Peculiear overheating issue

njmello

njmello

New Member
Jul 1, 2007
90
Hemet, CA
Boat Info
1978 21' Cuddy
Engines
Mercruiser 260 A-1 O/D
1979 SeaRay 21' cuddy w/250 Mercrusier (350 Chevy) w/A-1 I/O

Boat starts, runs fine. Temp normal at idle (700-750 RPMs) - stays normal until RPMs exceed 1200. After 10 minutes temp slowly climbs to red, reduce to idle and temp comes back down to normal after a few minutes.

Replaced impeller and impeller body, new 140 degree thermostat and cover. Hoses are OK with no pinch or collapse at higher RPM. Going to pull the water pump next.

Comments?
 
Last edited:
Could be several things.....obstruction in the cooling system (broken off impeller vanes, corroded exhaust riser, head gasket allowing compression into the cooling system.) I don't think your engine circulating pump is the issue. Check water flow first then take it from there....
 
Yep... classic impeller failure symptoms. Thats in the out-drive lower half on an Alpha.
 
Brand new impeller and pump body. Had the sme sysmptoms before and after impeller replacement.
 
njmello said:
1979 SeaRay 21' cuddy w/250 Mercrusier (350 Chevy) w/A-1 I/O

Boat starts, runs fine. Temp normal at idle (700-750 RPMs) - stays normal until RPMs exceed 1200. After 10 minutes temp slowly climbs to red, reduce to idle and temp comes back down to normal after a few minutes.

Replaced impeller and impeller body, new 140 degree thermostat and cover. Hoses are OK with no pinch or collapse at higher RPM. Going to pull the water pump next.

Comments?
Click to expand...


the water circulation pump is around $150 i believe so you want to be sure you need a new one before you buy a new pump....you could remove the existing pump from the engine and look at the vanes to see if any look worn and/or are broken....are you sure you installed the new impeller with the vanes twisted in the right direction?....if the vanes are twisted in the wrong direction the impeller will not move enough water to keep the engine cool at higher rpm's....you could use a water hose to backflush the engine water passages and the water hoses to see if there are any obstructions that can be forced out....check the water passages in the outdrive also to insure water can flow freely to the water pump...sometimes you can suck up some sea grass or plastic bag etc. that can block off the water supply....as was mentioned by another reply you could have a blockage in the exhaust risers and/or manifold...only way to find out is to remove them and take a look....

cliff
 
Pulliing the engine water pump today to check the vanes. Reasonably certain the new impeller was installed correctly, since the problem is the same before and after. Could be exhaust logs and risers - that's next after checking the pump.
 
The veins should correct themselves when the engine/shaft turns, shouldn't it? I am thought so.
 
I'm not familiar with the Alpha drives, does that have the belt driven raw water pump similar to the Bravo? If so, it could be an obstruction in the hose that carries water from the drive through the transom. I had a similar issue a few years ago. After chasing the problem for a few weeks, I finally found that the plastic insert and inner walls of that hose had collapsed where it comes into the transom plate. I couldn't stick a phillips head screwdriver in the hole that remained. I was amazed at how much water the thing was still pumping through that hole, but it simply wasn't enough at higher rpm's.

Sent from my DROID RAZR using Tapatalk 2
 
And yes, the vanes will correct themselves if installed in the wrong direction. (At least they do when dry, I tried it just to see)

Sent from my DROID RAZR using Tapatalk 2
 
If so, it could be an obstruction in the hose that carries water from the drive through the transom. I had a similar issue a few years ago. After chasing the problem for a few weeks, I finally found that the plastic insert and inner walls of that hose had collapsed where it comes into the transom plate. I couldn't stick a phillips head screwdriver in the hole that remained. I was amazed at how much water the thing was still pumping through that hole, but it simply wasn't enough at higher rpm's.
Click to expand...


if the OP has had the outdrive off recently he could have pinched the water hose that goes through the transom when he reinstalled the outdrive......there was a fairly recent thread about this...when the outdrive was trimmed up the water temp would go down because the pinched hose was open...when the outdrive was lowered it would pinch the hose and cause overheating....

cliff
 
CliffA said:
if the OP has had the outdrive off recently he could have pinched the water hose that goes through the transom when he reinstalled the outdrive......there was a fairly recent thread about this...when the outdrive was trimmed up the water temp would go down because the pinched hose was open...when the outdrive was lowered it would pinch the hose and cause overheating....

cliff
Click to expand...

I considered that possibility, but the outdrive to transom hose is part of the main gimbal and was unmolested in the changing impeller procedure. I did raise and lower the outdrive while running and it got hotter faster in the trimmed up position and was slower to heat up in the full down.

I got sidetracked with another issue yesterday and never got to the pump. Before I pull the pump I'll start the engine cold for a minute with the raw water hose disconnected at the pump to see what the flow looks like.

Close examination of the pump looks like the previous owner might have replaced the original with an automotive equivilent - if that's the case, it was just a matter of time before that failed.
 
I am confused. I believe all inboard boats (either total inboard or I/O) have two pumps. One is the raw water pump either driven off the engine (belt or on crankshaft end) or the raw water pump is part of the outdrive. The engine then has a circulating pump just like an automobile except marinized (stainless steel components). You said the PO might have changed to an automotive style. since there is no auto equivalent for the raw water pump I am left to assume you mean the circulating pump. If an auto
style pump was used it will have a short life unless you have closed loop cooling in which case it might last for a long time.

I suspect the failure of an auto style pump would be with the backing plate rusting out. If this happens the pump would be leaking all over. I doubt the internal vanes would rust away first to the extent that it causes an overheat.
 
Somewhere back many years ago I remember reading about taking the hose loose on the inside of the engine compartment that comes from the water pump. You then put it down in a gallon jug and while the boat is sitting at idle, in the water, you see how long it takes to fill the gallon jug. Sorry I cannot tell you the specs on this but I believe I read it in the big yellow Mercruiser Shop Manual. At least you can tell if the outdrive water pump is pushing enough water into the engine. If the restriction is not in the outdrive then it must be on the engine side of the circulation such as manifolds.
 
Got the pump off and apart - yep it's the automotive version, but couldn't find anything wrong. So back to square one. I know the new impeller was put together correctly so I started back at the outdrive and sure enough the hose between the outdrive and transom had a crack. I disconnected the tilt pistons and manually raised the outdrive to it's physical limits and used a mirror to look inside at the hose. Because I lifted the outdrive far beyond it's normal limits, the hose tore right at the outdrive connection. Hopefully I can get away with just changing the hose without doing a complete bellows thing.

Tournament coming up - I got a month to fix it.
 
"I suspect the failure of an auto style pump would be with the backing plate rusting out. If this happens the pump would be leaking all over. I doubt the internal vanes would rust away first to the extent that it causes an overheat."

You were right about that - When cleaning up the pump before reinstalling, the backing plate was very pitted. I see a new MARINE water pump in my immediate future. Right now, I just want it running without problems.
 
nj
You MUST get an IR laser temp gun and get the thing running too hot and shoot some readings at various spots, as someone mentioned above. Cheap tool that every boater should carry aboard. I'm guessing you are raw-water cooled so you can follow the flow of water and it may give you a clue where the constriction is. Could be anywhere along the system. My dad had a Cruiser's Inc., same year, drive and engine as yours. Same exact symptoms. We pulled everything, changed the impeller, checked the exhaust elbows. What it turned out to be was constriction in the thermostat housing. Changed that and all was back to normal. Unlikely this is your problem but if all the usual suspects turn up OK, keep it in mind.
 
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