7.4 sucking water with new risers??

[tedmalone]

Help! I have a weird engine problem that even Mercruiser seems baffled by.

I have a 7.4 Mercruiser engine which my mechanic told me needed new exhaust manifolds and new risers. It wasn't getting any water in it, but there was a crack in the exhaust manifold and the risers were very corroded (my manifolds are raw water cooled). Since the other engine had sucked in water and seized, it seemed like a good preventitive idea to replace these exhaust parts on the other side.

After pulling the manifolds and risers, the good news was that there was no signs of water getting into the engine. He installed the brand new exhaust manifolds and risers and started the engine. It was running rough and belching smoke, so he shut it down and pulled one of the manifolds back off. Now, the engine is sucking water in. WTF?

It seems simple, but we can't figure out how the water is getting into the engine. Any ideas or help would be greatly appreciated!!

 

[tedmalone]

Hmm. Learning a bit more about this boat and this problem. This EC has underwater exhaust and the engines sit very low in the boat. Because of this, there's a funky muffler setup that tries to keep water from coming back in through the exhaust.

In addition, the risers sit on top of a 6" riser block to lift them up higher above the water line. Because the exhaust is raw water cooled (52 degree Bay water), it's hard for the engines to heat up to the point where they're pushing hot exhaust out the back. This leads to heavy, ongoing condensation inside the exhaust system.

Also, these engines are tricked out (forged pistons, high compression, roller cams and partially overlapping valve timing), which means they're very sensitive to temperature and back pressure that can increase the chance of pulling water back up the exhaust (supposedly).

I'm evaluating 3 possible solutions (maybe all 3).

1. Install something in the exhaust lines to try to eliminate the water coming back up the exhaust. Some sort of valve?

2. Upgrade the exhaust system to a custom manifold/riser setup that manages the water flow in a way that is further from the engine.

3. Convert the exhaust cooling to fresh water cooling (FWC) so it's a closed system that uses a thermostat to help the engine heat up quickly (and stay hot) and only uses the raw water in a heat exchanger.

Any thoughts or advice would still be greatly appreciated!

 

[GMAN1]

Check compression on engine. Low compression will actually cause a vacuum effect on the cylinder next to it (Thus water injestion). If compression comes out good then pull and inspect each plug. Usually if one plug is black and others are good it could be a bad injector on that one cylinder. If all of them are black it can be a sensor issue. Any time exhaust is replaced it is a good habit to check the compression. If low than u will probably need a top end rebuild. Good luck.

 

[GMAN1]

I also forgot. You stated u just replaced only the risers? Are the manifolds ok? I usually replace all of them as a set. Just an fyi If one goes bad the likely hood of the others will be shortly thereafter.

 

[tedmalone]

I replaced both of the manifolds, risers and 6" riser blocks.

 

[Chris Dauth]

About 18 months ago, I read an article about the design of the exhaust elbows for the 454's with the cam overlap. It was a technicial paper about pressure waves in the exhaust system etc. I got on to it because when I rebuilt my 350 up to 383, we put in a new 212 degree camshaft (up from 208 deg) and I was worried about sucking in water like the 454's.
As it turned out, the 350 elbows were OK with the 212 cam.

I have just searched again and turned up nothing. I cannot remember where I read that, getting old. :smt089
Apparently, the 454 elbows are different to the ones for eg. the 350 motors. It is in the way the water enters the exhaust gas stream.

All I can suggest at the moment is to see if the elbows you have are for the 454.

 

[Bt Doctur]

edit

 

[EricW]

How are the cylinder heads? Could a water jacket in a cylinder head have corroded thru?

 

[PlayDate]

Sea Ray and Mercruiser should not be "baffled" by this problem.....nor should your mechanic. I really find it hard to believe that replacing a set of manifolds on a working engine with identical new parts would result in water intrusion if the new parts were properly installed and sound.

I have seen "new" manifolds leak water because of an internal crack or improper installation.

In regards to changing your existing exhaust system.....the simple answer is don't do it. Get the right fix from Sea Ray for your boat. Water reversion is a known issue with known solutions if that is truly the cause of this water ingestion problem.

-John

 

[Chris Dauth]

OK Guys, after looking for days, I found the article.

If it doesn't solve the problem above, it will still give a better understanding of what is going on in the exhaust system.

Here is the url for the PDF paper.

http://www.michel-christen.com/ExhaustElbows.pdf

And a bit more here :- http://www.perfprotech.com/store/articles/marine-exhaust-tips.aspx


Here is a bit more :- The marine engine makers probably didn't anticipate that valve overlap — fine on land vehicles — would pose a problem on the water. And, the boat builders simply follow instructions given by Merc or Volvo or Crusader and are caught short when engines fail.

Read it all here :- http://www.myboatus.com/consumer/EngineBlocks.asp

From the Michel Christen paper, it would seem your 454 (with greater valve overlap) needs the elbows with the part number 864591T02. They all look pretty much the same on the out side, but you need the ones with the vacuum breaker.


I guess the mechanics just grabbed a pair off the shelf and banged them on, not knowing there is an internal difference. (See the cutaway photos)



The above links are a good read, hope it helps :thumbsup:


BTW here is another link to an article about the restrictor gaskets (often asked about in this forum). http://www.perfprotech.com/store/articles/warm-manifold-cooling-tips.aspx

 

[tedmalone]

Wow, you guys are amazing. I hope this advice helps other boaters as much as it's helping me. Thank you so much!

I'm off to check my elbows and will report back. One additional thing I heard from my mechanic is that many of these manifolds are painted after manufacture and they get paint on the gasket surfaces. These cannot be sanded. The paint must be carefully removed with a razor blade (or similar) to protect the surface. If you don't do this, apparently the gaskets don't seal right and you can get a leak.