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Tried many things... still overheating

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surferjas

New member
Hello all. I have used the search function as well as searched through the internet and found a lot of useful information. I still have not solved my problem, and am hoping someone here might have some suggestions.

I just bought an old used boat that came with a Mercruiser 305 5.0 liter. I am unsure of the year, but the boat is an 84 so I'm guessing the engine is close to that.

Since we got it, it overheats under load. Idle is perfectly fine, but when I get it up on plane, the temp quickly rises to the red. I know people are going to probably ask exact temp numbers, sorry I dont have that yet. Here's what we've done and where we're at:

New impeller, and all components are good
New manifold and elbows
acid flushed engine (friend did it, I wasnt there... so dont exactly know how good it was)
new water pump.
Thermostat is checked and good (ran without it and same problem)
Pulled the hoses going into the manifolds while under load and they seem to have good flow. Right side has less, but still looks ok.
Pulled hot hoses off from thermostat housing and both right and left side seem to be flowing good...

Any ideas as to where I can check next? My friend seems to think that we may have a blockage somewhere in the block. Any ideas on maybe a way to check this theory or maybe there's a way to break corrosion off the water jackets? I'm trying to avoid pulling the engine because I dont have a trailer... getting the boat out of the water, on a trailer, and pulling the engine will be one hell of a feat.

Thanks so much for the help.

Jason
 
surferjas said:
Hello all. I have used the search function as well as searched through the internet and found a lot of useful information. I still have not solved my problem, and am hoping someone here might have some suggestions.

I just bought an old used boat that came with a Mercruiser 305 5.0 liter. I am unsure of the year, but the boat is an 84 so I'm guessing the engine is close to that.

Since we got it, it overheats under load. Idle is perfectly fine, but when I get it up on plane, the temp quickly rises to the red. I know people are going to probably ask exact temp numbers, sorry I dont have that yet. Here's what we've done and where we're at:

New impeller, and all components are good
New manifold and elbows
acid flushed engine (friend did it, I wasnt there... so dont exactly know how good it was)
new water pump.
Thermostat is checked and good (ran without it and same problem)
Pulled the hoses going into the manifolds while under load and they seem to have good flow. Right side has less, but still looks ok.
Pulled hot hoses off from thermostat housing and both right and left side seem to be flowing good...

Any ideas as to where I can check next? My friend seems to think that we may have a blockage somewhere in the block. Any ideas on maybe a way to check this theory or maybe there's a way to break corrosion off the water jackets? I'm trying to avoid pulling the engine because I dont have a trailer... getting the boat out of the water, on a trailer, and pulling the engine will be one hell of a feat.

Thanks so much for the help.

Jason
Click to expand...

Ayuh,.... Yer gonna be chasin' yer tail, til ya Verify that the gauge is Correct....

It's quite doubtful that there's a blockage in the block...
 
I agree,

Have you confirmed the temp is actually too high? Infa red heat gun or temp probe and special meter........

I once worked on a boat for almost three days chasing a hot condition. it turned out to be the sending unit on the motor.

the temp would rise normally to a certain temp but not settle at normal (150-160 degrees) and the gauge would say 190 ish.

Finaly checked the sending unit compared to a new using a pot of water on a electric stove top and a ohm meter hooked to it.

The original sender would rise to about 140 then shoot straight up to 190 +
Of course i was reading ohms not temp but you get the picture.

New sender was very linear and the ohms reading went up gradually and stopped where it was supose to.

If you do the test this way make sure you have a temp probe in the water and try to keep it under 200 degrees........
 
If the gauge check out to be good, Get the boat on the trailer and pull the drive. You will be looking for the old exhaust flappers in the Wye pipe or lodged in the drive.

What brand water pump did you install?
 
I will say the symptoms say bad water pump impeller.......

lack of high speed flow..........

Maybe when installing the copper tube the rubber insert inside the plastic part on the upper gear case got pinched....seen this many times......
 
kghost said:
I will say the symptoms say bad water pump impeller.......

lack of high speed flow..........

Maybe when installing the copper tube the rubber insert inside the plastic part on the upper gear case got pinched....seen this many times......
Click to expand...

Or the install of an aftermarket impeller. The aftermarket impellers with the brass hub are terrible. Especially if it is an SEI drive. The hub slips in the rubber at higher RPM's.
 
Pull the fresh water hose to the thermostat housing off where it attaches inside the gimble and see if the tube is not plugged w/corrosion.
 
I'm with Dock if the flappers fell down you will overheat forever.

I would also backflush the hose that feeds raw water to the engine with the drive off.

You need to know for sure you have good water coming to the engine.

With the drive submerged not on muffs pull the hose that feeds raw water to the engine and start the engine hold the hose straight up you should have 8-12 inch's of water, quick shut off the engine.
 
Ditto checking the temp gauge against a thermo gun, getting the boat on a trailer, making sure that the new impeller is correct for YOUR drive pump, the copper tube connection, and the exhaust back-flow flapper failure, etc.

Jason, if you'll use the tried and proven P of E (process of elimination) systematically and methodically, it won't let you down. You will find the problem.

Start by chasing out the sea water path.
If this is a Merc A drive, your sea water pump is in the drive.... actually, just above the lower unit.
Not only replace the impeller, but examine the pump housing.
Look closely at the tube that connects this pump to the upper gear housing.
Look closely at the curved hose that takes the sea water supply to the Gimbal Housing fitting.
The boat will need to come out of the water for this. There's no short-cutting this!
Examine this fitting and hose connection.
Make sure that when the drive is down, this curved hose is not being "pinched" in any way.

If there is a Power Assist steering system oil cooler, remove this, and examine the sea water passage.
If a previous impeller came apart, this is where the blade fragments can end up.
Follow this to the engine.

If Raw Water Cooled, first in line would be the T-stat housing sea water inlet fitting.
If Closed Cooling, first in line would be the Heat Exchanger sea water inlet fitting.

Once the sea water leaves either the Heat Exchanger, or the Engine T-stat housing, it will be diverted and directed to each exhaust manifold.
There will be some type of diverter or splitter in the system that will divide this flow equally.
You did say; "Right side has less, but still looks ok."
Check to make sure that this is diverting the sea water equally.
NOTE: some people will restrict the side that has the better flow in hopes that it will improve flow at the other side.
DO NOT restrict. Find the issue, and correct it.

When you replaced the exhaust manifolds and elbows, were the correct ported gaskets used???

When you say "new water pump", are you talking about the engine circulating pump?

Blockage within the engine is unlikely, IMO.... unless you have excessive rust scale, or a previous impeller came apart and has lodged pieces further into the sea water system, and eventually the engine block.
Not as likely if you have PA steering pump oil cooler. The oil cooler will generally trap and hold this pieces.

Have you checked to see if a Y-pipe back-flow prevention flapper has come apart?
These can come apart and lodge themselves down at the lower section, and restrict exhaust gasses at higher engine rpm.


You will find it, but it's going to be a process of elimination.


.
 
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"If Raw Water Cooled, first in line would be the T-stat housing sea water inlet fitting."
I once chased the same problem and found the passageway inside the housing,a hole about 1/4 in dia. was rusted closed. I found it accidentally while poking around with a screwdriver. It solved the mystery and stopped running hot. I eventual replaced the housing and could clearly see the hole I had missed before because of the rust and scale.
 
"If Raw Water Cooled, first in line would be the T-stat housing sea water inlet fitting."
I once chased the same problem and found the passageway inside the housing,a hole about 1/4 in dia. was rusted closed. I found it accidentally while poking around with a screwdriver. It solved the mystery and stopped running hot. I eventual replaced the housing and could clearly see the hole I had missed before because of the rust and scale.
 
Hello Everyone...

First off, let me apologize for my friend not replying to this thread. He's a little computer illiterate (I made the initial post for him...haha). Anyway, I'm tired of hearing him complain about not figuring out the overheating problem, so I'm going to help him find the problem...

We have gone through lots of things to try and figure out what the culprit might be, but cant seem to figure it out.

Sending unit and gauge is good... checked with UV temp tester

We've taken the lower unit off. Impeller looks good, intake looks clear, copper tube and gasket going into the upper part of the lower unit looked good. The plastic "guide" that goes over the copper pipe is partially melted and deformed....? Picture attached.

Manifolds and risers look great, looked into the exhaust to see if a flapper fell down there and cant seem to see anything out of the ordinary. Flushed the engine with acid (let sit for a day or so). Lots of crud came out. The Power Assist steering system oil cooler is not blocked in any way.

My buddy pulled the raw water line off while the boat was in the water to see how far the water came up... he said it shot up when he first turned the engine on, but then it was a constant stream about 2-3 inches high at idle.

He soaked the thermostat housing in acid for a while... and though it does look corroded inside, nothing seems to be restricting flow. I have attached a picture of the t-housing. It looks like where the thermostat sits, there is a rectangle opening. Seems to me, that if the thermostat is open, hot water will be able to flow back into the cold water area... does this look right?

Kind of at a loss at this point... any additional ideas of where we could check??

Thanks so much for all the help from everyone so far. Sorry again for my buddy's lack of responce. I will be checking and responding from now on... I'm ready to get out on this boat! haha.













 
At each end of this copper tube there should be a rubber sealing ring, if memory serves me.




The upper 4 bolt fitting attaches to the underside of the upper drive unit.

From the upper drive unit to the Gimbal Bell, will be an O-ring seal within the gasketed area.
From the Bell to the transom unit, will be a hose that must absolutely be the correct length as to make the bend. If this hose has been replaced, and if too long, it may become pinched.

From there, this hose connects to another copper fitting that penetrates the transom housing.

From this copper fitting, the hose ID changes, and this brings the sea water up to the oil cooler, if so equipped.

From there, this brings the sea water to the T-stat housing, where it becomes divided between the engine cooling demands, and the exhuast.

NOTE: most all sea water is being sent to the exhaust system at this point.
The engine circulating pump will pull from the T-stat housing, and will provide what cooling water the engine requires as per the thermostat.

Once into the exhaust, the "spent" sea water must be Non-Restricted as it passes through the Manifold/Elbow "transfer" ports.
(these are the ports between the manifold and elbow)

If the transfer ports are rusted and are restricting sea water flow, you can spin your wheels for weeks trying to solve the problem in other areas.


Follow the sea water path, and you'll find the issue.


.
 
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couple of places to look. lack of water flow to melt the guide tube means poor flow out of the engine. 2 places come to mind. Therm housing is clogged internally. You say impeller but no mention of complete water pump kit. sometimes the water pump base is warped allowing hot exaust gas to get sucked in. you can test this putting muffs on the lower unit only and checking for leakage at the pump base. The transom connecting water hose may be collapsed internally restricting flow. that leaves the therm housing.
 
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