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Thread: Exhaust manifolds..replace or not?

  1. #1
    Okay, here's the deal. I've got a 1995 Merc 350 Mag/EFI MP with Bravo 1 drive. This combo has been flawless for the past 8 years. Motor now has 520 hours on it. ALWAYS starts first try, even after sitting for months on end. I've done all the routine maintenance such as oil/filter changes, fuel/water separator changes, change outdrive oil etc. Anything beyond that I have a shop do it (i.e. pull the outdrive to lube the u-joints etc.)
    There are some things I haven't touched. I haven't touched the spark plugs. Original plugs from the factory are still in and she doens't miss a beat. I had the original sea water pump (and impeller!) on until a week ago when it finally let go when I was putting in at Willow Beach (Lake Mohave).
    I kind of figured I was due for a new seawater pump and bought a replacement pump just before the trip. During the launch I make it a point to have the hatch up and check the belts and hoses and check for copious water leaks right after the engine start. As I was pulling back up to the dock, I noticed the engine temps coming off the peg (120). I found this odd since it usually takes quite a while to warm up, esepecially in the cold waters at Willow Beach. A quick check over the back of the boat and no water was coming out the tail pipes! YIKES says me. I was only 15 ft. from the dock so I quickly shut the motor off and tied up. Temps never got above 130. Motor was only at idle for about 60-90 seconds.
    An hour later I had the new pump installed and boat back in the water. The original pump impeller came out looking like it went through a blender. After I got home I pulled all the block water plugs and exhaust manifold coolant plugs and drained the water and flushed the system several times. I had one of the elbows get a little warmer than the other prior to this and I suspect some of the impeller may have lodged in a passage. Water flow and temps are all normal now.
    This leads me to the exhaust manifolds. The manuals say they should be serviced (rodded out) or replaced every 200-300 hours. I've got twice that amount of time on these puppys. Should I go ahead and spring for a new set of manifolds and elbows or just "leave a sleeping dog lie"? I'd hate to pull the manifolds only to find they're in great shape and no replacement is needed. But at the same time I don't want the motor to eat water and cause a major malfunction.
    If I do replace them I'd like to stick with stock Merc stuff. I can't justify the price of the after market pieces plus they don't offer any performance improvements for my SB application. After looking at the Merc parts website I noticed my particular exhaust elbow part is no longer available which is an added rub.
    As a side note, I had the pleasure of meeting Mr. Barron, of Hallett fame, and several of his friends. Real nice folks. The group had 3 boats doing their annual Laughlin run. One guy had an absolutley mint 89 Hallett 270T, another a Cigarette (didn't get more details) and Mr. Barron was launching another 260 AC with a sick (in a good sense) sounding Pfaff EFI motor. Good thing I didn't have the check book cause the 260AC was apparently for sale.
    Sorry for the long diatribe...

  2. #2
    just my 2 cents...i would think if it runs that gd...leave it alone...maybe chng spark plugs for next season...also u could pull ur manifolds and ck them for ur own satisfaction...otherwise...i would just leave it as is...sounds like it runs very well and ur haveing fun with it

  3. #3
    just my 2 cents...i would think if it runs that gd...leave it alone...maybe chng spark plugs for next season...also u could pull ur manifolds and ck them for ur own satisfaction...otherwise...i would just leave it as is...sounds like it runs very well and ur haveing fun with it

  4. #4
    Dennis Moore
    Check out my website for exhaust manifolds.
    I worked for Nick Barron for 9 years as his engine and drive man. Thanks for sharing the story.
    Dennis Moore
    Mercruiser Master Technician

  5. #5
    Thanks for the info on exhaust manifolds. I am very interested.
    What has been your experience with manifold lifespan? I've run 95% in fresh water (local San Diego lakes and the Colorador river from Parker to Powell) and maybe 12 times in salt water. Each salt water trip has been religiously followed by an engine flush. BTW, my exhaust routes out via a Rex Marine Sound Selector after the one piece riser/elbow then via 4 inch hose out the tail pipes if selected. Otherwise through the Y-pipe.

  6. #6
    After my past experience of thinking my 5 yr old manifolds/risers were OK since everything else was and then having them leak.. I would at the very least pull them and have them inspected, but more likely after 7 yrs I would replace them, much cheaper than any water getting in the motor. Haven't priced small block mercruisers' parts but would have to guesstimate 6-800??

  7. #7
    I have agree with Dennis...I only found his site 2 weeks after paying double for 2 v6 manifolds...
    His prices are great...I paid $200+ each for my manifolds...
    But that is still cheap ..considering how much damage a bad manifold can cause..bent valves, busted pistons, even water in the oil..ect..
    My 2 cents is....replace..couple hundred bucks is cheap for piece of mind

  8. #8
    Dennis Moore
    Twelve times in salt water and flushed after every time? I woudn't even consider it to be used in salt water. Salt water isn't THAT harmful on a boat engine! Don't worry about it!
    You can use it every day in salt water and as long as you flush it out you won't see any harm. Just don't leave it in the salt water more than a day or two at a time.
    Dennis Moore

  9. #9
    I just had to replace my motor due to roughted manifolds and risers ( wish I thought ahead like you ), wish I took a pic of the hole in the piston, also wish I knew about the Moore manifolds, bought some off a local supplier, paid a similar price but just an Osco universal center rise manifold, my old setup was a log style so I'm sure it is an improvement but I like the look of his manifolds

  10. #10
    Thanks for all the responses. I was leaning towards getting new ones and I guess just needed that extra nudge. I don't think I'd be as lucky with old exhaust manifolds as I was with the freshwater pump.

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