txtraveller2002

I have a 98 32. By recommendation, I put 4 inch, 90 degree rubber downturns on the exhaust tips to mitigate some of the hull vibration at idle. They worked awesome, BTW. Also, I had a riser give out a couple of weeks ago, so i have new-ish stock merc exhaust on one motor. This is the first weekend that I have had it that I have really run it ballz out for an extended amount of time.
I have noticed that now, after a run (even a short one) that the cuddy smeels very stongly of exhaust. Enough that someone would probably pass out if they sat down there with it closed up. I have rechecked my work on the manifolds, and no leaks on those, or the various tubes and pipes going to the silent choice through the hull. I don't see how the downturns could be making any more back pressure than the pipes straight out, and even if they were, where would it be coming from? Most of the exhaust joints for the risers to the through hull are all wet joints, so I should be seeing water coming out as well. I know that the gasses are pushing up the drain tubes through the step and into the cuddy, so it's not a question of how it's getting up there, more of where is it coming from. Ay ideas would be greatly appreciated.

Interceptor

I'd suggest taking the down tubes off and running the boat to confirm the condition exists with or without the down tubes. With the down tubes fumes may be trapped under the platform and are somehow sucked back into the boat and migrating into the cabin.
ed

txtraveller2002

That's where I'm going to start. I was really hoping to keep them on there, just because of the vibration issue, but not worth it if that's what's causing the issue. I'm going to pull them and try it next weekend.

Sydwayz

Do you still have the large vent holes on the back of the boat? (edited, see below post)

Be VERY careful with this. I'd take a battery powered CO detector with you. It's probably obvious that it's CO; but the CO detector will tell when the PPM are too dangerous to be near.
A member here on OSO had a very bad CO scare with his boat a few months back; damn near killed him.

boatnt

do NOT tape those holes shut ...they are the vent holes for the blowers...

Sydwayz

My bad if they are the blower exits, but I've seen others when they holes have been removed, especially on the 32.

Could the blowers be wired backwards; sucking air in--exhaust?

txtraveller2002

It does it with the blowers off and on, and only under power...it seems I can idle in and out of the marina all day long without being able to replicate the issue. And this is the first weekend that it has happened. Like I said earlier, the only 2 things that have changed are the downturns, and the manifolds, and I can't find any leaks in the exhust system inside the boat.

Sydwayz

Its probably traveling up the gunwales somewhere. I don't the exacts of the Fountain layout, but can you find some way to block off the gunwale inner air pathways at the bulkhead? I wonder if the Fountain depends on this for fresh air for the engines. Anyone know?

txtraveller2002

That's about all I can comee up with too. I know there are some drains somewhere that help move water in from ot the step, but they are under the floor. I thought about trying to plug up the 2 small holes between the engine bulkhead, but I probably am better off trying to figure out where it's coming from in the first place. I rechecked the manifold bolts one the one that I replaced, and they were still tight. I kinda figured that if I had a manifold leak, I would be a ble to hear it pretty well.

Sydwayz

Do you have any rubber hoses between your manifolds and tailpipes? No leaks there? Rope gaskets?

Are your tailpipes sealed well as they pass through the transom? With the way those particular Fountains sit in the water, trapping the exhaust gasses under the platform, those gasses want to go somewhere.

Pure speculation here, but when you had just tips, perhaps gasses were escaping "better" out the back of the boat; than now with the rubber tips installed. You could have (CO) air pumping back into your bilge and then forward while off plane; sheerly because they those gasses are "circling" under the platform rather than heading straight aft--reentering the bilge around your tailpipes.

Do you get water in your bilge when idling? You could have water saturated with CO entering your bilge also around the tailpipes; and the CO is moving up to your cabin once the water is in your bilge.

There is a 38 Nortech that was built with virtually no cabin, side by side engines, and a molded in platform similar to the Fountains. It was purely a race boat. When idling, it would build up so much pressure it would actually lift the transom up and burp the CO/exhaust out the back of the boat every 30 seconds.

Just thinking out loud on the keyboard.

Next step might be running the engine(s), with the SWP disabled; (single belt removed, or if a serpentine, remove the impeller; reinstall SWP; and run the engine for just a minute or two)...
...while someone stands at the transom with two Nerf footballs lightly pressed into the tailpipes creating backpressure in the hopes you might find an/the exhaust leak.