The “flood” occurred when the engine was started from stone cold. So I don’t believe it’s a heat expansion issue.
Last night I checked my muffler, apparently some neighbors have had issues with Muskrats climbing into the exhaust, but no luck.
I’d throw a new riser at it, but they don’t make them anymore so I’d have to do all new risers an manifolds of a different style.
I’d call PCM and ask them for advice.
My problem seems to be solved, and it's a doozie. I pulled the manifold and riser at least a dozen times, went so far as to pull the entire exhaust system to the transom and pressure tested everything in the world(block, head, manifold, riser).
All this mess was caused by 2 CROSSED SPARK PLUG WIRES.
Wires for 8 & 2 were swapped. Seems this created the perfect condition for the exhaust from the opposite side of the engine to backfeed into the problem side, bringing lots of water with it. I'll be the first to say, this sounds ridiculous, and I can't articulate it well myself. However, 8 & 2 swapped created a flood... 8 & 2 in the right place.... dry as a bone.
omg! glad it's fixed, but what made you check the plug wires? and when / how did they get crossed? I'm guessing that was the last thing that was touched before the flooding happened....
have you done any repair - replacement work to the engines? Heads reworked? Head gaskets? new short-blocks or Long-blocks?
If you have checked and double-checked your exhaust manifolds and risers and found no problems...… There could be an issue with the exhaust valves staying open too long and literally sucking water back into the cylinders at idle speeds. But something would've had to change, such as the aforementioned engine change, head work, etc.
You started your question with "repowered with 454's" so I'm going with this idea.
If the engines were built or rebuilt to "automotive" specs, I would think the exhaust valves are a little too tightly set. Your 454's should be an hydraulic lifter engine and most engine builders will set the valve lash to 1/2 to 3/4 turn past seated at the rocker arms while twisting the push rods with their fingers. While this gets you the most valve-lift from the camshaft and a nice quiet running engine. I call this "automotive spec rebuild/reman). But this is absolutely a No-No with marine engines with wet exhaust.
This "tight" setting of the valves results in a nice quiet running engine but can allow overlap of the opening and closing of the exhaust and intake valves. This is not usually a problem at running speeds as the exhaust pressure is enough to keep the water pushed away from the engine and out the exhaust. BUT, at idle speed and low engine rpms, this valve overlap will allow suction to happen as the piston is partially in the intake stroke before the exhaust valve closes. This can and will literally suck water backwards from the riser, thru the open exhaust valves and eventually into the crankcase oil.
And on a large boat, you run at idle and low engine rpm's much of the time, such as the "no wake zone", docking, locking, etc. So there is plenty of time for this to happen.
I recommend, if you have run out of other causes, to pull the valve covers and loosen the rockers on the exhaust valves by 1/3 to 1/2 turn and see if this fixes your problem.
By the way, pull the oil dipstick and see if your oil looks like chocolate milk. If so, there is too much water in your oil. Do an oil and filter change before running your engines.
I ran into this issue years back with Jasper remanufactured engines. Even though they sell "Marine" engines they are still built to "automotive" specs and need the exhaust valves loosened before putting them in service.
PS.... I saw the "cross-wired" plug post. But the water ingestion is still due to too much valve overlap. I would still loosen the exhaust valves to prevent water ingestion.
Capt Mark Hunter, Ex-1973 Commander 41 Flush Deck owner. But I do work on my Dad's 31 Commander a bunch.
I appreciate all the info.
The problem was the valves, but maybe not for the reasons you had in mind. My engines are Pleasurecraft Marine SeaMaxx 7.4L MPIs.
I did the math and the way the wires were crossed, my 8 and 2 cylinders were firing at bottom dead center just before the exhaust stroke. This resulted in sucking the water back through. Correcting the wires completely eliminated the water problem.
Since then I have changed the oil twice and replaced all of my manifold gaskets. All seems to be well. Had it out Saturday for about an hour and a half and she ran better than I’ve ever seen. Very briefly(30 seconds) had them up to all most 4000RPM with no vibration, stumbling, backfiring.