1979 OMC 120 Sterndrive runs very hot at low RPM

I have a 1979 Sea Star with a 2.5 liter OMC 120 engine and 400 series Stern drive. The engine is GMC inline 4 cylinder. Model number of engine is 990243S. I worked on the boat over the winter and started putting it in the water this spring for the first time. The boat is new to me, never saw it run before I bought it. Had several basic problems which I repaired and was able to get the boat up and running on the rabbit ears in the driveway earlier this spring. Motor ran great after warming up and all was well. Problem after we started taking it to the lake we noticed almost right away the motor would start to get hot (Over 220). If you run it long enough the motor will begin to knock and diesel when you try to shut it down. Only would cool down a bit if you were cruising at about 3/4 throttle. Stopping and starting it just got hotter and hotter. I finally found a drum big enough to test in driveway so I could test without rabbit ears pumping water into the inlets. Found right away that it does not pump water below about 1800 RPM. Over the winter I had the Outdrive disassembled and replaced the water impeller and all the gaskets and worn parts. No trouble reassembling, and everything looks to be in good shape. Still puzzled as to why it will not pump water at lower RPM. I did notice however that in the past someone had used a fair amount of sealing compounds around some of the parts. I did not make any extra effort to add to these compounds as I was certain I replaced all the proper gaskets and seals. Has anyone ever heard of this model having internal leakage problems between the inlet water passages and the outgoing exhaust that would be causing a deficiency in the water flow?

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

Pull the hose from the outdrive and see if you have a steady stream of water coming up from the impeller. There are various things in the OD that could cause the impeller not to deliver water. You will know for sure if you can see the volume.

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

were the impeller vanes pointing the right way? contrary to popular belief, they do not right themselves if backwards.

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

Yes I was able to check the flow by removing the hose. There is no flow until about 1800RPM. Then water comes out the hose. Also removed one of the pivot cover caps and saw the same result. No water until about 1800RPM unless it is on the rabbit ears. As far as I know and I was careful when I reassembled the water pump the vanes were going the correct direction. Unless I can think of something else I am going to open it back up and check everything again one night this week.

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

lakedog said:

...... Unless I can think of something else I am going to open it back up and check everything again one night this week.

Click to expand...

I do not think there is 'anything else'. You tested it pretty well. I think you missed something when changing the impeller. Read the details of what that might have been, in the manual. I know they talk about just that.

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

years ago there were some aftermarket impellers where the hub insert would slip. I think it was GLM, but approx. a decade ago.

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

Yes I believe I missed something. I will take it apart again and see what I can find. I am still curious if it is possible that even with the proper seals and gaskets in place that it could be leaking inside somehow. That may explain the extra sealing compound I found. All of the metal parts looked to be in good condition, so I would not have expected a problem like that. This is my first I/O repair I have mainly worked on Outboard motors previous to this. I will double check the assembly and all of the gaskets and seals, and of course the vane direction on the impeller. I will also see if I can find a way to water or pressure test the unit before I reassemble it. I will post some photos of what I find. Thanks to all for the help so far.

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

OK I took it apart again and found everything assembled correctly. I took some time and traced and blew air or water through all of the water passages to confirm no obstructions. Also verified Howard Sterndrive suggestion that impeller may be slipping on center hub, it was not. I looked over the service manual instructions and did more research on the web. Found that I should have applied gasket sealing compound to the top of the pump assembly before putting it together. Did that and reassembled the stern drive. We placed the sterndrive in the 55 gallon drum filled it and started up the motor. At first we had no water flow then after I revved the engine a bit the water began to flow and filled our jug just as the manual indicates 1 gallon in about 25 seconds at about 550 RPM. Ran the motor in the drum for almost half an hour replacing some of the water slowly with the hose to keep it from getting to hot. Engine temp never got over about 125. All sounds pretty promising doesn't it? Took it to the lake this morning thought we would plunk it in and run around for half an hour or so and before I could even get to the middle of the lake the engine temp climbed to over the 200 mark. Pulled hose from inside and no water. Turned around and went back to dock. All toll in water about 15 minutes. Took her home put the sterndrive back in the drum and after slight rev to motor pumped water steady at 550 RPM and ran cool for 15 minutes or so no problem. I am perplexed.

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

the slight rev to get the flow going is an issue, it should pump steady after the engine starts. You should verify the parts are correct for your model O/D..you never know. Another issue is one that got mentioned on another thread recently but that's the swivel housing, if its not greased up it can suck hot exhaust gasses and overheat. Definitely grease that (back of the drive, with the leg up, look for a hole, a few inches in there is a grease zerk fitting)

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

Yes I thought about the parts being incorrect as well and will check them. I am familiar with the zerk because I changed the seals that go in the swivel bearing and found the zerk. I greased it after reassembly and when I opened up the lower O/D I found that the fresh grease was present in good quantity.

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

View attachment OMCWater.pdf

I have also noticed something about the pump assembly itself. Mine had a large O ring the diameter of the pump case. I am not sure it was in the correct place when I took the pump apart and so now i am confuused as to where it may go. On some schematics it does not show this O ring at all but with many of the pump rebuild kits the large O ring is included. I have asked 2 different companies that supply them if they know where it goes and so far no one is sure. Here is alink to one of the replacement parts sites SIERRA - Water Pump Kit for OMC Sterndrive&mfgno=18-3386

attached is diagram one company sent me stating the O ring was item 38. That is not the O ring in question. In this diagram the O ring is not even shown.

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

Its either #21 or #53.

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

Installed new water pump kit. Still can not get it to pump water. Rigged up test tank out of drum to see if we could get it to work before putting it back on the boat. Powered it with attachment made up for 1/2 drill.I found information on a website that stated water must be deep enough to submerge pump so not sure if drum will in fact work. However we were able to get this to work when unit was on boat. Boat will not pump water when you get to the lake though.

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

Was the impeller housing replaced as well?

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

that drum isn't even close to deep enough.
This is the depth at which the pump is designed to self prime:

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

Superbenk, Yes I bought the pump kit that included the housing the plate the impeller and O rings. I am still not sure where the large O ring they include with the kit goes. I put it between the plate and the housing.

Howard thanks for the photo. I had read that the drive needed to be submerged as you show. I plan to put the unit back on the boat and take it to a small lake near us on Saturday weather permitting. It seems we are getting rain just about everyday. I live just outside Dayton Ohio. When we took the boat to the lake a couple of weeks ago it of course ran hot right away. It was very busy that day and I had no opportunity to mess around with the motor near the dock. I am hoping for a better opportunity at this smaller lake on saturday.

Thanks for all of your help so far. Would like to get this licked and start using the boat for some fun. Have a great 4th everyone.:joyous:

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



if you mean oring 76, it goes here:

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

The pictures show the stages of assembly for the water pump. I used sealant per the service manual instructions. I placed the O ring between the impeller plate and the Swivel Housing as in Howard Sterndrive illustration.

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

These 2 pictures show the plate just before installing the swivel housing and after it is bolted together.

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

I have a bunch of pictures from when I did mine as well. Maybe they'll help identify something?

https://picasaweb.google.com/m/viewer#album/102782264640776793236/5448245489809728881