happen after running and attempting to start a warm engine... the steam settles, and into the heads or past an open exhaust valve into the cylinder.
realize this can also happen because of debris or even fluid (such as oil some folks insist to put on threads?)... it can literally hydro-lock the bolt in that case, and... it can show perfect torque specs but have hardly any clamping power.
if the decks on those heads has been shaved or even if you're reusing the bolts- they could have stretched just enough to slap bottom. I'm not saying this is 'your issue', but i am suggesting it could be. it's often overlooked.
PC BYC, Holland, MI
http://www.clubsearay.com/index.php?threads/warning-brand-new-oem-merc-manifold-casting-cracks.58912/
Interesting link!
PC BYC, Holland, MI