Mercruiser 350/260hp - run without thermostat?

[allingeneral]

I have a 1989 Sea Ray with a 350/260hp Mercruiser w/ Alpha One outdrive. Is there any reason that I shouldn't run it without a thermostat?

I'm having overtemp problems at idle - especially after running down the lake on plane for a few minutes, then dropping to an idle - temp quickly climbs to 220*F - which I know is way hot. Hopefully, I haven't done any permanent damage to the block or heads.

I'm going to replace the impeller/pump housing in the lower unit and see what effect that has. Current impeller is about to start its third year.

I'm also hoping not to have to mess with risers. Is there a good way to check them for blockage without removing them?

 

[1980Galaxy]

Re: Mercruiser 350/260hp - run without thermostat?

I had an overheating problem in my mercruiser 120 last year. After taking off the outdrive to replace impeller and looking for blockages in the exhaust manifold, I took apart the t-stat housing and found that I didn't have a t-stat in there at all (since the engine was just replaced). I put a t-stat in and everything worked fine again. I don't really understand why the engine would overheat without a t-stat but thats what was happening.

 

[Oldskool]

Re: Mercruiser 350/260hp - run without thermostat?

It is highly unlikely that your engine will ever come up to normal operating temperature with the thermostat removed. This will result in it running rich. You need to determine what is causing the engine to run hot.

 

[littlebookworm]

Re: Mercruiser 350/260hp - run without thermostat?

You may be afraid of them, but your manifolds and risers may be your problem. Were they ever replaced? You could check them with a thermometer gun. You aim it at the spot and it gives you a reading. If your risers and manifolds are original, that's probably your source of your overheating. If you are even a little mechanically inclined and have some basic tools, you can change them yourself. Download the proper manual; read the previous threads on manifold changing for the tricks (or PM me), get your parts and go to work. In a few hours all will be good again. Hy

 

[Rocky_Road]

Re: Mercruiser 350/260hp - run without thermostat?

I had an overheating problem in my mercruiser 120 last year. After taking off the outdrive to replace impeller and looking for blockages in the exhaust manifold, I took apart the t-stat housing and found that I didn't have a t-stat in there at all (since the engine was just replaced). I put a t-stat in and everything worked fine again. I don't really understand why the engine would overheat without a t-stat but thats what was happening.

Just to address your last question: the missing thermostat will allow the cooling water to pass through the block too fast to absorb all the heat. This is what would happen in an automotive environment, and will be the same in your boat.

The engine manufacturer designed the engine to cool properly...and reach the necesary operating temperature...using the correct thermostat. If it wasn't needed, they would have saved the production costs of putting them in!

Research the correct thermostat for your engine, and relace it if you are worried about it being an issue.

Happy boating!

 

[jtmarten]

Re: Mercruiser 350/260hp - run without thermostat?

Just to address your last question: the missing thermostat will allow the cooling water to pass through the block too fast to absorb all the heat. This is what would happen in an automotive environment, and will be the same in your boat.

The engine manufacturer designed the engine to cool properly...and reach the necesary operating temperature...using the correct thermostat. If it wasn't needed, they would have saved the production costs of putting them in!

Research the correct thermostat for your engine, and relace it if you are worried about it being an issue.

Happy boating!

+1
When the t-stat is removed, the temp gauge shows cool because the water is flowing too quickly to pick up heat. So your engine is overheating while you think its running cool.

The reason it overheats after a hard run is because at idle the water is flowing slower and can pick up more heat, so the temp gauge shoots up. Your engine is running hotter than that while running hard, so you could be doing significant internal damage, certainly shortening its service life.

 

[nofuss]

Re: Mercruiser 350/260hp - run without thermostat?

If you remove the thermostat, it may remove the symptom. (overheating) like taking a pain killer for a brain tumor, instead of getting the right medical advice, no pain no problem. however there are several things that could cause you to be overheating and from what you indicate the impeller could be one main culprit, being unable to supply enough cooling water at low rpm. but it could also be blockage in your manifold, from corrosion, which is only allowing water to flow through with the increased pressure produced by the pump at the higher rpm. Now if your manifolds are corroded, and they fail while running, "remember the brain tumor" well the effects could be just as bad, it could cost u an engine.
Change the impeller, if the problem goes away TOTALLY, then u might be ok, but if not check the manifolds, if in doubt, change them. if you can change the impeller in the water pump, you could definitely change manifolds and risers. its way easier. just download the right manual and u are on the way.
But remember always be sure of the cause before u treat the symptom, and if you treat the symptom to get the boat back to dock FIND THE CAUSE. I learned this the hard way, u dont need to.

Cheers
Pernell

 

[allingeneral]

Re: Mercruiser 350/260hp - run without thermostat?

Replaced the water pump (all except the base) and replaced the thermostat. Checked the port riser and it's in surprisingly good shape for a 1989 model. Still have to pull the starboard riser and hopefully it will be as clean as the port side was. It's time for a break right now though. May finish it up tomorrow. Then off for a test drive tomorrow and hopefully a happy summer from there on out

I appreciate everyone's replies. You guys are great!

 

[allingeneral]

Re: Mercruiser 350/260hp - run without thermostat?

Took it to the river today. It wouldn't shift into gear without dying. I knew immediately that I hadn't done something right when putting the lower unit back together. So, I brought it back home and took it apart, after checking out the Adults Only thread to get the link to the Mercruiser Manuals, and sure enough - the shifter in the lower unit had to be held in forward gear while mating things back together.

So, I drained my new $16 quart and change of lower unit lube back into the original container so I could reuse it. Pulled the lower unit apart - AGAIN lol.

Turned the shift rod in the lower unit clockwise, then held the prop counter-clockwise to keep it in forward gear. Lined it all up and zipped it back together.

Took it back to the river and did a few checks to make sure there weren't any leaks from pulling the port exhaust elbow and replacing the thermostat. All looked good, so I took it for a ride. I'm a happy camper again

The only thing I don't like is that the parts guy gave me a 160* thermostat. I may go back for a 140* thermostat since the motor still seems to be running a bit hotter than I like at ~195*

 

[Rocky_Road]

Re: Mercruiser 350/260hp - run without thermostat?

Took it to the river today. It wouldn't shift into gear without dying. I knew immediately that I hadn't done something right when putting the lower unit back together. So, I brought it back home and took it apart, after checking out the Adults Only thread to get the link to the Mercruiser Manuals, and sure enough - the shifter in the lower unit had to be held in forward gear while mating things back together.

So, I drained my new $16 quart and change of lower unit lube back into the original container so I could reuse it. Pulled the lower unit apart - AGAIN lol.

Turned the shift rod in the lower unit clockwise, then held the prop counter-clockwise to keep it in forward gear. Lined it all up and zipped it back together.

Took it back to the river and did a few checks to make sure there weren't any leaks from pulling the port exhaust elbow and replacing the thermostat. All looked good, so I took it for a ride. I'm a happy camper again

The only thing I don't like is that the parts guy gave me a 160* thermostat. I may go back for a 140* thermostat since the motor still seems to be running a bit hotter than I like at ~195*

Almost every Mercruiser uses a 160 degree thermostat...he gave you the correct part.

Going to a cooler thermostat will not change the operating temperature--it will only open at the cooler setting, and increase the time it takes to reach operating temperature. If you are concerned about the operating temperature, you need to do some more 'digging'.

Keep in mind that our gauges are not 100% accurate. If you have access to an infrared temp gun, shoot the thermostat housing when at operating temperature and see what you really are running!

Happy boating!

 

[Don S]

Re: Mercruiser 350/260hp - run without thermostat?

If it's running ~195? with a 160 stat, it will do the same with a 140? stat.
Do you run in salt or brackish water? Have you ever removed the exhaust riser and checked the passages to see if they are becoming blocked with rust and corrosion?

 

[allingeneral]

Re: Mercruiser 350/260hp - run without thermostat?

I do run in brackish/salt water a lot (Potomac River). I only pulled the port riser/elbow and it was really very clean. There was a little bit of rust, but nothing that caused me any concern. I went ahead and cleaned it up a little and reinstalled it on with a new gasket. Assumed the stbd would be the same, so I didn't disassemble it.

Both elbows were hot enough that I couldn't lay my hand on them. I could touch them, but not for long.

Maybe I should invest in one of those temp guns. Anyone have a preference?

 

[Rocky_Road]

Re: Mercruiser 350/260hp - run without thermostat?

I do run in brackish/salt water a lot (Potomac River). I only pulled the port riser/elbow and it was really very clean. There was a little bit of rust, but nothing that caused me any concern. I went ahead and cleaned it up a little and reinstalled it on with a new gasket. Assumed the stbd would be the same, so I didn't disassemble it.

Both elbows were hot enough that I couldn't lay my hand on them. I could touch them, but not for long.

Maybe I should invest in one of those temp guns. Anyone have a preference?

You can do what I did: I needed a decent RMS Multimeter, and chose the Craftsman #81079 w/IR thermometer. I am now able to trace all of my electrical issues, and can 'point' the beam at any spot for an accurate temperature readout.

Single tool that covers a lot of issues!

Happy boating!

 

[Don S]

Re: Mercruiser 350/260hp - run without thermostat?

Sounds like you have your work cut out. If you know your impeller and raw water pump are OK, and it's overheating still, then you need to pull the risers and inspect. You can get a temp gun if you want, but if you can't hold your hands on the risers for 5 seconds, then it's overheating and you need to pull the risers and inspect.
In salt water, manifolds and risers are expendable maintenance items. Sorry, it's just a fact of salt water boat life.
At this point removing and inspecting will do more good than running, overheating again, just so you can use a temp gun.
Then, since the engine dies when going into gear, my guess is you probably need a new lower shift cable. Wonder how old the bellows are? and is there water in the ujoint bellows??

 

[allingeneral]

Re: Mercruiser 350/260hp - run without thermostat?

Sounds like you have your work cut out. If you know your impeller and raw water pump are OK, and it's overheating still, then you need to pull the risers and inspect. You can get a temp gun if you want, but if you can't hold your hands on the risers for 5 seconds, then it's overheating and you need to pull the risers and inspect.
In salt water, manifolds and risers are expendable maintenance items. Sorry, it's just a fact of salt water boat life.
At this point removing and inspecting will do more good than running, overheating again, just so you can use a temp gun.
Then, since the engine dies when going into gear, my guess is you probably need a new lower shift cable. Wonder how old the bellows are? and is there water in the ujoint bellows??

Just replaced the raw water pump/impeller.

Port riser looked ok - plenty of life left and not a lot of corrosion.

I wonder if I have an obstruction in the block somewhere...

Or maybe I need to replace the circulating water pump on the front of the engine? It does seem to be squeaking a bit...

The engine died going into gear because I put the lower unit back together after replacing the pump/impeller without following the proper method. I took it all apart and re-assembled it and it shifts fine now. (BTW - I just had the shift cable replaced last year).

 

[nola mike]

Re: Mercruiser 350/260hp - run without thermostat?

+1
When the t-stat is removed, the temp gauge shows cool because the water is flowing too quickly to pick up heat. So your engine is overheating while you think its running cool.

The reason it overheats after a hard run is because at idle the water is flowing slower and can pick up more heat, so the temp gauge shoots up. Your engine is running hotter than that while running hard, so you could be doing significant internal damage, certainly shortening its service life.

this isn't true. the reason it overheats at idle after a hard run is because you have the same amount of engine heat, which is now being cooled by less water. the heat will actually transfer faster if you have more or cooler water flowing through. the t-stat doesn't regulate water flow once it reaches its operating temp. at that point (and all temps higher than that) the t-stat is all the way open. only purpose is to let the block heat up by limiting flow when cool.

OP, where on the potomac are you?

 

[allingeneral]

Re: Mercruiser 350/260hp - run without thermostat?

OP, where on the potomac are you?

Mike - I'm on the Virginia side near Colonial Beach / 301 Bridge (the Salt Line). Where are you?

 

[dirtyoldman]

Re: Mercruiser 350/260hp - run without thermostat?

I have a 1989 Sea Ray with a 350/260hp Mercruiser w/ Alpha One outdrive. Is there any reason that I shouldn't run it without a thermostat?

I'm having overtemp problems at idle - especially after running down the lake on plane for a few minutes, then dropping to an idle - temp quickly climbs to 220*F - which I know is way hot. Hopefully, I haven't done any permanent damage to the block or heads.

I'm going to replace the impeller/pump housing in the lower unit and see what effect that has. Current impeller is about to start its third year.

I'm also hoping not to have to mess with risers. Is there a good way to check them for blockage without removing them?

How bad was your raw water pump? If it was bad there could be pieces stuck somewhere, like your power steering cooler.

 

[nola mike]

Re: Mercruiser 350/260hp - run without thermostat?

Mike - I'm on the Virginia side near Colonial Beach / 301 Bridge (the Salt Line). Where are you?

about 5 miles down from CB, just before westmoreland state park--sounds like maybe 10 miles or so from you. we're at lunch often at happy clam or

also, i don't have the specifics, but you can measure the flow of the engine by hooking it on muffs and seeing how much exhaust water you get/minute.

 

[allingeneral]

Re: Mercruiser 350/260hp - run without thermostat?

How bad was your raw water pump? If it was bad there could be pieces stuck somewhere, like your power steering cooler.

The old impeller was completely intact. Good thought though. Thanks!

I'm starting to wonder about timing... if I have the timing advanced too far, that would cause it to run hot, eh?