Logo
FULL SITE MENU

Mercruiser 3.0 overheating

C

coldwhack

New member
Hey guys Im new here and new to boating. Recently purchased 91 bayliner capri 3.0 merc seems to have overheat issues i have replaced impeler and t stat(140 deg) on muffs, temp runs about 140 to 150 got out on water and temp went to 180,185. i shut it off let cool down put it back on trailor. first off im not sure what temp should be runnig at. had it up to about 3000rpm on water i know nothing about boats or motors for that matter. please help
 
Last edited:
Welcome aboard !

With the drive submerged, remove the long hose that comes from the transom that hooks to the t-stat housing, start the engine for a few seconds and hold the end of the hose straight up you should have about 8-12 " of water shooting out the end of the hose quick shut off the engine.

If you do have good water to the t-stat housing then i suspect your elbow is clogged up.

Common problem and if the boat was used in saltwater the elbow lasts tops 5 years.

When you have the elbow off look for a flapper shutter, They sometimes fall off and fall down blocking the exhaust.

http://www.marineengine.com/parts/m...9901994/0c856559-thru-0f353099/exhaust-system

# 6 in the diagram. You may have to use a mirror to see it if it fell down. It has to come out no if and or buts.
 
Last edited:
Thank you for getting back to me, I picked up gaskets today, tomorrow I will remove the elbow and inspect it. I will let you know what I find, again thank you so much for your reply

QUOTE=Master mechanic;414431]Welcome aboard !

With the drive submerged, remove the long hose that comes from the transom that hooks to the t-stat housing, start the engine for a few seconds and hold the end of the hose straight up you should have about 8-12 " of water shootingout the end of the hose quick shut off the engine.

If you do have good water to the t-stat housing then i suspect your elbow is clogged up.

Common problem and if the boat was used in saltwater the elbow lasts tops 5 years.

When you have the elbow off look for a flapper shutter, They sometimes fall off and fall down blocking the exhaust.

http://www.marineengine.com/parts/m...9901994/0c856559-thru-0f353099/exhaust-system

# 6 in the diagram. You may have to use a mirror to see it if it fell down. It has to come out no if and or buts.[/QUOTE]
 
This is the flapper I removed. Was still in place but not in to good of shape. How important is this can I remove it completely or do I need a new one
 

Attachments

  • IMG_0805.JPG
    IMG_0805.JPG
    39.7 KB · Views: 1,590
Last edited:
Replace it or you stand a good chance of a backwash induced engine lockup because the cylinders ingested lake/sea water.
 
That question has been discussed I have a 85' no shutter.

What I do when I come off plane I do it very slowly and turn slightly.

It is up to you.

How does the elbow look?

volvo does not install flappers and recommend if they are installed to remove them.
 
Last edited:
The elbow doesn't seem clogged up. It has a bit of rust not bad though and a bit of carbon or soot so I sprayed it real good with parts cleaner I'm also going to replace the temp sender I figure that can't hurt. Won't get a chance to put it in the water till next weekend. But hopefully all will be well
 
Master mechanic said:
Let us know if you have good water to the t-stat housing.
Click to expand...
hi new here as well .
i have a 1988 marlin with a 3.0 merc. having the same issue. .i recently changed oil and used silver star 25w-40. and th filter i put on was a little smaller but it says
there interchanable. anyways . temp is fine running out side with hose . and get in to the water and temp spikes . . what would happen if there was no water at the t-stat housing .
thanks
 
Stock 3.0l engines did not come with a heat exchanger, some came with power steering as a option.

The water enters the transom thru a fitting.

Say your impeller went in the drive pieces could clog up that fitting on the outside of the transom.

But seaweed and other objects can be sucked up also. Say a plastic bag.

So first we have to find out if you have good water entering the transom.

The first stop for the water maybe the t-stat maybe the power steering cooler.

Say there is no water or very little water, it's time to check or in what i do is change the impeller. And while the drive is off it doesn't hurt to back wash the hose and see what pops out the drive.

Good water coming in then the next stop is the t-stat, change it out with the correct one. Still overheating next stop is the elbow. And the flapper shutter.

Maybe someone before you installed a new flapper and never found the old one.

Keep us posted.
 
coldwhack said:
on muffs, temp runs about 140 to 150 got out on water and temp went to 180,185.
Click to expand...

does the overheat only happen when you are running fast on plane?

that's a sure sign of an impeller sucking exhaust gases. The Alpha Gen 2 impellers are tricky to seal and love to pull in exhaust. there's a Merc bulletin with some tips and tricks.

Replacing a section of the water supply hose between drive and thermostat housing with clear hose for a test run can help on the diagnosis.
 
I had an overheating problem. Changed the impeller. Inspected the risers (all ok), replaced the thermostat. Turned out the impeller I replaced was defective. The metal casing around the impeller slipped and was rotating withe the impeller. Engine heated up very quickly while acceleraring due to no cooling water coming in. Had to replace new defective impeller. All is good now.
 
Back
Top