350 (Merc 260) Overheat Issues - please help!

S

Starvin48

New Member
Aug 11, 2008
64
New Baltimore, MI
Boat Info
1978 SRV 240
Engines
Mercruiser 260
Hey there -

I have been learning much about motors this year. I have been out twice ALL YEAR and both times there have been issues. Here's the deal, and I humbly ask anyone out there for thoughts and solutions:

Boat runs infinitely fine at idle. Sounds great.

Temp consistently rises as RPMs increase.

I can run to about 3k before I get dangerously close to overheat, but can run at 2500 indefinitely at a slightly higher than normal temp range.

Impeller and all that completely replaced.

Thermostat replaced (when I went to look, there wasn't one in there!).

Slight overheat once brought out what I think to be lifter tick, which disappears after the motor cools (a minute or so).


Thoughts? I'm out of them :smt009
 
Sounds to me like there is a blockage in the system somewhere. When the water pump was replaced, was it the circulating pump on the engine or the one in the drive. The symptoms you describe sound very much like a bad drive water pump impeller. When you replaced or added the thermostat, it's possible that you put it in backwards..it's easy to do and you wouldn't be the first to do it.:thumbsup:
 
Thanks so much for the speedy reply!

To answer - there was no thermostat prior to my inspection, and the boat was doing the same thing. I dropped it in spring-side-down. Hope that's right :)

The pump on the drive was replaced. Dumb question, but what's the probability of it being the one on the motor? This is a mystery craigslist motor, BTW. I know. Poke fun now at my misfortune based on absolutely foolish practices.
 
have you checked the water lines from the drive to the engine driven water pump?

What do you know about this motor?
 
I do know that the motor runs well and sounds well below 2000 RPMs.

I know that it begins to overheat above that range. I know the thermostat is functional and the line from the drive to the engine driven pump is sending water without issue.

I had her out over the weekend just putting around within the specified range (under 2000Rs) for over four hours. Motor is fine and no other issues popped up.

Now what?
 
Have I stumped the site? My mechanic said I could have a head issue (developing "pockets" or something like that) which may have developed during an overheat condition. The water pump on the motor is supposedly good (he tested it).

Any other thoughts?
 
No way you stumped the guys on this site. Try posting some more details, hours on the engine, any exhaust work & the like. In due time they'll chime in with the answers/advice that you probably won't want to, but need to hear.

The PO probably took the thermostat out in search of, or in spite of the original problem. That(the original problem), is what you need to find & fix. Then you can move on to any damage that may have been caused by running the engine without a thermostat.

You say "my mechanic tested the pump" I'd be interested to hear how he did so. I've been reading a ton of threads lately about overheating. Perhaps you can use the search feature & get some more ideas that way.
 
If you didn't find everyone of the pieces to the old failed impellor when you replaced the it with the new one, there's your blockage. The pump on the motor is a circulating pump simialr to the one on your car. It's impellor is a fixed blade type and not prone to failure.
 
Thank you all for your replies -- I appreciate all the help I can get!

Has anyone ever heard about a "clear hose test" you can do to find out if there are circulation issues with the cooling? If so, how? This is what I understand:

1) replace cooling lines with clear lines.
2) run boat under load
3) look for bubbles in the lines, showing potential head issues or cooling blockages.

Thoughts?

Again, I wish I knew more about the situation. Thank you all for the insight.
 
In reality this is simple stuff. If the cooling system circulation is clear there will be no overheating. If it is blocked or impeded in any way there will be.

I lean toward the theory of clogged exhaust/circulating manifolds....or logged water jackets in the block. When you flush it after use is the water rusty?

Just for kicks...check the serial number on this engine to determine if it is an automotive model or marine.
 
Yes it is plugged up manifolds; if they look rusty, no paint outside due to overheat, believe they are worse inside. check the coolant flowout after crimping off each side hose; if no change it what blows out (on the work rack with muffs on) that side is plugged. if risers/elbows too hot to touch you need all new, $ 1000 per side
 
There are soo many things that will cause your problem. Starting with some basic tests, you can figure this out. You have to test the water flo to the engine. To do this you must remove the large water input line from the drive. Get a 5 gallon bucket, start the engine, run at 1000 rpms for 15 seconds should produce 4qts h20. If the flow is much less, it'll prove restriction. Next step if is water flow is less, is to pull your lower unit and inspect the impellar housing for cracks or seal issues. Most importantly, and a few guys posted this, the 1/2in line from the drive. Small pieces of anything starting with impellar pieces will clog that line. To clear that line: remove that large input line to the engine, (with lower unit off), hook up a hose with descent pressure, a 5 gal bucket to catch the debris under the drive and run through until it blows out. It won't take much, that reverse flow generally does the trick if it's blocked. In my experience, boat mechanics will replace parts until it runs instead of spending time giving A solid diagnosis. I hAd this problem with mine, stumped four mechanics, every one of them tried the parts replacement method, and in the end it was me that fixed it with the help of csr.

If it needs risers or manifolds, there are tests for that too, but prob not the case. Just start with the basics.
 
WOW! I am going home this weekend to do these procedures and will report back with the results.

I am so grateful to be a part of this community, and thank you all for your invaluable input!
 
I had a similar problem with an 84 21 W/A with a 3.7 4 cyl.
Over 2800 she would jump right up in temp.
Mechanic gave me a New Riser to put on. Did not cure it.
Mechanic said to take off Heat Exchanger and have cleaned.
This did not work either.
Bought New Heat Exchanger from Basic Power. Did not cure.
Mechanic said we'll hook up a clear line for the water input.
Stuck the end in the bucket and timed it and looked for bubbles. Sure as heck it was sucking air.
Mechanic previously did a water pump.
Mechanic said he will remove the drive and check the pump again. Found gasget missing I believe.
Re-did pump in the drive and it was a go.
 
UPDATE!

Found this in the waterline going from the impeller to the motor when I reverse-flowed it with the outdrive off. Would this be the culprit? Going to test now :)
 

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Did you run the impellar test with the bucket? That would tell me alot!! That would isolate it to a water flow issue. I need to be able to give you more ideas for solid tests to run, if not the drive. more info on your steps please!!
 
Zklink5150 said:
Did you run the impellar test with the bucket? That would tell me alot!! That would isolate it to a water flow issue. I need to be able to give you more ideas for solid tests to run, if not the drive. more info on your steps please!!
Click to expand...


Can I do this using muffs? It's on a trailer, so not very easy to do it at the docks when others are waiting to go in. I plan on doing it for sure.
 
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