3.0L Merc high speed overheat.

[brucetech09]

Soooo I found this site while trying to fix my boat. I have a 1999 3.0L Merc, freshwater only. Last season I was getting an overheat at high RPM 200+ degrees. I have a 160 thermostat. I assumed it was the impeller so I had that changed (housing as well) and I changed the thermostat myself. Low speeds it sits at 160 and high speed it USE to sit at 170 or so. Now it goes to at least 180 and climbs from there. I pulled the riser and it looks perfect, so I reinstalled with a new gasket. We have had the boat in the family since new. I have ruled out impeller, thermostat. The riser and manifold looked clear. the water circulating pump didnt look to be leaking. The hoses looked good. My thought was maybe the water intake hose is collapsing???? I really do not know. the season before it was perfect. last year it started overheating.

 

[Bt Doctur]

using a temp gun is it really that hot? gauges and senders are usually the culprit if inpellers were changed. The elbow may look clear but did you try to rod it from the outlet side (where the water comes out)
Your raw water is pressure from the housing upwards simply changing the impeller and top but not the base can cause a hi speed overheat .

 

[tschmidty]

Also can have bits of the impeller clogging things up.

 

[brucetech09]

I have not used a temp gun. I did not run a rod or anything through the elbow, they looked really good both sides. The base was also changed with the impeller housing. I am going to try to change the water intake house/ gimbal hose.....after that I will have essentially checked everything I am familiar with....I am going to try and pull that house through the transom so I dont have to pull the drive (if that is even possible?) [h=1][/h]

 

[brucetech09]

Also can have bits of the impeller clogging things up.

I bought the boat new and the impeller has never been broken at all. This change I expected to see it broken because of the overheat and was surprised it still looked being 5 years old. I have been thinking about a new boat the last season or so, may be time to trade her in.

 

[alldodge]

Have you looked inside the exhaust tube going to the drive to see if the flappers have broken off and our blocking the exhaust some?

Next, remove to bottom large hose off the circulating pump and see if the vanes on the pump have come loose from the shaft

Pulling the drive is a 10 minute job, not talking about splitting it

 

[brucetech09]

The flappers looked good i checked when i pulled the riser. So when i pull the hose to the pump the vanes should be connected to the shaft? Is pulling the druvw that easy? I need access to the transom behind the bell housing

 

[alldodge]

Yes the vanes are connected to the shaft which is connected to the pulley

To pull the drive
Raise the drive and disconnect the small tube, this is the speedometer tube
Lower the drive and then put throttle in full forward (WOT) position
Remove the bolts holding the trim cylinders
Remove 6 nuts from drive and pull drive off

 

[Bt Doctur]

everything you need to know on how the elbow works http://www.michel-christen.com/ExhaustElbows.pdf

 

[brucetech09]

checked the water circulating pump last night and it looked great. One thing I havent checked is the power steering cooler, could that thing be clogged up???? After the water enters the transom that is the first place it goes to.

 

[brucetech09]

Anyone else? Anyone in southern Maine want to try to figure it out? I have a check ready for you.

 

[alldodge]

The water cooling system is pretty simple, you either flow enough water or you don't. Not flowing enough water is either a restriction or a pump not able to pump enough water from leaking or a restriction. From the write up your have checked or replaced everything, so it should work but it doesn't.

Only thing left is something is not what it appeared or there is something with the pump

Only other thing is there is a head gasket leak

 

[brucetech09]

I have pretty bad luck so at this point I would assume it is a head gasket....I will change the circulating pump and see if that helps. Thanks for the reply’s.

 

[alldodge]

I have pretty bad luck so at this point I would assume it is a head gasket....I will change the circulating pump and see if that helps. Thanks for the reply’s.

I wouldn't change the circulating pump, I would look again at the drive water pump or a restriction. The drive pump housing is just as important as the the impeller. The Gen 2 drive uses pins to keep the pump body centered during install.

 

[brucetech09]

Thank you. We are going to pull the drive this weekend and change the drive to transom hose. Maybe it is clogged or collapsing. Also going to check the water pump again. I may check the manifold and block drain holes and see if they are junked up. I will follow up with the results.

 

[brucetech09]

Pull the freeze plug, nice clean water. Pulled manifold plug....nothing came out. Jammed a wire in there and all that black sand and some rust came shooting out. I am assuming I fixed the problem. I pulled almost every hose looking for obstructions and flaws. The picture is only some of it the rest is in the bilge. That is many years of sand. Only the water test next week will tell. Any advice on flushing the manifold without pulling it?????? I found pouring water in the top and jamming wire in the drain hole whole it drains works ok. I hooked a hose to it and I think it just pushed it around because it wouldn’t come out.

 

[alldodge]

That could be the problem

 

[brucetech09]

I hope so. Temp gauge is now sitting where it use to. Water test next week though.

 

[brucetech09]

I will post a follow up. Are the end caps on my manifold for cleaning the manifold??? If that is the case I may pull those too.

 

[Maclin]

I think the best thing to do is leave the bottom drain plugs out on the manifold, take the hose off at the thermostat housing that goes into the manifold, and keep pouring water in there and poking the drain holes until it flushes and all you see is clear water coming out the bottom. Leave the end caps intact, no need to disturb those.