What did I just do? Exhaust flange / coolant leak?

[High Current]

Hi all,

I need a little hand-holding.  Is a potentially leaky exhaust flange gasket a risk to the engine, i.e. from coolant flooding the block?

I just replaced my motor mounts yesterday.  While lifting the engine, I had issues with the exhaust riser getting in the way.  After cursing about the extra gasket replacement work I'd be creating for myself, I tried unbolting the flange.  It appears quite new, and the nuts came off easily, but the flange wouldn't budge.  Then I noticed it dripping antifreeze and put on the brakes.  As the rest of you already know, the freshwater coolant manifold is also sealed by that flange, and I didn't drain it first...

I never actually pulled the gasket apart, and was able to stop the leak by re-torquing the nuts.  I was also able to replace the mounts without removing it after all.  My only concern now is whether coolant might be able to find its way back into the engine if the gasket is compromised.  Is a full coolant drain + gasket replacement necessary, or can I just keep an eye on it to make sure it doesn't start dripping?

Thanks!

[Ron Hill]

High : Yes, it's a surprise when you suddenly find out that that flange also seals the fresh water cooling (50/50 Preston mix)!! That has been written up a zillion times in posts and in the Mainsheet tech notes!!! 
If you only started to break that gasket seal and then retightened the bolts - you should be OK. Just keep an eye on it!!

A few thoughts

[Dave Spencer]

I agree with Ron.  I had my engine out and the exhaust flange off last fall to replace my transmission.  I replaced the gasket but what you described seems harmless enough.  Watch your coolant level carefully over the next few hours of engine operation to make sure it's not leaking.  I don't see any risk that the small amount of coolant that leaked could adversely affect your engine.

[High Current]

Awesome, thank you everyone for the quick help!

[KWKloeber]

Just so that you understand the mechanics of the "plumbing," if the gasket is compromised, the most likely occurrence will be that as coolant heats and expands, pressure (about 15 psi) in the closed cooling system will push coolant past the solid half of the gasket (that seals against the exhaust manifold water jacket) into the very end of the exhaust side of the manifold and out the flange/riser.  While that's possible it's likely a low probability because you didn't remove the flange and so just loosened the gasket seating against the exhaust manifold water jacket and didn't tear it.

The telltale of that leak is if your coolant level keeps inexplicably running low and/or you have an antifreeze odor in the exhaust.

(A dramatic coolant loss also happens if the gasket is put on backward -- as soon you start the engine, all coolant is pumped out the riser/muffler/exhaust hose.)

To have coolant from a leaky gasket get back into the engine, you would need to run/shut it down, turtle the boat (manifold upsidedown,) the closed system pressure keep pushing coolant past the gasket, coolant make its way up (now down) into the exhaust ports to an open exhaust valve, then right the boat so gravity can dribble coolant down into a cylinder.

[Noah]

Ken—It will take only slightly less effort than you described to "detect the smell of coolant in the exhaust". ;-)

[KWKloeber]

Ken—It will take only slightly less effort than you described to "detect the smell of coolant in the exhaust". ;-)

Noah and if he turtles the boat I suspect coolant in the cylinder maybe one of the last thought-of worries. then  then then then  

[High Current]

Hahaha!

Thanks Ken.

From my simplistic vantage the coolant reservoir appeared to be one of the highest points on the engine, so it wasn't clear to me what if anything would stop it from draining inward.  I confess I still don't have a good mental picture of how it's plumbed inside, but I trust what you're saying.

Before I posted I found plenty of posts about leaking flanges and none about flooded engines, so I guessed it was OK, I just didn't want to make a potentially disastrous assumption.

So much for my planned turtling drills though.  They'll have to wait until I can install a new gasket, just in case.