Overheating again.

jmagdaz

jmagdaz

New Member
Sep 27, 2010
97
southwest missouri
Boat Info
1983 sundancer 260
Engines
350 mercruiser w/alpha drive
Ok, was overheating a few months back pulled the drive, changed impeller, housing an the like. Changed out tstat as well.
All has been good for a few months, now overheating again. Pulled stat out yesterday, an took for a ride. Still overheating wtf? It will overheat coming up on plane to 3400rpm, if I pull throttle back to 3000 it takes a minute but cools back off rapidly. I can then increase rpm back to 3200 an will run fine. If I so much as tap the throttle above 3200 it rapidly goes to hot an have to start process all over again.
Any thoughts?
 
Did you replace the water pump base? That is so important.
Many folks will just change the impeller and upper housing. If the engine has ever over heated in the past if can damage the base.....
 
No I didn't. Looked ok. Not sure why it would have run fine for a cpl of months an now all of a sudden type of thing.
 
I had one that had a crack in it. Replaced the top and impeller and it too ran fine for a while,,,,,,,,, then started up again. The 1/2 fix got it back in range and only allowed for the crack in the base to get worse.
When I pulled it again could see the seal in the base had melted. Also the crack was now visible.

Just a FYI, Not saying that is your problem but If you can eliminate that variable you can move on with confidence.
 
I had a very similar situation on my 268, with a 260 Alpha set up. Swapped out everything you did, and still had the problem. I spoke with a friend who is a certified Merc. mechanlc, and he said to check the short molded hose that comes from the transom assy. and feeds the motor. Come to find out, even though it looks fine, what happens is the hose gets weak, and will collapse under a load. Back off on the load.....and the hose relaxes back into shape, and the motor cools down again. That was my problem.
 
That sounds more like what I'm experiencing.cause when I back off the throttle a bit it cools down so fast. It takes about 15 seconds to start the plunge on the gauge, but once it starts it goes all the way down to about a qtr mark on the gauge in less than 5 seconds. I will pull the drive an change the impeller base this fall, but gonna hope a hose is my issue. Since the boat is wet an a trailer is not readily available.
R there any parts store options on that hose, or is it mercruiser specific item?
 
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Yes, A heater hose will work but you will need to pull the boat and you will need the some special tools.
 
Mercury Marine Mercruiser 91-78310
 
Check for sediment and crud in your hoses. Bow high the crud slides back partly obstructing the passage. Level off or back off and the crud settles to the low point in the hose letting water flow. You wouldn't think it possible given the water pressure but cooling isn't under a lot of pressure.
 
All great thoughts, no one has said anything about the engine mounted water pump?? Do these usually not fail?
 
A circulatory pump will leak 99% of the time if they are going bad. But Impellers have been known to spin off.. I would focus on raw water pump and hose first. Personally I think... becasue by your own admission you had a over heating problem and you did not replace the base that means no mater if that is your problem or not it should be replaced. I harp on this becasue when they over heat, that heat blows right down on that pump and melts the seals out of the top and bottom not matter what.
 
Well I am thinking flapper. I had the same thing happen to me replaced everything and it was still over heating at idle or above 3200 rpm. It ended up being the flapper, the pin corroded away and the flapper fell down into the Y pipe and was restricting the flow. It's an easy fix and and an easy item to check you just need to pull the hose off at the top of the Y pipe. I used a laser temperature prob to narrow the problem down.

Ken
 
Spookeay said:
A circulatory pump will leak 99% of the time if they are going bad. But Impellers have been known to spin off.. I would focus on raw water pump and hose first. Personally I think... becasue by your own admission you had a over heating problem and you did not replace the base that means no mater if that is your problem or not it should be replaced. I harp on this becasue when they over heat, that heat blows right down on that pump and melts the seals out of the top and bottom not matter what.
Click to expand...

I appreciate the help. An I am not arguing at all. I love having all this experience out there to pull from. This is a great site, an thanks for all the input.
If the lower housing is suspect, why would it have ran perfectly for several months after changing the impeller an top housing? This problem didn't slowly happen over time. It ran 165 ish for 2 months an several hundred miles, an then one day on a return trip it was running cool an then spike! Like something let loose. It never gets hot at idle. It will not overheat at any rpm under 3200-3400. ???
In my limited experience I would think if btm housing were cracked, it would run warmer at all rpm until a point that it over heated. ???
At 3200 all is good. Not warm, not close to hot, I mean perfect.
Tap the throttle to 3400 an it spikes .
 
I am ruling out the engine mted pump. That doesn't add up. If the circulatory pump impeller failed, pretty sure I wouldn't have been able to run at any speed for any distance. Let alone 65 miles at 3200 rpm an a 165-170 degree engine temp.
 
Not to hijack this thread, but I'm having the same issues as well on my '81 Merc 260. I'll check the lower end of the pump housing tonite. THANK YOU for that suggestion!

This seems to be an ongoing issue for a few folks in here. I had a post a few weeks back where my boat was doing the same thing. I found that the hoses to the thermostat housing were reversed, so I swapped 'em out the right way. Thought that would fix the problem, but it didn't. Engine ran cooler at idle - 1500rpm, but then heated up again like it did before. Changed out thermostat, no fix. I dropped my drive last nite, and pulled the impeller. It looked ok, just a tad bit of wear, etc., so I'm not optimistic about this fixing the problem.

Is there a way to flush out this entire system? I'm worried that there's rust/scale, etc. jamming up the shutter from when I pulled the risers to swap the hoses a month or so back. Everybody is saying that's not the problem, but I don't see any other thing that could cause this. Pretty coincidental that it all started right after I pulled the risers. Figured I disturbed something in there...

Also, temp on exhaust manifolds is approx 130, a little hotter on the risers, and a little cooler on the thermostat housing - with both sides being within a few degrees of the other. To rule out the gauge and/or sender, what's an acceptable reading on risers/manifolds, where the thermostat is 143 degrees?

Thanks
 
I think you guys are boating in fresh water? All the same corroded or clogged exhausts can have an over heating effect. When were they last inspected? Raw water flows through the block or heat exchanger and leaves via the exhausts. If the water can't get out you'll start overheating. Running a pressure gauge is a good idea. As the pressure increases over time things are getting blocked or if you loose pressure your pump is wearing or you have a leak.
 
Changed impeller in June, had it on the trailer then, started an ran on the muffs, good exhaust flow then.
 
That means nothing, one flapper could be stuck or missing, the water flow is coming out of the "Y" pipe and one side could be restricted. It took me 2 years of dealing with over heating to finally check the simplest and cheapest thing and it was a flapper blocking one side of the "Y" pipe. Hope you don't spend 2 years hunting this down like I did.

Ken
 
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