mercruiser 3.0l overheated, then back to normal! well kind off please help..

alex3012red

Seaman
Joined
Nov 20, 2006
Messages
65
Hi to all first of all, haven't been on here in a while, haven't been using boat much, sat for 12 months. never had problems at all till now. 300hrs on motor saltwater used all of its life, took it out yesterday, as usual started right up, put it in the water let it idle for about 25 min. temp 130 took it out for a short trip 3500 rpms for about another 25 min. no problem. went for another trip, about ten miles of running at 3800 rmp, temperature rised to 220 deg. shut her down about 3 min, started back up and temp came down, then it started to go up again 180, shut her off, started, made weird noise, so I checked oil o.k, out drive o.k, started up o.k 1000 rpm temp 140 for about 10 min, then 2000 rpm for another 10 min temp 140... got back to camp. sat for an hour went for 5 mile trim 4000 rpm ran at 140.... what could have gone wrong, changed impeller last year, oil, and outdrive oil too. no water in oil, from overheating, seems to have the same power.
for being saltwater used all of its life, risers and motor look great. all original never changed riser or manifold.

where should I start looking or testing. want to keep the cost low due to economy but want to keep boat running great again...
could weird noise be because of overheat, sounded like a bad motor for about 5 seconds never heard before on boat, then sound normal again, it idles and revs fine..
 

dingdongs

Chief Petty Officer
Joined
May 29, 2009
Messages
649
Re: mercruiser 3.0l overheated, then back to normal! well kind off please help..

you may have picked up a bag over your intake on the drive but you may still have been lucky.An impellor replacement is a must to check as that would now be destroyed.Dont take a chance on putting back in the water and hoping,replace and hope nothing else has failed.if its a mercruiser pull the lower unit after draining oil and replace impeller and its housing if its bad.2 hour job and its diy to save on marina bills and give you some more gas.
 
Joined
Oct 21, 2012
Messages
61
Re: mercruiser 3.0l overheated, then back to normal! well kind off please help..

It looks to me that the rust in manifold and riser from sitting unused for 12 months finally plugged the riser and now overheats only at high rpms. I would pull the riser to make sure it has clean passages.
 

alex3012red

Seaman
Joined
Nov 20, 2006
Messages
65
Re: mercruiser 3.0l overheated, then back to normal! well kind off please help..

i will change the impeller and order the gasket set for the manifold to check it out thanks...i'll do the acetone test on manifold to see if it leaks... thanks
 

alex3012red

Seaman
Joined
Nov 20, 2006
Messages
65
Re: mercruiser 3.0l overheated, then back to normal! well kind off please help..

finally took it apart exhaust elbow was clogged, one other question, I didn't see a flapper in the exhaust tube? does it need one?
 

Don S

Honorary Moderator Emeritus
Joined
Aug 31, 2004
Messages
62,321
Re: mercruiser 3.0l overheated, then back to normal! well kind off please help..

finally took it apart exhaust elbow was clogged, one other question, I didn't see a flapper in the exhaust tube? does it need one?

Yes it needs the flapper. Probably burned up and fell off. Look in the exhaust passages and bellows area, and in the drive for pieces.
 
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