Mercruiser 3.0L Overheating on Plane

NicksterFL

Seaman
Joined
Oct 29, 2014
Messages
61
Hi Folks
First post here, but have read through many posts on this form for months!
Hi, I have a 1971 Mercruiser 3.0 that was overheating on plane. I have only owned the boat for 3 months so I know no prior history other than it sat for a year. When I got the boat, the first thing I did was replace the impeller (entire housing, gaskets, etc) and then also the thermostat (140 marine thermostat). We took the boat out and it ran fine for about 30-45 minutes. Then while on plane it began to overheat. I dropped down to idle or a little over, just to keep moving, and the temp dropped back down to normal. I got back on plane and temp creeped back up.
Brought the boat home and replaced the exhaust manifold and riser (they were pretty bad) with new ones and also had to replace the impeller again (the plastic tube was melted) and top housing had bubbles in the plastic. So I completely reinstalled a new one. Now, I have it all re-assembled and have it running with the outdrive in a barrel w/water, and the riser is hitting 152-164 with my IR gun at idle. I cannot put my hand on the riser for more than 1 or 2 seconds before it's too hot.
I also checked compression and all cylinders are between 150-160.

I don't want to drop the boat in the water and burn through another impeller kit.

thoughts??
 

Grub54891

Vice Admiral
Joined
Jun 17, 2012
Messages
5,908
Check the water tube that goes from the drive,up to the transom ring.it's next to the bellows,port side. It may be cracked,and not getting enough flow under power to keep it cool.
 

John Poet

Recruit
Joined
Feb 27, 2014
Messages
5
I have a 1987 3.0 that had a simular problem. Mine was from the blades of the main water pump being corroded. I had replaced the impeller and the thermostat before finding the problem. My manifold and riser were in pretty good shape. I did replace the flappers. Good luck.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,297
guessing by your screen name, your in florida. is this a salt water boat. if so, your manifold and riser could be plugged preventing water from freely flowing.

I would pick up a new riser elbow gasket and pull the elbow to inspect the inside water passages.

you have a water flow issue somewhere. either exiting or entering the cooling system somewhere.

the raw water pump should flow just under 20 gpm, so you should be able to fill a 6 gallon bucket in about 16 seconds from the line from your raw water pump to the thermostat housing.
 

fishrdan

Admiral
Joined
Jan 25, 2008
Messages
6,989
I have it all re-assembled and have it running with the outdrive in a barrel w/water

Ditch the barrel and use a set of "ear muffs" to attach a water hose to the drive. If the water level isn't over the waterpump, or the pump gets starved of water, the impeller could get ruined. If the pump runs dry, it's going to get destroyed quickly.

With a new impeller and housing being installed, and the the plastic housing and plastic tube getting melted, you have a lack/limited/blocked water flow. Possibly the old manifold/riser was plugged up, or maybe you have broken pieces of impeller stuck in the water lines (drive's pump up to engine)

Look up in the stickies ("adults section") as there are some troubleshooting guide for overheating problems, also numbers for how much volume the water pump should deliver within a period of time.

A bad engine mounted water pump could cause the engine to overheat, but it wouldn't cause all the plastic parts in the exhaust system to get cooked. The drive's water pump will deliver plenty of water to keep the exhaust cool and the plastic parts from melting. (Supposedly, the drive's water pump delivers enough volume to cool a 3.0L/140HP even if the engine mounted pump wasn't working.)
 

NicksterFL

Seaman
Joined
Oct 29, 2014
Messages
61
Thanks!
So i think my problem is I have the wrong impeller kit. Any one know which impeller kit (top and bottom housing) would go to my MerCruiser 3L. The serial number is: [FONT=arial, sans-serif]2902609. [/FONT]

[FONT=arial, sans-serif]I pulled the drive off and put another impeller kit with a new base and top housing. After I assembled the impeller housing. I blew in to the guide pipe that comes out of the top and air leaks out of the side of the pump. [/FONT][FONT=arial, sans-serif]Wouldn't[/FONT][FONT=arial, sans-serif] this also "Suck" air when i'm on plane? this tells me I may have the wrong pump![/FONT]

[FONT=arial, sans-serif]This is what I ordered: [/FONT]http://www.amazon.com/gp/product/B005R46UQ2/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1

I haven't located the SN on my outdrive, but I no it's a prealpha and has a preloading pin at top.
 
Top