Merc 5.7 overheating...

[Aspidaso]

2004 22V
fine up until the other day then overheated at idle, revved cooled it down, the day went on an wak taking 4k in neutral to cool it.

Pulled impeller and pump housing looked perfect, was not unbounded.
Saw grooving on cover plate so replaced pump housing.

Flushed tranny cooler, flushed all the way up to circ pump, got some trash out, ran on trailer with the hose that feeds the circ pump hose disconnected, it was putting out a big flow.
rehooked it up, ran and eventually overheated.

Checked circ pump, no play in pulley, no grinding turns freely and no water at weep hole.

replaced tstat
Still over heated
strange to me was riser are cool. Manifolds are HOT

So now I figure getting enough cooling water to motor but it is not getting out.. I pulled the 2 block drains 1 on each side...nothing came out...dawned on me I might have sucked up sand from beaching, so I picked both out while running and cleaned a bunch of sand out until water flowed freely.

Figured I solved the problem but nope....same deal..
On a while I Pulled the line off the block drains again while one the lack,, very hot water came spewing out and temp went down and stayed down, water was much less hot coming out of drains.

Did I just come to the conclusion I need to pull the manifolds and clean the water exhaust holes?

I assume the hot water from the motor exits from holes in the block though the exhaust manifolds into risers and on out to the lake....

[freeheel4life]

Follow the hoses from the top of the tstat housing. Should be one hose each running to the manifolds coming off of a T at the the top of the housing. Hoses run to a fitting on each manifold.

Where the manifolds bolt to the heads there are just exhaust ports.

May be the temp sender is bad. A cheap laser infared/temp gun at harbor freight or the parts house would help confirm

[Aspidaso]

Ok, so if I pull the hose. Just pulled one from the T stat housing that goes to the exhaust manifolds (goes down makes a L and goes into what looks like a check valve , it's threaded into the bottom of the exhaust manifold, temp comes down big time, and I can see the flow deminish, tstat closing....and flow come out big time when tstat open....

So I am convinced not all hot water is not getting out, I know it's good up to that check valve thing on both sides....I need to pull them and see if water is getting through them.

We are at the lake so I'll pick up a infrared gun when we get home.

[Aspidaso]

Ok, verified temp sender is good, hauled boat home and pulled port riser, riser is clean and great water flow.
if i pull a hose to the bottom of the exhaust manifold (where the hose from the Tstat housing goes into) the flow is weak getting to the top of the exhaust manifold into the riser in my opinion, going to pull the riser from SB side tonight and see if it is the same...

[Aspidaso]

Just in case anyone is following...pulled sb riser and exhaust manifold, manifold is plugged up, a very small trickle of water is making it from bottom of manifold to top, zero rust but it has a built up of scale, looks and acts like lake scum on hull after not cleaning it for weeks, hard water deposits...it Chios away butting getting it out of water passages will be a chore..

[Aspidaso]

Cast, risers are in great shape, all the build up is in the actual exhaust manifold, plan is buy some 3/16 cable and cut it to about 12" chuck up in a 3/8 drill and go to town, figure the cable with fray and start knocking the scale off and should be small enough to get through the passages.
Starting to think this had nothing to do with sucking up sand but rather me not flushing with hose water every so often and letting the boat sit with manifolds full of lake water.... bottom of boat gets same **** on it from this lake....

[Aspidaso]

How does it work on gel coat? I keep a few cans of it in the camper to do the AC a few times a year.

[Aspidaso]

So update (late one), the overheat issue was due to scale build up in manifolds, it was blocing the hot water from going through the manifold and getting enough out through the exhaust.
Ended up getting a gallon of muriatic acid and soaking the manifolds (fun stuff- BE SAFE)! it worked like a charm, rinsed and used baking soda to neutralize the acid.

Worked like a charm, temps rock solid.

[boatwakes]

How did the flanged surfaces look between the manifold and riser? It's very important to make sure those surfaces are clean, flat with no pitting, RTV sealant or anything else is on the gasket surface or you'll hydrolock the engine from water ingestion through the mating surface.

[Bamer]

Muratic acid is diluted hydrochloric acid. It works great on scaling in pools. You should mix it 4:1 with water. 1 part acid to 4 parts water………


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[Bamer]

And yeah, always pour acid into water and not water into acid because with the case of hydrochloric acid, it can get super hot real fast if you just add a little water to the acid….


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