Logo

Mercruiser 260 overheating

rmbanas

Member
Howdy all...

I had posted here before, I acquired a boat with a freshly built 350 mercruiser that, after a few days of scratching my head, finally pulled to find out the crank was smoked due to oversized bearings and oiling issues... whoops. Anyhow, got a fresh motor that was built properly, stuffed it in, rebuilt the carb, went through everything and it ran good. I took it out on the lake with a 17" pitch prop (15.75" dia), the boat is a 1982 four winns and the outdrive is a pre-alpha mercruiser. Anyhow, with that prop it would max out at around 4000rpm / 35mph on plane and run around 165 deg F. When idling however, it would overheat.. we hit it with an IR gun and the heads were around 170 or so by the spark plug and the intake manifold was around the same, so I questioned the sending unit and gauge.

Since the motor was such a whack job, I decided to pull the lower unit and found that the impeller was 'set' in shape, although not burnt up terribly. Either way, the bottom housing seemed to have melted a little, so I replaced the entire assembly (bottom housing, upper, impeller, etc), threw a 21" prop on it (for faster top speed) and took it out tonight on the lake. Now, it does the exact opposite--it overheats on plane but runs cool when idling. I noticed that it would run around 150F or so, but as soon as I get on it to get out of the hole and plane, it slowly will start creeping up until I lay off it, I never let it get past about 215 or so. In an attempt of desperation, I tried to remove the thermostat, but it wouldn't come out, so I "beat it open" with a screwdriver. In this state, it would run at about 130 deg F at idle and still overheats quickly when I start to plane--but I did put about 2 hours on it idling and had no problems.

The block is an 010 chevy 350, good shape, it had antifreeze in it prior to being put into the boat and was flushed--the current cooling system is raw water only (oh, and this is has ONLY seen fresh water, great lakes). I checked the power steering cooler/exchanger, and it's completely clean (new). I also checked to make sure the water hose from the transom is clear and that the passages in the water neck are clear. The only thing out of ordinary that I found was that the drain plugs on the bottom of the exhaust manifolds had "grime" in them, and were plugged. I jammed a screwdriver up there and they cleaned out and started to drain properly.

As far as I can tell the water pump (belt drive) and impeller are working properly, as I would expect with them being new. Any ideas? I'm wondering if gunk in the exhaust manifolds would cause this... I plan to remove the risers tomorrow to check. I also wonder if the ball spring/valve assembly on the front of the water neck is not working correctly, and if this could also cause the overheating? To note, it only takes about 30-45 seconds when I put load on the motor and open the throttle before it starts to creep up rather quickly. I think the higher pitch prop is putting more load on the motor and would explain why it's overheating now prior to when it had the 17" prop on it and bad impeller.
 
Is the engine water circulation pump belt tight? Install a brass sleeve and new thermostat w/copper pellet down--143 F. should be stamped on the old one. Pull the water inlet hose from the thermostat housing. Using ear muffs and garden hose on full waterflow run the engine for 15 seconds while holding the hose over a bucket. It should quickly fill the bucket w/a strong water flow. If not then there is a blockage between the hose and the drive. Find the inside transom connection for the same hose. Remove it and check the hose coupling for blockage.

If it does fill the bucket well then reconnect it and restart the engine. Pull the hoses off the exhaust manifolds one at a time and look at the water flow. It needs to be a consistent flow coming out for each hose. If not then check what is coming out of the thermostat nipples. If the water flow is good then there should be good flow to the manifolds and risers and elbows. If the manifolds and risers and elbows are too hot to lay your hand on them for a minute w/o discomfort then they are probably partially restricted and need to be cleaned or replaced. Post back what you find.
 
Last edited:
guyjg... will do those tests tomorrow. Tonight I was too intrigued and bored, so I went and pulled the risers anyhow. What I found was rather amazing... they appear to be relatively new (i.e. like one year). The passages are very clean, minor oxidation. I pulled the brass plugs, flashed some light through the passages, etc and couldn't see any crust or debris that would warrant a blockage.

One thing I did however discover is that in the down elbow for the exhaust, there's some fairly beat up, highly corroded butterfly valves, whatever you want to call them... they don't turn very well and appear to be broken and lodged in there at about "half throttle" if compared to a normal butterfly valve. Could these things be causing the problem? I highly doubt it... but either way I'd think these guys need to be removed (if not essential) or replaced.
 
Could these things be causing the problem?

They could contribute to increased engine temps. and reduced RPMs at higher engine speeds if they are actually restricing exhaust gas /water flow.
 
Ok, I'm stumped. As far as I can tell, everything is working properly but my engine is still overheating, even at idle in the yard (~190-200 deg F) with no load. So here's the deal... I swapped out the old 143 deg thermostat, and put in a 160 deg (all that was available at the parts store). I checked the flow from the out drive to the transom aluminum elbow, from there to the p/s cooler, from there to the inlet to the water pump, good flow all the way. Flow was going to the risers, they were cool to the touch. Flow was also going through the ball valves to the bottom of the exhaust manifolds, also cool. The ONLY hot thing was the suction hose (spring one) on the water pump, which was very hot. It almost seems like, the pump isn't taking any water from the water neck into the system? Any ideas?
 
...they were cool to the touch....The ONLY hot thing was the suction hose (spring one) on the water pump, which was very hot.

Did you put the thermostat in w/the copper pellet down? If not it will never open.

Is the circulating waterpump shaft turning? It may have a defect; broken shaft or impeller is loose internally. Block openings behind the pump may be blocked. W/engine off grab the pump pulley and try to move it side-to-side, in/out and up/down. Any noticeable play in the shaft could be a problem. Was the thermostat housing all corroded? Pull the hose off and look into the thermostat housing for a blockage. Also use a mirror and flashlight to look into the circ. waterpump. You need to get a 143 F. marine thermostat. If you replace the circ. pump get a marine pump.
 
Last edited:
guyjg... yep pellet down. The shaft is turning, pulley doesn't wobble and there's no play. The inside of the t-stat housing was somewhat corroded, but I fully inspected it when it was off and there were no blockages in it.

I'm just absolutely stunned at the variety of this... every time I mess with it, something changes. At first, it overheated only when idling, and cooled fine at 3000rpm, then after replacing the impeller, the exact opposite, now it overheats at idle with no load in the yard just by changing the t-stat.

Sigh... I've spent 3 weeks now working on this boat and have not even been able to use it yet once.
 
Ok.. spent another hour and a half or so working on it. I pulled the lower unit (again), tore apart the brand new impeller housing, checked it, and all is good-it's exactly as it was before. I did notice the plastic stand tube to align the copper tube for water flow was completely melted, so I replaced it with the old one, which was only partially melted. I backflushed the system and watched water pour out of the copper tube. I checked all hoses again, after re-installation, with a good stream of water coming from the impeller at the water neck. I ran the engine at idle and it went up to 190 deg F, under no load, increasing RPM, it creeps up. Well, what the hell? I've also verified the belt driven water pump works by turning it by hand and inspecting the impeller and verifying it is indeed, pumping water. Again, all hoses are cool except the suction hose on the pump, which is red hot. Shouldn't cold water from the lake be going directly into that water pump and into the heads? I know I already said this, but it seems like it's not, like it's being diverted, although I did look inside the water neck and there's NO obstructions, so it makes absolutely zero sense.

I give up, I hate this boat.
 
It was suggested that maybe a flapper could have disloged .. so I remembered the starboard side flapper was gone when I removed the riser. So i went searching for it.. ran a fish tape up the exhaust through the Y pipe, no flapper. Doesn't appear to be anywhere, either way exhaust seems to be unrestricted and traveling freely on both sides through the transom exit... so, again I've got nothing to go on.
 
I did notice the plastic stand tube to align the copper tube for water flow was completely melted

Did you insert the key on the shaft when you did the impeller; or maybe it fell off. The melted alignment tube says it got too hot in the leg. The only water flow into the engine might be from the garden hose pressure.
 
I had to mark the impeller (to see where the keyway was) in order to fit it on the shaft. When I removed it and replaced it both times, it would only go on where it was marked--i.e. it's been keyed the entire time. I did however notice that right above the impeller, the shaft was shiny, polished and scratched/grooved--looks like something was rubbing on it--not sure if this occured pre or post "new impeller kit".
 
Last edited:
Pull the thermostat out and see how it goes. It is apparent that cooling water is not getting into the block. Water coming out of the "T" w/check balls is usually cool while the water going to the exhaust mans. is mixed w/hot engine block water. Could there be a wrong gasket in the cooling system? Look at the diagram and see if the thermostat was assembled in the same order.
http://www.mercruiserparts.com/Show...r=1077&bnbr=160&bdesc=STANDARD+COOLING+SYSTEM
 
I had orginally pulled the t-stat on the lake when it started to overheat, and it didn't do anything other than lower the idle temp from 160ish to about 120... it still just as quickly overheated under load and acceleration. I've looked at the diagram, and everything was installed in the exact same order. The t-stat cover, metal gasket, brass cup, seal, tstat. The nipples on the t-stat cover itself run to the bottom of the exhaust manifolds and the nipples on the T run to the risers--both are cool to the touch. Again, it seems as if the inlet water is bypassing the belt driven pump's path and going to either the risers or manifolds... since it would follow the path of least resistance, is this possible? In that case, the water pump would just be recycling its own coolant over and over, causing the overheat. I know this sounds really bizarre and unlikely, but everything else checks out as far as I can tell--if I break loose any of the hoses, there's ample flow coming out.
 
The gasket was #10 I believe, it had all the ports open on the gasket, except the ones on the right/left which had small holes in them, likely factory. Either way, the coolant passages were open in the riser.

The engine came from a pickup truck. The previous engine in the boat, was also from a pickup truck but the exhaust manifolds/accessories/fuel pump/etc. were from a mercruiser 228. The previous engine did not overheat.. essentially I swapped everything over, including the drain plugs.
 
The only thing I can think of is there is a bypass hole inside the thermostat housing that I think allows mixing of cold water w/hot water. Perhaps a piece of impeller or corrosion buildup is blocking it. If there is a pipe plug on the front of the housing then you have access to it. I think the hole is visible through one of the housing openings. If there isn't any plug there is a procedure for drilling/tapping one. I just don't remember it and no long have a manual.

Do you remember if the water pocket seal was intact on the underside of the upper half of the outdrive? It is what the copper tube is inserted into. If not, I believe it will affect engine cooling by allowing exhaust to enter the cooling water system which may be why the plastic alignment tube was melted.

If I can come up with any other idea I'll insert it.

Anyone else? Please jump in, the water is hot.
 
There was a rubber seal that the copper tube inserted into, similar to the replacement rubber seal that went into the upper impeller housing. As far as the thermostat, there was a few brass plugs on it, nothing seemed obstructed and I removed the plugs and replaced them just to see what they went to. I'm going to take it to the boat launch and run the outdrive in water to see what the actual flow rate of the impeller is--since on a garden hose it's not realistic. It just seems odd that a brand new impeller and complete kit would not be working, especially when the prop is spinning in gear under full load.
 
Update: tore down belt driven water pump as suspect. It's a late 60's car pump, short shaft with an impeller with 'straight' blades, i.e. it will work in standard or reverse rotation, doesn't matter. Pump is in good shape, replaced. Checked out the coolant passages and hoses etc one more time, along with the water neck. Going to take it to the lake to test it on a not-garden-hose, and put the 17" prop back on there to see if it doesn't overheat with it.
 
Update: Took the boat out on the lake... behavior is much different this time around, but still overheating nonetheless.

With the 17" prop, the boat comes up to an operating temperature of about 90-100 deg C (about 190-215 deg F)--I only use the C because I can see it with better resolution on the gauge whereas only 130/160/190/240 are marked in F. It will stay at this temperature either idling or under load up to 3000 rpm, after which it begins to climb into the 220's+. The only other quirky thing is while on plane, going down the lake at 3000 RPM, the temp sometimes will creep up to like 212-215 (boiling) then back down to 190-200'ish... and keep doing this, although rpm/speed does not change. If I bring it off plane, it'll cool back down to the 190-200 range.

To note, this has a 160 deg F thermostat in it... also I did the following;

1. I pulled the inlet to the t-stat housing from the outdrive, it was pumping water about at the rate of a garden hose or so.
2. I 'stretched' the springs in the t-fitting ball check valves for the risers which made the temp go down a little bit, not sure what the spec is on these.
3. I checked the exhaust with the boat running on muffs and in the lake, and it seems fully unrestricted through the transom exits, I wasn't putting my head down near the prop, so no idea how that is.
4. Risers maintain cool feel at the 190-200 F operating temp, but under load heat up to the point it's uncomfortable to touch them, didn't have an IR gun so no idea, but I'd guess probably 200 ish. It also seemed that the 'bow' side of the riser was cool and the 'stern' side was hot. Remember I had these apart and they were clean.
5. I tried trimming up and down with no difference in temp climb or idle temp.
6. I pulled the temperature sending unit from the manifold and cleaned a slight tint of oxidation off it with a wire brush, seemed very clean and "new" based on appearance. There were no obstructions.
7. I noticed that the middle of the manifold was boiling water on it, while the front and back was not--I assume this is from the exhaust passages in the manifold to activate the choke.

I thought of a few more things that could possibly be causing this...

1. The marine 'stuff' from this motor is from a 228 (305)--is it possible that the heat output from a 350 is great enough to offset the ability of the equipment to cool?

2. Along with #1, is the outdrive undersized for the engine? I'm not sure if maybe the boat originally had a 4 cylinder 470 or something in it and someone dropped this v8 in? I wasn't even "asked" by the dealer what engine I had when he pulled the impeller repair kit, so I'm assuming that it's all universal. I believe the outdrive says mercruiser 165 on it.

3. The pulley for the water pump, I assumed was part of the unit since it had black urethane paint--but is it possible that the water pump simply isn't turning fast enough? Maybe if I put an underdrive pulley on there it might fix it? Does anyone have specs on the mercruiser water pump pulley diameter?

Really at my wits end here guys... I appreciate all the help by the way, hopefully I can get this thing figured out.
 
Last edited:
yep wrong gear ratio but will not over heat the boat.

Got the boat to plane better with a 17 p prop but won't overheat the boat.

But you HAVE NOT FOUND THE RESTRICTION IN THE EXHAUST YET !
 
The serial # on the gimbal housing is 6338317 on the outdrive itself, 3518838, which from what I've been able to find says 1970-77 165. It also says the gear ratio for a 165 outdrive from this era is 1.65, and the 260 (v8) is 1.5, so I'm losing a little top end I guess with the taller gears, but in all reality, this should take load off the motor and make it less likely to overheat!

As far as the exhaust, chiefalen... I took the risers off, inspected them and the manifolds which are clean and flow at good rates. I took the flapper out of the port side, could not find the starboard side flapper, but I ran fish tape through the exhaust pipes, took the outdrive off and the exhaust bellows and did not find anything. It could have been removed by the previous owner perhaps? I don't know where else to look for exhaust restrictions.
 
Last edited:
take off the drive see if it's held up in the exhaust tube bye the long bolt that goes thru the tube.

I know what i speak of , i seen it before.

See if it went into the drive then the water can still exit will not over heat the motor. And i have seen the flappers come thru the prop..

Have faith my friend i wouldn't steer you wrong.

Let me ask you where do you think it went martians ? Obama came and took it out.
 
chiefalen.. where's this "long bolt" at? I will certainly take a look as soon as the rain lets up... I know I have the "newer" Y style exhaust, right past the first elbow from the riser, there's a metal pin that the flapper sat on , then it goes straight down into the transom to an exhaust bellows. I removed the bellows and looked up in there, and looked down through the aforementioned elbow, didn't see any bolts. I guess I might have missed something.

Is it #8?

http://www.mercruiserparts.com/IMAGES/CRUISER/99985/56.png
 
Last edited:
and you didn't see the flapper ? maybe it passed thru.

I'm stumped and i should not be joking around with you it is a problem your dealing with.
 
Hey, if you can't take a joke... you got a problem, I personally thought it was pretty funny, trust me I need humor at this stage because I'm ready to scuttle this boat.
 
I even thought about your problem when i was fishing with my good buddy today.

I asked him he said good water to the t-stat, good water thru the heat exchanger, good water thru the risers. No obstructions at the y pipe thru to the drive.

He asked me good water coming out the center of the drive ? Could the flapper be blocking the drive itself ?

Is it ?

Your a good sport really whats left ? But it should not be overheating the motor in my mind cause the exhaust bellow should allow the water to exit no matter what .
 
When I backflushed the drive (I stuffed a garden hose on the water neck side of the hose that goes to the power steering cooler), water was streaming out of the copper pipe that goes to the impeller just fine, I tried both ways with equal results, it seems that there isn't any restrictions anywhere. I checked the exhaust leg while I was in there, but didn't really probe too much. A trickle of water comes out of the prop on the hose, not sure if there's anything in there.

One other thing... I've got a buddy building another 350 mercruiser in my garage right now and I was looking at his motor, it's about 5 years newer than mine... on his, the temperature sending unit is in the thermostat housing, not the intake manifold, and the springs on his riser check balls is about the same as mine. I wonder if I put my thermostat in the water neck... I know that damn manifold has exhaust ports in it that cause it to run hot for the choke and such, maybe the heat transfer through that is affecting the water temp? Seems odd, but who knows... and just because water is boiling on the intake manifold then, it doesn't necessarily mean the heads are that hot.
 
May I make a suggestion? Since you just built this 350, could it be possible that you have one or both of the head gaskets or intake plenum gaskets upside down on top of the cylinder block or heads? Just flipping one gasket over and not physically checking to be sure that the passages line up exactly can cause a blockage or restriction to coolant as well as OIL! I have seen this before on a Toyota 3.0L V-6. The comment you made about water boiling on top of the intake as well as the suction hose (spring) being very hot leads me to believe that a cooling passage is blocked inside the engine block. You've eliminated everything else, could be worth a look. Good luck!
 
Back
Top