Rocky buildup in exhaust manifold?

[scrumper]

Hoping the brain trust here can help. The rest of the internet can't. What am I looking at here? It's kinda hard, almost rocky. It looks like calcium (and it fizzes a little in vinegar so I think it is) but... how? Seawater leaking past the block-off gasket and creeping into the manifold leaving deposits behind?

Backstory is I pulled a riser in the course of chasing a coolant leak and investigating some light rusting around the riser/manifold gasket. It's a little up inside the riser too, but not really. Makes me think it was coming in through the gasket.

I pulled plugs, cylinders are dry. Risers and manifolds were "just done" before I bought the boat in 2020, so in theory they are 4 years old. Sadly I think the flat surface on the manifold is too pitted to recover so I plan to change the whole thing.

 

[370Dancer]

dried salt in my experience. you're getting reversion, or a leak

 

[scrumper]

Thanks for the response - 370Dancer. So I just learned about reversion because of your post - there's a good article and a fun video here: https://www.hardin-marine.com/t-what_is_exhaust_reversion.aspx and a nice solution here: https://forums.iboats.com/threads/exaust-reversion-problem-solved.706475/

The salt deposits pictured in the second article look exactly like mine. So not calcium, but sodium chloride.

Fascinating that all is, but from the state of the gasket I'd still guess 'leak' first. The riser itself is pretty rust-free on the inside surfaces (the water jacket & out-pipe are grim and they'll be replaced. The riser on the other side of that engine has no leaking or rust around it, so I'll pull it and see if I've got the same problem. If so: reversion! And I'll hammer in a stainless steel tube to make the problem go away, like the guy in the second article did for his customer.

 

[scrumper]

You may be absolutely right here with the reversion theory, 370Dancer. I just had a look and the gasket I pulled didn't have the turbulator lip on it. Might've corroded away, might have been a cheap 3rd party gasket. Either way that little lip which collects reversed water (and condensation) wasn't there. And you see the results.

Should I assume that manifold is toast? Or can I chip the salt away and refinish it. It's a closed cooled manifold so I actually do expect it to last a long time.

 

[Brite Idea]

If they're closed cooling what's with the rusty brown water in the cooling passages? IMO I'd be tossing those manifolds, risers and all associated parts in the garbage! If you're not having engine trouble yet you dodged a big one!

 

[scrumper]

Ha yeah, I am going to replace. But the brown stuff is actually red, the lighting is weird. It's good red dexcool, just looks like ass in this photo.

 

[scrumper]

I pulled the riser on the other side of the engine and the manifold doesn't have that salt build-up but is rusty and - agh- there's little pools of water (not coolant) in each of the four wells inside. So that's getting replaced. The exhaust run is a different shape on that bank, and from what I am reading that does suggest reversion.

The risers are watertight, I have tested both, so I don't think that the risers are leaking into the manifold. Could be the gasket. Once again, the turbulator lip is missing; I am wondering if it's corroded away or if the PO used cheaper knockoff gaskets without the turbulator.

 

[bfernald0]

The exhaust portion of the manifolds should be black with soot, with no rust or salt deposits. You've apparently got both, which means salt water is getting in there. Since you've got freshwater cooled manifolds, it's not weeping through the steel or coming in through the wet joint, so it must be coming in through the exhaust, which means you've got a problem with the exhaust set up--based on the build up, this is not a one time occurrence. According to your profile, you've got 6.2 MPIs in a 310 Vanguard Express, if I'm reading it correctly, which means this was a repower. I'm betting you don't have sufficient down angles from the risers to the exhaust tubes, which in that boat I'm betting are not water lift mufflers, but rather the old horizontal collector set up. The installation manual for your engines will specify how far above the waterline the risers need to be and the downangle from the risers to the exhaust tubes. Generally, you'd have a 4-6 inch spacer between manifold and riser in order to ensure sufficient riser height and exhaust downangle to prevent water from backing up into the manifold. A turbulator is really only there to catch condensation running down the inside of the riser into the manifold. Based on that salt build up, this ain't a condensation/turbulator issue. There are also at least two service bulletins on this issue--not for this boat, but they may be helpful. Mercury's is No. 2001-13. SeaRay issued one for a number of models in October 2001--#02.07. If you PM me an email address I'll email them to you. If I were you, I would get someone that knows how to properly install marine engines and exhaust, as I'm betting whoever repowered this boat did the install wrong, which is why you're getting water in the manifolds.

As you probably know, water in the manifolds leads to corrosion on the valves, which leads to compression issues and if it gets bad enough a stuck valve, hydrolock and all sorts of horrible stuff. I would assess the entire engine installation and exhaust system before just throwing a new set of manifolds at it.

 

[bfernald0]

Sorry, based on the photograph you've got a dry joint manifold/riser set up. If there is no spacer installed between the manifold and riser, I'd be very suspicious. Not knowing the riser height relative to the waterline, it's impossible to assess via chat, but again I'd have someone that knows what they're doing take a look.

Just took a look at Merc's installation manual for the 6.2 mpi horizon. There are six pages devoted to all of the exhaust measurements needed to ensure you don't have a water inversion issue.

https://www.manualslib.com/manual/1546125/Mercury-Mx-6-2-Mpi.html?page=26#manual

 

[scrumper]

bfernald0 - thanks for the detailed and thoughtful reply! I'm about to send you a PM for the SBs, I'd really appreciate seeing those. And, yeah, no surprise it's a repower done by the PO in around 2004.

Attached is a photo of the installation taken shortly after I bought the boat.

- I posted a photo of the outside manifold (closest to the stbd. hull), the exhaust run on that one is closer to standards - a long straight pipe before the Y fitting. There was no water, but of course plenty of salt build up.
- The centerline manifold I pulled yesterday, which had water and rust in it, has a very non-compliant exhaust run with a a tiny 8" straight run from the elbow to a fiberglass bend that then goes to the Y pipe. You can see that in the photo.
- I'll take measurements of waterline-to-top-of-elbow height but I can tell you it's going to be close to the 13" requirement, over or under.
- There isn't room for a 3" riser in there without raising the entire cockpit. I'm not really in a position to do that or build out a whole new exhaust system so I think, depressingly, this might be the end of our boating career. And that's assuming the engine hasn't suffered damage from the water.

Could you explain a bit about how water gets sucked back into the engine? It seems like it's more of a danger at idle? I don't really understand the mechanism, since the pipe is pointing downwards. Where's the vacuum come from? I see some recommendations to run up to 2,000rpm for a minute or two before shutting down, to blow it all out. Perhaps some sort of mitigation is the best we can manage.

 

[bfernald0]

I've frankly only heard about this on big block V8s, but during the course of normal engine operation at idle, you have brief periods of vacuum if you have any overlap in the opening of the intake valves and closing of the exhaust valves. If you look for videos on Youtube. They use a clear exhaust tube and you can see it in the water. Those brief periods can create a siphon like back pressure, which is exacerbated if you don't have proper elevation on the risers. Looking at your set up, I don't see ANY downangle on your exhaust set up. The other thing I find odd is the straight shot out of one side of each engine, and the angled exhaust on the other side. Generally, you want to keep the exhaust runs even on both sides, so you don't get pressure differentials. In any event, I received your email and will send you the bulletins, but I would DEFINITELY have someone that knows what they're doing look at this set up. It doesn't look right to me, but I'm not an expert by any means.

 

[bfernald0]

Emailed you the two bulletins and a copy of the 6.2 mpi installation manual.

 

[scrumper]

Thanks again. Yeah I am going to call in some expert help. You're describing reversion, at least as I understand it.

Immediately now I am focusing on saving the engines. I assume the port engine is in the same state, so I am pulling manifolds and risers, will crank over the engine with plugs out to make sure there's no water in there, squirt a little oil in the cylinders, crank again, hopefully stopping any corrosion in its tracks.

 

[bfernald0]

That's a good idea. I would test the oil and I would also do a compression check on both engines. Hopefully you don't have low compression on any of the cylinders, but unfortunately I wouldn't be surprised if you do. The cylinders toward the stern will be most susceptible as the water naturally runs downhill and thus the exhaust valves toward the stern will suffer the worst. I dealt with this on a 7.4 mpi. In my case, it was after market manifolds. The water literally weeped through the steel and bubbled in the exhaust portion. Did a deep dive at that time on all of this crap. I had elevated sodium in the oil and lower (not fatal) compression in the cylinders toward the aft. Luckily, I caught it before major damage was done.

 

[bfernald0]

Make sure you THOROUGHLY plug those exhaust lines once you get the manifolds and risers off. A very easy way to sink the boat by leaving them open and unattended.

 

[bfernald0]

Avoid cheap, Chinese manifolds! Water literally weeping through the porous iron.

 

[scrumper]

That's astonishing. It's amazing to me that something as simple as cast iron can have varying quality like that. How can water seep through solid iron?!

I pulled the manifolds tonight and the outboard side - the one with the salt deposits in my first post - was bone dry, carbon black inside, other than the salt it was textbook. The inboard side where my exhaust run is too short was soaking wet and there was water in three of the cylinders. I cranked the engine over, oiled the cylinders through the sparkplug holes and exhaust chambers, cranked again, oiled again, and stuffed rags in the exhaust ports and put the plugs back. All the exhaust valves were operating.

I'm going to work on the port engine next on the assumption that the same exhaust design is causing problems there.

Once the engines are working well I'll need to think about how to mitigate this badly designed exhaust.

Cautious fingers crossed that I can save the engine without a teardown or rebuild, but it's too early to tell.

One other interesting thing. I took a photo down the exhaust pipe. Every component of this is above the waterline - even the discharge port in the hull side is a few inches above. But here's what looks a lot like water sitting in the pipe high up in the engine room! It flows out fine when the motor is running, so why's this not running out... Must be a flat spot (or it's just a shadow.) To investigate.

 

[scrumper]

Treating this thread as a bit of a diary on diagnosis and repair; maybe it'll help someone in the future.

So far I've pulled manifolds and elbows from both engines. The left (port) banks of both engines had water in cylinders, not the two inboard banks which was what I was expecting given the exhaust design.

Best theory now is:

- reversion prone exhaust design
- low idle caused by clogged IAC mufflers (fixed that back in June)
- aging elbows with reduced seawater flow as rust began to clog the tubes
- lot of recent idling around

all conspired to cause a bit of seawater suction on two of the four cylinder banks. I was lucky it wasn't all four.

I have purged the water and oiled and filled the wet cylinders with corrosion inhibitor to preserve the engines while I get ready for recommissioning.

Next steps are:

- see why water is pooling in the starboard exhaust, and try to understand the whole system better
- change oil
- compression test
- new plugs
- reassemble with new manifolds and elbows, fill coolant, check hoses etc
- pour out a mug of very good scotch for Poseidon and fire up the engines

 

[bfernald0]

Glenmorangie 18

 

[scrumper]

Yep that's not a bad choice. In my bar now is an Oban 14, and a 25 and 10 year old Scapa (well worth seeking out if you can find it - not easy, I've had my stash for years), and a Hibiki which while not technically scotch is absolutely wonderful and fully deserves appreciating alongside any other single malt. Poseidon will receive a generous slug of Oban.