1979 OMC 120 Sterndrive runs very hot at low RPM

[lakedog]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

Took the boat out this afternoon. Seems to be cooling better now. Still gets a little warm when you are maneuvering around the dock slowly. But out in the water temp stayed around 140. So much better.

 

[Boomyal]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

What did it get up to around the dock? What thermostat are you running? IMHO 140? when you are under load is way too cool for optimum combustion chamber efficiency.

 

[lakedog]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

It would get to over 200 around the dock. It does not have a thermostat in it. Previous owner must have removed it. I do not know much about the previous owner except they had the boat for a long time and seemed to have took pretty good care of it.

 

[Boomyal]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

Put a thermostat in it it. 140? is way too cold for normal operation. It does not allow the cylinders to expand properly and increases wear on the bore. It also does not make proper use of the fuel. If you do not run in salt water, use a 180? stat. If you do run in salt water, follow the recommendation in your manual. If you still have a temperature issue, get it figured out with a thermostat in place.

Also, have you verified the temps with an IR gun?

 

[bruceb58]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

My Volvo in my signature uses a 140? thermostat. Its not uncommon for raw water cooled engines to have that or a 160?.

 

[Boomyal]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

My Volvo in my signature uses a 140? thermostat. Its not uncommon for raw water cooled engines to have that or a 160?.

Both those temperatures are too cool bruce. They were picked as compromises in the event the boat was used in saltwater. That being said, changing the thermostat in a carbureted engine works just fine. You cannot do it in a Fuel Injected engine unless you can re-map the program. The computer is looking for cool temps and tries to richen things up if it detects higher temperatures.

 

[lakedog]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

The manual I have calls for a 160? thermostat. I will order one. I have not used an IR gun on it. I could borrow one from work. Never thought about that. I will try that as well. I would think though that the outside surface temp of the engine would be different that the reading the the temp gauge on the dash gives or the water temp the thermostat senses.

 

[bruceb58]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

The 140? thermostat is the stock temp thermostat for my engine. Remember that the temp of the thermostat is where it starts to open and usually fully open 20? above that. A 180? is way too high if you don't have closed cooling. I have a carbed engine.

His thermostat specified for his engine is part #0383307. It is a 160? thermostat.

EDIT: Looks like lakedog and I posted at the same time. I would do the IR temp gun too. The temp of housing where the temp gauge screws into should be within a few degrees of what the gauge says after temps equilize. Its common practice to measure that way.

 

[Boomyal]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

For years I ran a 180? stat on my raw water cooled OMC 190hp 302. It ran right on 180? whether idling or pulling 5 water skiers. Subsequently, I added a closed cooling system (not including exhaust mani's) It still runs dead on 180?. Maybe different engines behave different than mine but that is my experience and I'm sticking to it.:smug:

 

[bruceb58]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

Ha...I believe you!

 

[lakedog]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

Installed the 160? thermostat on Saturday and we took her to the lake on Sunday. Came right up to 160? and pretty much stayed there. Occasionally it would run up to about 200? if we would slow down for a few minutes at the quiet end of the lake or around the docks. But it never went over that. I believe it may be good to go. Thanks to everyone who responded the input helped a lot. Have great summer all. Hope to speak with you again soon maybe on another project.

 

[bruceb58]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

If its hitting 200?, its not good to go. You still have a problem.

 

[Boomyal]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

get an IR temp gun and verify the temps. 200? is too hot. You have an endless resevoir of 65-75? water. There is no reason that your engine should not run at thermostat temperature, except that something is still amiss.

 

[superbenk]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

Mine runs well below 160? at idle & hovers around 160? at cruising speed and up. Sounds like a problem still. I still question your impeller or vacuum through your drive.

 

[lakedog]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

Well that is wht I would think as well. Sort of out of ideas. I will check the sending unit and try to confirm the temp with a ehat gun. I showed a local boat mechanic the photos I took some of them I posted here. He says that everything should be OK he is puzzeled as well about the temp hitting 200. Back to the tool box I guess.

 

[Boomyal]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

Have you timed and measured the water flow coming from the line from the intermediate housing to the thermostat housing? Not sure what the exact volume should be but I do believe it is a published figure.

 

[lakedog]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

Yes we have checked the water flow from the hose. My manual says 1 gallon in about 25 seconds at 550 rpm. I have seen it do that no problem. Checked it at the lake a week ago. Was even able to do that from the drum you saw the photo of on my driveway. Had wife hold the jug and I gave it a bit of gas to get it started. After settled in at idle put jug under and no problem 1 gallon less than 30 seconds. "I am not saying that was a good way to test but I am just covering what we have tried" At the lake with unit fully submerged just as in one of the photos I believe you posted the water will pump but sometimes it still can be intermittant.

 

[lakedog]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

Was able to put the boat back in the lake Sunday. Ran it for almost an hour slow, fast and let it stop and we would idle for a bit. Never got above 160?. Used the heat gun but it gave various temps depending on where you would aim it. If I aimed it at the temp sensor it read about 225?. Finnaly we took it back to the dock and had to putter around a bit waiting to get out. It finally did climb up to about 200? shot it with the temp gun but it read about 214? at the temp sensor. Talked to a couple of guys about the temp gun here at work they are not sure if a surface reading really tells you much as even moving air can cause a big swing in the reading.

 

[Boomyal]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

lakedog, unless you were in arctic temperatures I don't think the air can move fast enough to significantly affect the temp reading on specific locations like the water pump return hose, thermostat housing and the area immediately around where your temperature sensor is located.

How is the temp on your exhaust manifold? You should be able to hold your hand on it. Not being familiar with your engine and setup, is there any chance you have any restrictions on the manifold outlet? That could back water up and not allow a sufficient intake to keep things at thermostat temps.

 

[lakedog]

Re: 1979 OMC 120 Sterndrive runs very hot at low RPM

I bought the boat last year. Over the winter I worked on it and found the manifold had at one time frozen and cracked. It had been repaired but was leaking. I aquired a used one that was in excellent condiiton from A guy in Georgia I believe. I have had all of the manifold and hose assemblies off including the water circulation pump cleaned anything that was dirty and reassembled. The engine itself and based on pictures I have seen of other engines of it's era is in pretty good and clean shape. As far as the manifold temp when it was running I did shoot a reading on it but do not remember the temp. Yes I believe you can hold your hand on it while it is running.