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Thread: 5.0 cracked manifold 22' seasport

  1. #1

    Default 5.0 cracked manifold 22' seasport

    I was coming back into Whittier last night and smoke came billowing out of my engine compartment. After clearing the smoke I determined that the exhaust manifold had cracked and the lower rubber boot had gotten so hot it is like crispy rubber now. I was just wondering what can cause the manifold to crack this time of year and how many bolts hold the manifold on. I have removed the 6 that I can see and was wondering if it should come off now. Any help would be greatly appreciated.
    Thanks

  2. #2

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    Sometimes the elbows on the saltwater cooling jackets become rusted or corroded, especially if you operate in saltwater on a regular basis. this can lead to overheating problems and if it is not noticed quickly, things will fry. I don't know if that was what caused the problem, but it might be something to look at. I had twin 350's in a 34 foot cruisers inc. and I had to have the elbows boiled every season.

  3. #3
    Member Dupont Spinner's Avatar
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    Lack of water thru the risers because of them being plugged will do it.

    A couple of things you can do. 1st thing is a good healthy rinse of freshwater. I run mine at idle on the hose until I see my water temperature rise. I do check mine everytime I hit the water. When I have it warming up at the dock, I pull the lid, place my hand on the riser and I expect them to be cold and I will then do a check after a hard run by doing the same check. For winter storage I pump RV antifreeze into the system until I see it come out the exhaust. RV antifreeze has rust inhibators in it and is enviromentally friendly. To date, 8 seasons, my manifolds are still in very good shape. I check them by pulling the hose and looking at the riser outlets.

    ON EDIT: 350 chevy engine usually has 8 bolts, 2 per cylinder. So did the riser crack or the lower manifold? When you said rubber hose I was thinking riser. The lower manifold, the one connected directly to the engine has antifreeze in it if closed system cooling system. The upper part that connects to the rubber exhaust hose has raw water.

  4. #4
    Member patrickL's Avatar
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    Isn't his 5.0L engine a 305 cu in and the 5.7L the 350? Just wanted to clarify to help him out. All good advice here.

  5. #5

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    The advice is great as always. I was thinking this is the 302. The actual manifold itself cracked. The strange part is that the temperature gauge never went over normal temp (180 ish) which I was thinking maybe it reads from the opposite side of the block. It was the lower hose from the elbow that was completely fried and where most of the smoke was coming from...kinda scary, but had extinguisher in hand :-)
    One suggestion from a friend was that if it has a sand trap, check to see if it was plugged. I haven't done this yet but there was a lot of stuff floating out there over the weekend. My main interest is that listing all the things to check might keep this from happening to someone else. Thanks again for the help

  6. #6
    Member Dupont Spinner's Avatar
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    Sorry....the 5.0 is a 305 but they are basically still the same.

    Now I understand better.....Volvo or Mercruiser? Open or closed cooling? No need for a sand trap....If it goes thru the intake and plus thru the pump, it will pass thru the motor.

    Motor temp will not change much if at all with a plugged manifold. The rest of the engine continues to run as normal but the manifold gets hotter, as much as 1100*F, with out any type coolant running thru it. The surface of the engine manifold will get hot, about 180* to 200*, but not get car engine manifold hot. The reason they can run rubber hoses on the manifolds with hot exhaust running thru them is because of the water passing thru at the same time to keep them from burning up.

    Now for possible causes....are the pipes coming from the manifolds to the rubber hoses, brass, copper, or steel or maybe even a coated steel. If they are steel or iron even on the freshwater side (closed cooling)they will corroed and plug water passages. Cooper or brass are much better choices. Even stainless will eventually have issues. Remember all the different metals in the system will cause galvanic corrosion.

    A jet unit requires a sand trap because proxmity to the sand bottom and the jet intake can ingest things as big as a peach pit pushing them possibly into the cooling system or just a large amount of debris.

    Useless piece of triva for you.......if you want to make sure that you are using real stainless steel take a magnet to it. If it sticks to the magnet it's not pure stainless steel. Many of those building blue water boats set all their stainless parts on a magnetic table and tilt it 90 degrees, what stays on the table is trashed.

  7. #7

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    Mine did the same thing. Corroded riser elbow, plugged up and melted the big rubber hose and caused a bunch of smoke. Changed it out and kept going. (Hundreds of dollars later of course.)

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