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Mercruiser exhaust manifolds and risers: How do you know?

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  • Mercruiser exhaust manifolds and risers: How do you know?

    See explanation below, but how do you know that the exhaust manifold/riser is clogged enough to replace???

    1991 4.3lx Alpha One outdrive (rebuilt engine installed in 2000 and relatively low hours since). Boat is a Cobia 19' bowrider and is new to me 1 year ago. Boat has only previously been used in freshwater. Now 70% of time it is run in saltwater.

    I have been trouble shooting an overheat problem. I have done the following:

    - At end of last season, temp gauge was showing a trend of increasing temps at idle and while running. Gauge would read 180-just under 200.

    - As part of maintenance, impeller changed at start of this season. I have done many merc impeller changes and this is now all set. Previous impeller needed replacing, but not the worst that I have seen in the past. No missing pieces, but almost all of the blades and splits from the hub out to the end.

    - While the outdrive was off, disconnected the inbound hose from the t-stat housing and back flushed from there out into a bucket. Water flowed freely. No blockages and only some sand collected in the bucket.

    - After, temp gauge again read numbers as noted above, but now started to spike when run at higher rpms. Temp would cool down (sometimes to 165 or so) when returned to idle.

    - Picked up a infrared temp gun and looked around while on the muffs (forgot to bring it on the water). Once the engine was warmed up, temp at the base of the t-stat housing and around the sender read about 155-165. Temp on the risers was about 150 and manifolds (where they meet riser) was higher.

    - Since the boat is relatively new to me. I decided to tear into the t-stat housing (check ball type) and replace the t-stat with a new 140 t-stat. Interior of t-stat house looked more corroded than I expected, but I didn't see any blockages.

    - Took the boat to local water and no change in behavior (not that I really expected it). Unfortunately, I forgot the infrared gun to get some temps while in the water rather than on the hose.

    Again, not knowing the history of the manifolds/risers. I suspect I have some good corrosion in there. In the boat's maintenance folder, I don't see any history of any change outside of the time the rebuilt engine was installed. I'm assuming that these are 9+ years old. I have no experience with sterndrive setups in saltwater use (all previous boats had outboards), but from what I have been reading, it sounds like I'm do for manifolds/risers.

    Tonight, I took off the port side riser. There is some flaking blocking one port and the interior is clearly corroded. However, I was expecting to see more gunk. How do you really know if it is "time" for a change?

    Thanks in advance for your thoughts.

    Dan


  • #2
    Re: Mercruiser manifolds and risers

    Ayuh,... I live in Sweetwater,...Manifolds Freeze, but not really Rust,...

    Clean the gasket faces up real good,+ drag a Flat File across them,...
    It's usually the Gasket Face that causes Problems...
    But,..
    My motto is usually,... If in Doubt,... Replace it....
    Any Grease is Better,..... Than No Grease at All.......

    Comment



    • #3
      Re: Mercruiser manifolds and risers

      Thank you for your response Bond-o. I always learn when reading your posts.

      I'm not sure I follow you. Can you explain a little further? Thanks

      Comment



      • #4
        Re: Mercruiser manifolds and risers

        Ayuh,... Which part,..??
        Any Grease is Better,..... Than No Grease at All.......

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        • #5
          Re: Mercruiser manifolds and risers

          This part........'It's usually the Gasket Face that causes Problems...'

          What issues does this create?

          Comment



          • #6
            Re: Mercruiser manifolds and risers

            Originally posted by fishingdan View Post
            This part........'It's usually the Gasket Face that causes Problems...'

            What issues does this create?
            He's talking about leaks, not clogs....
            If the temp was good in the beginning of the season & rose towards the end after repeated use in salt water, I would replace them....
            That said, if you are getting scale (flaking) in the exhaust ports it will restrict flow out & raise core temps....
            You can try rodding them out, but it will continue to clog now that they are begining to degrade....
            It is really important to flush with fresh water EVERY time after salt water use.....
            After 9 years they are due for replacemnt, if they leak internally, you will be looking for a new block as well...
            Disclaimer!: In my inexpert opinion......

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            • #7
              Re: Mercruiser exhaust manifolds and risers: How do you know?

              Thanks Haut. That is the information I was looking for. If I'm tearing apart, I'm not putting the old ones back on given the uncertainty about their age.

              Dan

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              • #8
                Re: Mercruiser exhaust manifolds and risers: How do you know?

                Thanks Bubba. I have been flushing, but on the frequency that I previously followed for my outboards (ie not necessarily every trip).

                Comment



                • #9
                  Re: Mercruiser exhaust manifolds and risers: How do you know?

                  Some people have posted about good results from Trac Barnacle Buster.

                  This will be tried on my raw water system soon.
                  2000 380DA

                  Comment



                  • #10
                    Re: Mercruiser exhaust manifolds and risers: How do you know?

                    Originally posted by Bubba1235
                    Salt water use, figure on replacing the exhaust manifolds every 3 or 4 years depending on use. As others point out, flush the entire system after EVERY use.
                    I find 5-8 years about max life for salt water manifolds and risers. At 3 years, I start doing yearly riser removal and inspection. I NEVER clean them out.
                    If they plug up, they are dead. It's an early warning system before the manifolds and risers destroy the engine.
                    Don S.

                    sigpic

                    Please, no PM's (Private Messages) regarding boat/engine problems.
                    That is what the forums are for.
                    Only forum/moderator issues will be answered in PM's.

                    Comment



                    • #11
                      Sign up today
                      Re: Mercruiser exhaust manifolds and risers: How do you know?

                      Today, I changed the manifolds and the elbows. Not a difficult job, except for the weight of the manifolds.

                      The starboard elbows was pretty badly clogged. The port side was better, but when I took a closer look it wasn't in great shape either.

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