Exhaust Troubles

[Gilded_Splinters]

Good Morning,

Had some trouble last night on a slow cruise with my wife.

I have twin 1990 OMC Cobra 5.7s. We cruised at a relaxed 1600 RPMs for a while, the boat was running nice and cool. There was a sudden tapping sound that seemed to indicate trouble. Within seconds the boat was billowing smoke from in the engine compartment, in the cabin, etc.. The boat immediately lost power in the starboard engine and I shut them down. Mind you this all took just a few seconds.

I opened the hatches. let the smoke clear and ensured that we were not on fire. What I found was a roasted rubber exhaust connector, the one attached to the riser. The tapping sound was the exhaust leak apparently. I have to assume that there was no water there at all, but I was bringing water on board via the raw water pump and the other side was intact.

The really odd thing is that the temp gauge read only a few degrees above normal. The gauge and sender do work.

After things cooled down a bit I briefly started that engine and the exhaust leak notwithstanding, it didn't sound too great, nor was it running well. The oil seemed OK, not burnt smelling or in appearance.

I'm going to get back in there tonight. Any thoughts? I suspect the manifold or an obstruction between the thermostat housing and the manifold.

I should mention that I put new risers on it last year as they were very clogged, but the manifolds looked OK at that time.

 

[Bondo]

Ayuh,.... Donno why, but 'bout the only way to burn up the rubber parts in the exhaust is from a lack of water flow,....

 

[jerryjerry05]

If the water doesn't flow and fill the water passages.
The sensor might not register how hot it's getting.
IF??? You changed the risers??? You should have changed the manifolds too.
The top where the exhaust goes through and into the riser. It can look clear but the corrosion/ crud/sludge settles to the bottom.
You still have the flappers in the exhaust tubes?
They melt and drop to the bottom cutting off flow and can cause overheating.

Start with checking the impeller and then the rest of the system.

Unfortunately a lot of people throw trash overboard and it ends up shutting off the water flow.
"Catching a bag" happens and is hard to figure out unless the bag gets caught in the prop.

 

[Gilded_Splinters]

"You changed the risers??? You should have changed the manifolds too.
The top where the exhaust goes through and into the riser. It can look clear but the corrosion/ crud/sludge settles to the bottom".

I agree. They did look clear as much as you can see. I will be doing the manifolds now, but am concerned with the condition of the engine in the aftermath of this event.

I'll do a compression check I guess, but what else would you all look at to ensure that the engine isn't shot before I start bolting parts onto it?

Thanks

I have to ask: You mention that crud and corrosion settles to the bottom. The bottom of the manifold has a nipple on each end, which as you know gets capped at the back. I flushed them out and water freely flowed from front to back, but of course that doesn't mean much as to the condition of the other passages within the casting.

 

[Gilded_Splinters]

Well, last night's compression test was encouraging. Good numbers in all cylinders and oil that appears in good condition.

Also, spark plugs came out dry with no evidence of water in any cylinders.

Ordering manifolds and exhaust rubbers today.

I'm getting married on this boat on August 1st, and the plan was NOT to do that dockside.