Cooling Issue

[nickzlb77]

Hey all. Have twin MPI 5.0s. FWC.

boat sat in a bad spot at low tide, silt, debris etc. boat running a bit warm occasional overheating never let get above 200z

Both raw water pump housings and pulleys were wiped out. Swapped both. Did barnacle buster flush on raw water side, cleaned HX tubes.

replaced tstat and circulator pump as well, circulator was making noise. Oil clean.

Motor overheating now. It appears there is
a blockage on the closed cooling ckt. With top of inlet hose of circulator off and full of coolant level does not decrease. Return hoses disconnected as well. Shouldn’t I see the coolant being drawn in while cranking? Thanks in advance!

OW024480 SN

 

[nickzlb77]

I apologize for sporadic info, was still on the boat covered in grease.

A little backstory, boat is a friends. I assist him regularly. The boat is an 89 Slick Craft 31sc. He got the boat 3 years ago and since I have completely rewired the boat, custom dash, custom upholstery, JL sound all over the thing. I brought the motors somewhat back to life last season and he had a trouble free season and put about 150 hrs on it. He never hauled and winterized the boat (very lucky we had a mild winter). Recently pulled for drive service (bravo's) and bottom paint. Since back in its been running a bit hot.

Both raw pump housing were leaking badly, so those have been replaced. Thermostat seemed to be sticking, that has been replaced. Completely flushed raw water circuit for an hour with barnacle buster (with a sump pump, basically routed exhaust hoses into bucket with pump and connected pump output to inlet of power steering fluid cooler) as HX tubes looked a little clogged.

Put it all back together and overheating persisted. The circulator pump was very loud prior and that has been replaced. Still overheating.

At this point I disconnected and removed the HX completely along with the entire thermostat housing. When pouring coolant into the inlet of the circulator pump I can see it rise up to where the thermostat housing normally is but when cranking the motor there is no suction on the inlet of the pump. It seems indicative of a blockage somewhere on the coolant supply side. Ive honestly never ran the circulator pump with the inlet disconnected to see how they flow but given the simplicity of the pump I'd expect to see some suction. Maybe an obstruction at the manifolds?

Note the tstat housing had significant buildup around the temp sensor and sender but not much around hose ports.

 

[tpenfield]

You seem to be working on the closed cooling side (circulating pump), but do you absolutely know the problem is not on the raw water side? Have you back-flushed and cleared out the oil coolers on the raw water side?

Chances are the closed cooling side is fine.

I assume that these are replacement engines, given the age of the boat (1989) . . .

 

[nickzlb77]

Everything on raw water side has been gone thru, a lengthy flush was done as well. Good flow on raw ckt.
Yes replacement motors, they have 780 hrs on them.

 

[alldodge]

The circulating pump doesn't suck, it just moves water around.

Everything is cleaned or replaced on the antifreeze side, and raw water pump is replaced. The motor and its components should be good. The raw water side is where I would look further.

Have you looked for Bravoits?

 

[nickzlb77]

Raw water is brought in with thru-hulls/strainer basket.

Thank you! Last thing I can think of is maybe antifreeze passages in HX.

 

[alldodge]

Don't see it on the inside HX unless antifreeze types where mixed. Normally that would stay clean

Silly question: The circulating pump was replaced with the correct one? Marine serpentine model rotates opposite as some cars

 

[nickzlb77]

Yes OEM merc. CCW rotation.

 

[nickzlb77]

It’s just odd To me, I would’ve thought with all hoses disconnected (except for the circulator inlet so I can pour into it) and thermostat housing removed the new circulator pump would be forcing coolant thru The t stat hole and out of block.

 

[alldodge]

No the circulation pump sends water into both sides of the block below the head, then water exits out the thermostat housing

 

[nickzlb77]

That’s what I mean, I have the tstat housing off and HX off,l with the inlet hose of the pump connected on one end to the pump other to nothing so I can pour in coolant. I fill the hose with coolant and crank engine. There is no suction on the inlet and nothing pushing out of thermostat housing like I’d expect, perhaps just cranking doesn’t generate enough pump rotation to really pump?

 

[alldodge]

The circulation pump is a pump in about name only, its not positive displacement. It does move (pump) fluid but not much more then like your hand in water swimming (so to speak). Here is the typical flow diagram, why older then yours and is a V6 but note the flow

 

[mr 88]

Depending on your raw water cooling circuit ,I would be looking at the risers and or the exhaust manifolds as being your issue. I believe they are good for anywhere from 4 to 6 years in salt water. Clogged up they restrict your water flow and cause overheating especially shows up as you increase your speed . May even show normal temps until you get up on plane.

 

[nickzlb77]

So flushed all antifreeze looks like types may have been mixed. Thank you for the information on the pump It just didn’t make sense to me!

These are dry joints, manifolds are good but man are the elbows full of crap! At the moment every component has been soaked in barnacle buster and cleaned. Going to reassemble today and cross my fingers!

 

[mr 88]

IF it was me I would just replace "elbows " [ risers ] . In most cases the salt eats away at the chamber walls and you may risk water intrusion into your exhaust manifolds ,the part that does not have any water in it. You can see if that's the issue by temporarily cleaning and putting the old ones back on. I think they go for about 125 + per side depending on where you buy them from.

 

[nickzlb77]

Thanks yes I took them off thoroughly inspected them, boiled and repeatedly cleaned with barnacle buster flushed they’re actually pretty good.
put clear hoses on exhaust and have good flow out.

Really looking like an issue on closed side.

manifolds possible. If they look anything like the tstat housing looked I’m not optimistic