2002 Diesel Mercruiser 4.2L 220 AC Exhaust Riser / Elbow

[darren.m]

Hi All. Im new to inboards and have just bought a 2002 Maxum 3000SCR with Twin Mercruiser 4.2L 220 AC Diesels. I’m not a novice when it comes to engineering having been a mechanical engineer in industry for 30 odd years, but I have very little marine engine experience which is frustrating me. So, I’m after some advice.

I noticed a small trickle of water in the engine bay coming from the Starboard engine down by the transom. It turned out there is a hose, about 1 1/2" diameter that connects onto a hose tail which is part of what I now understand to be the exhaust riser. This riser is cow horn in shape and comes from the Turbo. Upon investigation the hose tail is badly corroded and the hose was not sealing. After wriggling into the near impossible space for my large frame I managed to clean up both the hose tail on the riser and the hose. I then put the hose back on and tightened the clip, all now seems to be water tight but who knows for how long. I have no history of these being changed.

Questions:

1. I assume the boat needs to be out of the water to change the riser as water is probably sitting half way up the riser / bellow?

2. I guess the riser/s should be changed. I’ve read various comments from every 5 to 10 years. She sits in sea water on the south coast of the UK. What’s your opinions here.

3. The hose going to the hose tail on the riser, it looks like this is part of the closed cooling water, I did notice that the closed system expansion tank was out of water twice before I found the hose leaking. I’ve now topped this tank back up and am hoping it holds water. Where the hose fits onto the tail, does this go through into a jacket or waterway inside the riser, I’m assuming it does but I can’t find any diagrams on the web

4. Are there after market versions of these risers available as the Mercruiser ones seem expensive at around $700 so about £500 each

5. Is there any other advise you can offer?

Thanks for reading. Darren

 

[Highfive]

Hi Darren, I'm no expert, but I do my own maintenance/repair work and I've two of the 2.8L versions of this engine in my Searay 290 and previously owned a 4.2 Version, also in a Searay.
1. On both of my boats, I could remove the riser with the boat in the water. The water level came up about 2 inches lower than the top of the exhaust tube below the riser. I suppose it depends on the boat, but I would imagine they are designed like that, because if the water level was much higher, it would be in danger of flooding the motor.
2. That 5 years might apply to the risers on the petrol Mercruisers, as they are cast iron and rust like hell in salt water, I know, I have one of those too, in a smaller Searay! The ones on the Diesels are aluminium.
3. Yes, I think that pipe goes into a water jacket. It's where the cooling water and exhaust gasses mix before they exit the boat, hence,the term "wet exhaust"
My expansion tanks seem to be alyways empty but the level in the heat exchanger is fine, and I've run it like that for years, so I'm not worried about it
4. I have seen stainless ones for sale but, for BIG bucks!

If it were me, I would undo the clamps, and wiggle the riser around a bit, to make sure water wasn't going to get in. If it does, it won't rush in in an uncontrollable torrent, but just spill over the top of the exhaust tube, and you can always jam the riser/snail back on and do up the clamp.
Any decent engineering shop should be able to weld on a new flange/stub if they have the riser on the bench.
My worry would be, what will the rest of the riser be like, if the tube stub is that bad?

Mark

 

[simonfraser]

if you boat is on a trailer, rinse it all out with fresh water to slow the corrosion.
def have that repaired or replace the part, seawater slopping about in the engine room is not good
did you rinse the engine after spilling salt water over it ?

 

[Andy Cox]

Darren
On some of these engines the expansion bottle is actually a catch container designed to take excess coolant blown out of the system and not a reservoir allowing coolant to flow both ways.

Have a look on the container and you may see something embossed in the plastic along these lines.

Agree with Mark re the 5 year rule on the manifold and riser. That relates to the petrol units.

Cheers
Andy

 

[darren.m]

Hi Mark. Many thanks for your comprehensive reply. You've given me a little more confidence than I had. Ill have to experiment with breaking the seal on the rubber bellows and see what happens, I might even attempt to take a few measurements first to see where the water line sits from outside to inside. Thanks, Darren

 

[oldgit]

This might be bit dated now but worth a look, explains a lot, even after a decade or two, the grief lives on.

http://www.yachtsurvey.com/exhaust_risers.htm

Cardinal rule of the marine industry appears to be , if a mistake has been made, why bother to learn from it !

Lots of after market stuff around at sensible prices.

 

[darren.m]

Thanks Simon for your reply. Yeah Ill be getting the part replaced once I understand it and grow my confidence a little more. Also I need to try and source one, maybe an aftermarket one. Thanks, Darren

 

[darren.m]

Thanks Andy. I think the expansion tank is part of sealed system as it has antifreeze in. Thanks, Darren

 

[MapisM]

if you boat is on a trailer, rinse it all out with fresh water to slow the corrosion.

Good suggestion, even if the OP did specify that the boat "sits in sea water".
In fact, it is possible to rinse the cooling circuit also with the boat in the water, because the raw water coming from the pickup of the Bravo outdrive (which I suppose is what the OP has in his boat) flows inside the engine bay through a hose, eventually connected to the engine.
In between, it is possible to install an "L" operated 3-way valve, with a connector for a fresh water hose.
An easy modification, well worth the installation effort - which depending on the engine bay space can actually be relevant, mind!
But with such setup in place, rinsing the whole cooling circuit is just a matter of connecting the fresh water hose, turn the valve towards it, and let the engine run for a while.
Once done, the valve can be left closed, hence leaving only fresh water inside the heat exchanger and exhaust manifold+riser.
There's only one, but very important, warning: before turning the engine on again, whenever the boat will be used after rinsing the engine, never forget to open the valve and let sea water flow!

PS: while fitting the rinse connection, it might be worth considering also the installation of a sea water strainer, unless already fitted by the builder.

 

[Riggy]

Hi Darren, I'm no expert, but I do my own maintenance/repair work and I've two of the 2.8L versions of this engine in my Searay 290 and previously owned a 4.2 Version, also in a Searay.
1. On both of my boats, I could remove the riser with the boat in the water. The water level came up about 2 inches lower than the top of the exhaust tube below the riser. I suppose it depends on the boat, but I would imagine they are designed like that, because if the water level was much higher, it would be in danger of flooding the motor.
2. That 5 years might apply to the risers on the petrol Mercruisers, as they are cast iron and rust like hell in salt water, I know, I have one of those too, in a smaller Searay! The ones on the Diesels are aluminium.
3. Yes, I think that pipe goes into a water jacket. It's where the cooling water and exhaust gasses mix before they exit the boat, hence,the term "wet exhaust"
My expansion tanks seem to be alyways empty but the level in the heat exchanger is fine, and I've run it like that for years, so I'm not worried about it
4. I have seen stainless ones for sale but, for BIG bucks!

If it were me, I would undo the clamps, and wiggle the riser around a bit, to make sure water wasn't going to get in. If it does, it won't rush in in an uncontrollable torrent, but just spill over the top of the exhaust tube, and you can always jam the riser/snail back on and do up the clamp.
Any decent engineering shop should be able to weld on a new flange/stub if they have the riser on the bench.
My worry would be, what will the rest of the riser be like, if the tube stub is that bad?

Mark

I am far from being au fait with the technicalities and knowledge shown by you chaps on here but having had to get up to speed very quickly and going by recent experience on my 2007 V. Penta 4.3 petrol engine, I can only say if the riser stub is badly corroded and has no maintenance history, get it changed immediately as the inside of the riser will be far worse. The riser on the starboard side appears to take the brunt of the corrosion for reasons yet unknown to me.
However, when I bought my (9 years old) boat 18 months ago, the seller guaranteed that all maintenance was up to date and engine in tip top condition. Now and obviously, the risers were not part of his maintenance remit and I am paying the cost of a complete engine rebuild after seawater has invaded/flooded the engine. At the very least, with my new found knowledge, I would have both risers off and check for corrosion rather than risk knackering the whole engine. It may make a difference if the risers are ally, I don't know, but mine are cast iron.
Only yesterday I stripped (with the guidance of a good German mechanic) the engine down and took the heads off. What greeted me was a sad and expensive sight.

 

[simonfraser]

A sad sight
As far as I understand this is why you’d want a heat exchanger on sea boats so salt water is less likely to cause major corrosion to the internals of the engine ?