Merc 260 5.7L overheating

[lenny2113]

Regards to all !
I have a 5.7L Merc in my boat. When the engine is running the temp gauge shows 140-150. As soon as I stop the engine, temp gauge starts rising to 190. Risers and exhaust manifolds are very hot to touch.
Water pump impeller was replaced before last season (30 hours).
Out of the thru hull exhaust I get a normal amount of cooling water comming out.

What could it be?

Thank you !

 

[Fun Times]

Re: Merc 260 5.7L overheating

WOW this is kind of a good qustion, Before you turn the engine off are you letting it idle for a little while? Every engine will get hotter after running it B/c it is not circulating the water any more, But I have not myself seen it go up that high myself that I have noticed, But it could. What temp thermostat do you have and is it new? And have you ever changed the circulator pump yet? And do you run fresh or salt water?

 

[lenny2113]

Re: Merc 260 5.7L overheating

Thanks,
Well...
I shut it down as soon as I stop.
It is salt water, T-stat is not new.
Water pump is 4 years old.
Impeller is 30 hours old.

 

[Fun Times]

Re: Merc 260 5.7L overheating

Thanks,
Well...
I shut it down as soon as I stop.
It is salt water, T-stat is not new.
Water pump is 4 years old.
Impeller is 30 hours old.

Well as a start you might want to put a new thermostat in, And maybe a new circ, pump in. I am thinking you are getting some rust bulid up on the inside of the engine and exhaust manifolds and/or risers. And that might be holding extra heat. But I am only guessing at this point. Try letting it idle for a minute before shut down. [Has any one eles here on the fourms seen any thing like this before to help out with this one] thank you

 

[lenny2113]

Re: Merc 260 5.7L overheating

Ok, here we go:
This weekend I did some diagnostic work.
Changed Termostat - no changes.
Took off risers. They have clean water passage.
Manifolds are clean too.
Ran the motor on the garden hose "flush system" eliminating most of the outdrive water (spring controlled valve). This would eliminate faulty impeller clue........NO CHANGE.

Temp gauge is showing 130-140 when running. Riser and manifold are way too hot to touch. When I turn the engine off, temp gauge rises above 180.

Any suggestions, please.

 

[rfelectron]

Re: Merc 260 5.7L overheating

Can you explain how running on the hose muffs eliminates a bad impeller?
On my Alpha Gen 2 I can turn water on to the muffs all day and I don't get one drop of water up to the engine until I start the engine which turns the impeller. My guess would be that water flow to engine depends on the condition of the impeller whether on or off the muffs. Also IMO the riser temp directly depends on water flow to the engine as raw water is fed to the bottom of the center rise manifolds and up through the risers. IOW more water from the impeller= lower riser temp. Are you running restrictor riser gaskets or the open gaskets? If your manifolds and risers are clear then the only things I can think of would be impeller and hoses (kinked?) between impeller and manifolds.

 

[Showtime22]

Re: Merc 260 5.7L overheating

Lets recap -
Engines overheating
Thermostat is good.
Risers have flow.
Impeller questionable...

I would take the lower unit off and change the impeller. 4 years seems like a long time to be working perfect. Next I'd remove water hoses and check for restrictions...

 

[lenny2113]

Re: Merc 260 5.7L overheating

Thank you fellows for your answers.

Impeller is 2 seasons old - 30 hours at most.

rfelectron,
"Can you explain how running on the hose muffs eliminates a bad impeller?"

I have a fresh water flush valve by "flush-o-matic" (as I can recall).
This system is a T-like plastic housing with a spring inside. It stays in the open position at normal operation and lets the water from the outdrive flow into cooling system. When the garden hose is connected to the flush valve's fitting, water pressure pushes the spring down leaving only very little amount of salt water going in. (That is done so that outdrive cooling water does not get back pressure and blows gasket in the impeller housing.
So, when I connect to the garden hose, the impeller cooling is no longer a key factor. Garden hose without muffs
What's next? Can timing be one of the overheating causes?

Thank you

 

[lenny2113]

Re: Merc 260 5.7L overheating

Are you running restrictor riser gaskets or the open gaskets? QUOTE]
Open gaskets.
Next thing, I will be measuring exhaust water temp. on both engines (ONE RUNS COOL).
I would have started by changing the impeller and eliminate that as part of my guessing game in 2 hours and about $50 worth of parts. The boat however is stored in the water for the season, hence I would like to avoid bringing her to dry dock ($$$$$$).
Can some type of mechanical failure cause overheating? If yes, what could it be?
1. Oil clear and up to the level
2. Engine block, manifolds and risers have 135 hours on them - 4 seasons.
Like I have already mentioned, there are no restrictions in the manifold /
riser.
3. Belt is adjusted on the water circulating pump.

Thank you in advance !!!

 

[Cptkid570]

Re: Merc 260 5.7L overheating

This is defininitly an interesting topic. Most are from people whos engines are hot when running.. this one is about an engine that is hot when not running.. Is it possible to have enought flow to keep the engine cool when running, but engine doesn't actully hold much water inside of it to keep a low temperature when just turned off? Just seems odd.. Mine heats up some after I first turn off the engine... If you turned on your blower and pumped out some of the hot air in the engine compartment would it help? (I know, that sounds silly)

Any blockage could maybe be from the old impellor.. maybe a blade came off and got sucked in there.. you could pull te water hose from the inner part of the transom assembly.. blow into it and see how easily the water exits the engine... you could also leave it off, start your engine on a hose for a second and see how rapidly water shoots out of the hose fitting too. Just trying to think of some ideas that are inexpensive and easily to perform.. (don't know how good of ideas they are)

 

[Cptkid570]

Re: Merc 260 5.7L overheating

This thread could help you:

http://forums.iboats.com/showthread.php?t=333469

 

[lenny2113]

Re: Merc 260 5.7L overheating

.. this one is about an engine that is hot when not running..

When I said runs hot, I mean I know it is hot because I cannot touch riser and manifold. When it is running the gauge shows 140 - 150 (measuring flow of cooling water). When it is stopped, the temp climbs up to 190.
That tells me that the actual operating temp is 190 (Overheat).

Thanks.
Lenny

 

[lenny2113]

Re: Merc 260 5.7L overheating

timing issue ?

 

[Glastron_V210]

Re: Merc 260 5.7L overheating

I don't think you have a problem at all.

The engine itself is likey in the 190F range when it is running. Remember that your sensor is a COOLANT sensor, not a combustion temperature/engine metal temperature sensor. The coolant stream, by definition has to be cooler than the engine for any heat transfer to take place, and the coolant stream is not 100% efficient. In other words, there will always be a differential between the engine temperature and the coolant temperature if the water is flowing.

When the coolant flow is suspended, the coolant heats up to aproximately the engine temperature.

When the flow resumes, the temperature plumets precipitously. My feeling is that this is normal, and further it is demanded by thermodynamic laws to be the case.

Most temperature sensors are electric, therfore the operator never sees the temp increase due to the fact the guage is powered down. How are you seeing it...thermocouple guage, or leaving the key in the run position after the engine is off?

Chay

 

[Glastron_V210]

Re: Merc 260 5.7L overheating

Oh, yeah, and 190 is not overheated. Those of us with closed cooling systems generally run 190F thermostats. No big deal.


Chay

 

[lenny2113]

Re: Merc 260 5.7L overheating

I don't think you have a problem at all.

How are you seeing it...thermocouple guage, or leaving the key in the run position after the engine is off?

Chay

Leaving the key in the run position with the engine off.
I switched risers from the engine that runs cooler to the engine that runs hotter. No change. Engine that runs hot still runs hot. Risers are not the issue. I am thinking either circulating water pump or timing. I am clueless if it could be something else I should look at. I am tempted to give up and let the marina mechanic start looking at it.

Thanks !

 

[Fun Times]

Re: Merc 260 5.7L overheating

Leaving the key in the run position with the engine off.
I switched risers from the engine that runs cooler to the engine that runs hotter. No change. Engine that runs hot still runs hot. Risers are not the issue. I am thinking either circulating water pump or timing. I am clueless if it could be something else I should look at. I am tempted to give up and let the marina mechanic start looking at it.

Thanks !

Key word is runs hot, or does it sit and get hot? You might not have any thing wrong like Glastron_V210 said.

 

[lenny2113]

Re: Merc 260 5.7L overheating

It is a twin Merc setup. One engine runs much cooler, I can keep my hand on risers and manifolds. The other engine runs so hot that it steams and water boils if it gets on the riser and manifold.
While the engine runs it gets a flow of cooling water which sends a reading to the gauge at around 140 - 160.

 

[dvan1901]

Re: Merc 260 5.7L overheating

When you replaced your impeller, was it intact or in pieces? You might have a flow restriction somewhere due to a broken impeller piece. Try replacing the water pump. Have you pulled all the hoses and checked for restrictions in them?

 

[studdy05]

Re: Merc 260 5.7L overheating

First, I am going totally with Glasstron and say your engine is well within tolerance, and this is dead of summer, and with no water flow pushing over the sending unit, you are getting a simmer effect on your temp guage, IF it is reading correctly. And this is the dead of summer, the hottest part of the year. I suggest replacing the temp guage and sending unit, cheap enough, and getting an Infrared thermometer at harbor freight to double check your temps are accurate, but still the engine is fine.

If you are suspect of this being like a prostate - the commercials call it a "growing problem" then you might just have to wait out the symptoms getting a tad worse to point you in a better direction. It's better than throwing a ton of money into it if you can keep it from gnawing at you.

If it's keeping you up at night, give it to a professional. Otherwise, it sure doesn't seem like it is that bad of an issue right this moment.