165 Merc in-line six Overheat Problem

I`m on my second thermostat in 3 months.
I originally had an overheat issue that I noticed while charging my batteries in the driveway. Good find I figured. Better on the trailer than in the water.
I pulled the engine circ pump and it was in excellent condition. I then pulled the thermostat and the lift pin under the part that rises when the temp comes up was pushed through the top making it impossible for the thermostat to open. So, thermostat got replaced and the engine still overheated. So, the water pump impeller in the sterndrive got replaced and all was well, for about four hours. I was out yesterday doing some trolling and the engine started getting HOTTT!
I shut down and let it cool off a little and decided to pull the thermostat. When I lifted it out the lift pin fell out from inside the thermostat. I put the housing back together w/o the thermostat and it would still overheat at speeds less than 1000 rpm, but over that and it would cool OK.
I got home OK and pulled the raw water supply hose off at the thermostat housing for a few seconds while doing a flush and it appeared that there was good water flow from the outdrive.
I`m stumped. What`s up w/the thermostats?
The engine is a raw water cooled Merc 165 hp in line six w/ alum manifolds and riser.
Has anyone had any similar experiences?

Re: 165 Merc in-line six Overheat Problem

Are you in salt water? Was it a marine thermsostat?

Re: 165 Merc in-line six Overheat Problem

Saltwater? Yes
Marine thermostat? Yes
I also am looking at the upper stat housing to check for obstructions keeping the stat from opening. It appears that there`s plenty of clearence.

Re: 165 Merc in-line six Overheat Problem

Was the thermostat installed properly? In the right place?

Re: 165 Merc in-line six Overheat Problem

It's also possible you have a defective NEW tstat. Head to your local Merc dealer and get a good quality OEM stat and give it a try.

Re: 165 Merc in-line six Overheat Problem

If you are buying quicksilver t-stats, even they can be total crap. I had mine in for a total of 20 minutes and upon next startup, it was stuck open. Another one solved the problem.

*Just a thought*
Not that this is your problem, but do a search on 165 HP and there is a thread about a cracked exhaust manifold, it shows you how to re-plumb the hoses for better water flow thru the manifold at bow up cruising, this may help. It is really to address the cracking, but I also assume more complete flow as well. Maybe someone could elaborate on this.

Re: 165 Merc in-line six Overheat Problem

The engine is a raw water cooled Merc 165 hp in line six w/ alum manifolds and riser.

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manifold(s)? on a 165hp L6? got any pics of that, i've never seen that before...

have ya gone to the service manual? the trouble shooting section. it has a real nice flow chart as to possible problems with overheating. and there are many causes too.

ya could check your tstat out to be sure it works too. a pan of water and a thermometer on the kitchen stove...

Re: 165 Merc in-line six Overheat Problem

Yes it is possible that I got a defective stat just like the defective engine coupler I had put in a couple of months ago.
The last stat was a mercury stat not a off brand I`ve got a new one sitting next to me ready to put in after I do some more investigating on the boat.

Re: 165 Merc in-line six Overheat Problem

Hi I have 1973 inline 165hp had the same problem so last winter i took off all the parts that the water flows through it. I de rust all the parts from the log to the temp houseing and it was a lot of work to do it but know it runs better than it came from the factory. I got a merc t-stat and all the gaskets and i all so strip of all the paint of the log and got about 90 of the rust out of it and took to my local powder coat shop and they put a high heat paint on from the outside to the inside of it and cost about $180 but well worth it u can run the engine hard and u can put ur hand on the log and its warm but not burning hot. So i would recomend clean all the parts and making sure theres no clogs any were. Can u tell me when u pull the log drain pug is it clear water or with junk in it.

Re: 165 Merc in-line six Overheat Problem

Porker, I normally pull the manifold drian plug many time during winter season because I`ll use my boat during the winter and if freezing weather is forcast, I`ll pull the drains to keep from freezing, including the block.(I`ve installed valves to make this easier) The first time I drain I might have some gunk that keeps it from draining but a piece of wire shoved up in the hole clears that up. The water comes out clean after the first few oz`s.of water the first time and the drain runs open from then on after that first cleaning. BTW, I have an aluminum manifold on my 165 that was originally made for a 160 HP.

Re: 165 Merc in-line six Overheat Problem

How many times a year do you find "gunk" in your block?

Re: 165 Merc in-line six Overheat Problem

Update..
I put a new tstat in and my temp gage started climbing again. The tstat housing didn`t feel that hot though so I started thinking sender or gauge. I replaced the sender and all is good as of now.
I still can`t figure why I had two stats go bad. There is plenty of opening in the housing for the stat to rise.
I`ll be getting it back out on the water this weekend to give a good op test.

Dean

Re: 165 Merc in-line six Overheat Problem

The thermostat will go in 4 different ways. Only one is correct. Your statement that it has room to rise makes me question how you have it in there.

Re: 165 Merc in-line six Overheat Problem

The thermostat goes in one way,Spring loaded side down, just like an automobile thermostat

Re: 165 Merc in-line six Overheat Problem

dpoff said:

Update..
I put a new tstat in and my temp gage started climbing again. The tstat housing didn`t feel that hot though so I started thinking sender or gauge. I replaced the sender and all is good as of now.
I still can`t figure why I had two stats go bad. There is plenty of opening in the housing for the stat to rise.
I`ll be getting it back out on the water this weekend to give a good op test.

Dean

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Please share your sender part #. I need one for my 120HP. I put a cheap autozone gauge in the hole and found my engine was never overheating. I felt dumb.

But t-stats are junk now in 2009. My original ford t-stat from 99' was built 10times better than my new one I put in. Same with marine parts, getting cheaper and cheaper.

Anyone see the halloween candy this year? Absolutly unacceptable. WAY smaller and same price as last year. How about smaller = less price.

Re: 165 Merc in-line six Overheat Problem

Sorry FD I can`t find the packaging for the #. It is a sierra part and it was stamped "made in Mexico" Uh Oh!

Re: 165 Merc in-line six Overheat Problem

Even OEM parts now are comming from Mexico and China now and its a sad state of affairs. 'nutcase