1980 omc 120hp highspeed overheat need help

hey guys i know a few people have had this problem before but i just got a 17 foot runabout for dirt cheap i hope if the repairs are minimal i got the engine running recently fresh gas fresh oil and cleaned and set the points as well as changed gear oil the problem imhaving is when i start to accelerate past 1400 rpm i get a very rapid increase in engine temperature maxing out gauge within 1 minute or less once back off temp drops in smooth and fast manner
and holds steady at 145 degrees

the engine water pump is fine and water is definitly being circulated properly the ball gearsare getting lots of water and
the water is exiting the propller normally i dont want to replace the outdrive impeller if i dont need to so if you guys have any tip on flushing the water system in reverse id like to try as there are lots of rusts flakes

thanks for your inputs

impeller issues usually cause overheat more at idle, not 1400+ rpm

One common issue with higher rpm overheating is exhaust gases being sucked into cooling water in the lower unit. The solution is to grease the swivel bearing in the lower unit. There is a hole on the front side of the outdrive to access the grease fitting. Give it 6 or 8 shots of your favourite grease. You may need a small tip for the grease gun like what is used for u joints.

However, plan to change that impeller asap. Always on a new-to-you boat, and especially on an OMC stringer. You need to grease the splines on the water pump impeller shaft. That's the main reason for doing it every few years. The impellers can last 10 years in Ontario if treated well (not run dry on land). However, the spline grease washes out every 2 or 3 years and then the shaft strips and you are paddling.

hey thanks for your reply could explain what the grease is used for and where it goes and to clarify its the grease nipple facing the bow of the boat right by the shifter housing and cable on the driveshaft housing on the lower unit

thanks lucas

that's it - in through that access hole on the lower unit.

The grease:
1. lubricates the steering (notice the lower unit swivels when steering, and the upper half of the drive is fixed straight on your style of sterndrive - unlike mercruiser , volvo etc)
2. seals exhaust from getting sucked into cooling water entering the impeller housing


at idle there isn't enough exhaust to worry about, but when you give it hell, the exhaust volume and pressure as it tries to exit the prop goes way up.

okay i really hope this solves the problem when i dropped the lower unit recently i noticed the area where it meets the impeller housing was dry i really hope this solves where does the exaust flow after the intermediate case