Mercruiser 8.1 - Do your risers need replacing?

[juggernaut1]

Five years ago I had my manifolds replaced and reused the ss risers thinking they lasted forever. Fast forward to two weeks ago and the Merc tech pulled a couple of risers which showed evidence of rusting in the manifolds. He said the risers could be reused.

I inspected the risers and only one of them showed even exhaust soot in the riser. The other three showed something had been washing off the soot in certain parts of the inlet of the riser. I have not had the risers pressure tested but made the decision to replace all of them anyway. We started up the motors yesterday and one thing I noticed when the motors warmed up was that the digital tach showed a rock solid 650 rpms at idle and the very slight hunting had disappeared. Before the recent replacements the idle rpms on the tach would fluctuate slightly and the motors would slightly hunt.

I guess the point of this post is that a fluctuating idle rpm potentially indicates water in the exhaust system and possibly from your risers.

The other thing I noticed with the 14 year old risers is that they rust flakes in them, presumably flowing from the manifolds and which may have been blocking the riser water cooling outlets as one of my exhaust hoses had blistered internally, yet was fine externally.

I will have my risers tested and report back.

FYI I had the 700 hour motors cold compression tested which generally averaged 190 psi with the lowest being 175, which was interestingly the same cylinder on each motor.

 

[SeadawgVB]

I run in salt water and my 2003 360DA has been in salt water its entire existence. I swapped out my stainless risers last year. They do not last forever, but these seemed serviceable @ about 410 hours. A marine mechanic told me that where they fail is down near the flange, it starts to pit and corrode eventually allowing raw water to dribble straight down the exhaust manifold.

15 years in a saltwater environment is probably the max life expectancy..... Wish I could get 15 from the manifolds!

 

[juggernaut1]

I agree they would fail where the riser is welded to the flange. My turbulators did not look flash either which I suspect water leaked out of the flange weld and sat in the turbulators. Two of my turbulator/gaskets were also weeping externally which is what initially prompted me to dig further.

The positioning of the water inlets on the risers also means some salt water will sit in the risers at shut down.

I'm now keen to explore a freshwater flush setup plumbed up to one of the rear hatches for easy access.