Low Engine Temp??

[SHickey]

Good Morning and it's been a long time since my last post...
I have a very old girl, 1976 Starcraft American 20......She has a straight six, 165 HP Merc sterndrive..., This spring I changed the impeller in the lower unit because it had not been changed in 4-5 years. I have a fuel pump and electronic ignition to help her run well and start easy.....the first time I took her out she ran great and registered an average temp of @ 150 degrees.....the last two time she's been out she has been running @ 115-120 and stays there....plus she runs very rough just puttering around or at idle speed...

My novice mechanical experience would tell me to replace the thermostat or possibly the temp gauge.....so I was wondering if this kind of this happens to boaters in general....usually motor temps go up with poor circulation or a chewed up impeller.....I'm also thinking it's running rough because it's not making it to it's operating temperature......

We just moved near the S end of Canandaigua Lake in NY only 5 minutes from the state boat launch so any advice or recommendations would be welcome..... Safe Boating!!

 

[Scott Danforth]

if your thermostat is stuck open or failed open, it can run cool

 

[Lou C]

Even just a flake of rust or a bit of sand can keep it from closing

 

[SHickey]

if your thermostat is stuck open or failed open, it can run cool

Thanks for the response....I can remove it and test it.... I believe thermostats are closed when cold and open at a certain temp....probably best just to replace it and hopefully eliminate the problem.....I think there were 2 different thermometers with different opening temperatures listed for my motor....Which would be best for upstate NY climate.... in your opinion...

 

[Scott Danforth]

140 F with a carb and raw water cooling
160 with a carb and heat exchanger cooling

 

[CaptnKingfisher]

My first step would be to verify low operating temp by using an infared temp gun on the engine. If that confirms you're running cool then inspect your thermostat. This is what a "stuck open" condition looks like. If stuck open, replace with part recommended by your service manual.

 

[SHickey]

140 F with a carb and raw water cooling
160 with a carb and heat exchanger cooling

Awesome and thanks!!

I took the thermostat out of the housing and it already was open 25%...it's pretty old so I think I'll clean it up, order a new one, reinstall the old one and see if it keep acting up....if not, I'll switch it out when I winterize the old girl...

I'll go with the 140 F considering I don't have a heat exchanger....you guys are the best!! Thanks so much!!

 

[Lou C]

I love those old inline 6s, I remember when that was the standard engine on most GM midsize cars, the Chevy 250 cu in six.

 

[SHickey]

I love those old inline 6s, I remember when that was the standard engine on most GM midsize cars, the Chevy 250 cu in six.

Yes Lou....
It's a workhorse.....I bought the boat and trailer 15 years ago for $600 just for the trailer and I was going to use the trailer for a different boat but kids and life forced me to keep the boat anyway, redo the seats and all of the vinyl and she still gets us around.... the motor had a tick from day one which annoys me but I can't kill it no matter how much I take it out..... When it finally goes then we'll get a newer open bow craft probably @ 22'-24' and keep it at a slip....but when it will finally go is anyone's guess....

 

[Eric 3.7LX Alpha 1]

I love those old inline 6s, I remember when that was the standard engine on most GM midsize cars, the Chevy 250 cu in six.

Yeah, Ford and Jeep also made some great inline 6 engines.

 

[Earl Cordova]

Just throw out the old and don't waste time playing with it.

 

[SHickey]

Yeah, Ford and Jeep also made some great inline 6 engines.

So getting back to my thermostat situation....I tested my old one and it worked but it opened at 165 F......It was recommended I buy a 140 F thermostat for my motor....that being said if I use the new 140 F, will my motor run right at low speeds and idle or will I need to goof around with the carb mixtures and possibly the timing to compensate for the older thermostat operating temp over the last 10 years?......as always thanks for the replies...this motor is a dinosaur and no marinas want to work on it anymore...

 

[Eric 3.7LX Alpha 1]

So getting back to my thermostat situation....I tested my old one and it worked but it opened at 165 F......It was recommended I buy a 140 F thermostat for my motor....that being said if I use the new 140 F, will my motor run right at low speeds and idle or will I need to goof around with the carb mixtures and possibly the timing to compensate for the older thermostat operating temp over the last 10 years?......as always thanks for the replies...this motor is a dinosaur and no marinas want to work on it anymore...

No. There should be no issues changing to the 140. There are no different specifications in Manual 2 for closed cooling or freshwater cooled as far as timing, point gap, dwell or carb settings. If your engine has issues running, it won't be due to the lower thermostat setting.

 

[SHickey]

No. There should be no issues changing to the 140. There are no different specifications in Manual 2 for closed cooling or freshwater cooled as far as timing, point gap, dwell or carb settings.

Awesome!! and thanks so much....I'll order a new thermostat today and see how it works....

 

[Searay205]

I had a 250 in-line six with a tick for about 80,000 miles so good luck outlasting it, lol. The tick was caused by a lifter, that being said don't try to change it out, you won't get it out, sure it will move up and down in the bore but the part of the lifter that never enters the bore is coated with oil varnish and will probably have to be driven out from bottom. Once out easily replaced...

 

[Lou C]

Yeah, Ford and Jeep also made some great inline 6 engines.

Yep, we've had a '98 Jeep 4.0 since new, one of my favorites.

 

[Lou C]

BTW, I just changed my 1988 OMC 4.3 over to a 140* stat. I wanted to years back but could not find one that fit. After doing some research I found that OMC & Volvo used the same thermostat housing for years. On the V6s and small block V8s they both used 160* stats. However, Volvo used a 140 stat on the 7.4 liter big block engines from '94 on up. I bought one just to see if it would fit my stat housing and it did, looked like its made by the same supplier as the OE stat as it has all the same patent markings on it etc. So with this when warming up it runs at 135-140, on plane 140, after coming off plane it doesn't go above 145. Before with the 160 stats it usually was at 160 warming up, on plane, 160-165, and after coming off plane about 170-175. I did this to reduce calcium deposits due to raw water cooling in salt water. The only thing I was concerned about was would my well type choke work as well, but the stat does not affect exhaust temps only cooling water. So its working out quite well. In fresh water you can use a 160* with no worries but for us salt water folks I think the 140 is better.