Exhaust Steam
 

I have a 502 MPI in my boat and have never had any cooling questions or problems until yesterday. When we got back to the marina to pull the boat out, we noticed steam coming out of the starboard exhaust tip. After I tied off, I raised the hatch and found the starboard exhaust manifold was much warmer than the port side. It wasn't overly hot, as I could put my hand on it for 10 seconds or so without burning, but it was hotter than usual. I also looked at the exhaust tip from the dock and it had approximately the same amount of water flow as the port side. We pulled the boat out of the water and started cleaning her up. When I went to the back to clean the transom area, I just happened to open the rubber flap and took a look inside. Looked like the exhaust flapper was closed. I stuck my hand in and hit it with my finger and it popped loose. It was stuck in the closed position. Haven't run it since then, but will run it on hose or lake and see if the problem still exists. By the way, temp gauge was normal. Impeller has about 8 hours on it, but has been in since last year.

If the flapper was stuck closed, I would think it would create some back pressure, so would that cause the affected side to be hotter than the other side and cause the steam? What kind of damage could that do?

Hmm, thats odd. With the fresh impeller, and comment that it looked like the same amount of water was flowing out on both sides, not sure how it could be stuck closed. They are under spring tension of sorts to be closed when water flow is not passing it on way out to tip. When not running, both sides should be closed. I would maybe hit it with some rubber friendly spray lube (just try to hit the hinge), and try again....

Oh, just saw you are in Houston. Im in Katy...., boat exclusively at Canyon Lake. My boat is kept in my garage at the lake. You?

It may have just been me, but it didn't feel like it was under any tension. When it popped loose it seemed to flip open and closed freely. Unfortunately, I didn't check the port side at the time. I will check both again when I go back up this week. I think I will just back it in the water and let it run till it gets up to temp and check the manifolds again, then check to see if the flapper is working correctly and hopefully, it doesn't get stuck again. Will hit them with a little bit of lubricant first.

I live NW just north of Jersey Village. For now, keep my boat at Conroe and boat there most of the time (during the week). Haven't been to Canyon since my college days. We have a lot at the Legends on the river just above LBJ. Hope to build and move there in a year or two.

Pretty common starboard exhaust on Mercs to run hotter. My 454 does

Exactly. My 502mpi's do it as well. It is just more noticeable with cooler Spring water and air temps

Even though it seems to be normal, I don't remember the starboard riser ever being that hot before. I will take my IR temp gun up next time I run it and get some numbers.

Would it be worth the effort to check the first cooler downstream of the sea water pump, just on the off chance it could be clogged up? How many coolers are there on a 502mpi? Merc manual shows 3. Power Steering, Fuel and Oil. It looks like they are in the prior order on the diagram, but I don't really know yet.

As others stated it is common and there is probably 20 threads covering it. But do your due diligence and make sure you don't have blockage. I believe this why there is the little spring loaded ball setup out of the thermostat housing on some engines.

Mines the same normal for the 454 mpi raw water cooling, its routed to the left side first then on to the right and picks up a little heat, should be ok.

I have read all the threads that I can find about the issue and there are plenty. However, there are a bunch of differing opinions as to what the problem is. Everything from normal to cool water and weather to trash in the cooler to impeller to blown head gasket. I really don't know what to think right now.

Guess my main question right now is, and I am just brainstorming (fishing), could an exhaust flapper stuck in the closed position cause the exhaust gasses to be trapped and unable to escape thru the tips and consequently heat up the manifold and risers, even with the proper amount of water to cool them? Sorry, not a very technical question, but I don't know any other way to ask it.

Fix known issues first and then go from there if you have to.

And yeh, if you have some time, alays good to check frt of oil cooler and such for debris...especially if you barely every have or never have.