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Old mercruiser

Lrtracker

New member
Hi just got on the forum I have a 1980 merc 120 with a 470 out drive I am having a overheating problem. I have replaced the pump in the stern drive no help. It will run 180 while idling but on the lake will overheat I'm about 15 min can any one help
 
welcome to the forum! for future reference, you'd be better off post mercruiser questions in the mercruiser area....were all of the vanes still connected to the old impeller? 180 at idle is really too hot...any idea when the exhaust elbow was last replaced? you should be able to put you hand on the elbow at cruising speed - if you can't, not enough water flowing thru it...an IR temp gun is great for making the measurement and can save the skin in your hand too..
 
thanks will go to right area in future. new at this. at shop now mech. don't know how to fix. had boat very short time don't know when elbow was replaced. is this a normal replacement part. i will look at it when i get it back. if it does get hot is there a blockage or just problem with elbow. thanks for your help.
 
if mechanic "don't know how to fix" time to get new, competent mechanic....

if a vane was missing, it could obstruct the water flow (=overheat)....if the elbow clogs (water outlet where water gets 'dumped into' exhaust), reduced water flow = overheat....similarly, if the exhaust baffle fails (usually due to lack of cooling water) if can constrict the exhaust and impact engine performance...


Routine replacement of the exhaust is normal in any water other than very low mineral fresh water lakes.... most of the creeks on the chesapeake offer an environment that yields ~ 7 years of normal service life, with typical usage....you may be able to clean out the elbow once or twice but you won't see much service life after that.
 
BTW, unless somebody changed the gears in the upper drive, a drive from a 470 uses a different ratio that the stock unit behind a 120...in other words, unless the gearset was changed in the drive, your engine will work harder than it was designed to (yields shortened life).
 
Within the exhaust manifold and elbow, you will have seawater transfer ports.
These ports allow the seawater to leave the manifold and enter the elbow.
If these transfer ports have become rust scale restricted, you will have over-heating problems.


Pay attention to what Mark said about the 470 drive behind a 120 engine.
There is NO substitute for a correct over-all gear ratio for a particular Engine.
 
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