3.0l over heats in the water but runs like a champ on the muffs. What the hell?

[SSGRHYNO]

So two weeks ago I overheated and I blew my head gasket. I have since replaced the manifold and riser along with the thermostat housing, thermostat, and impeller. boat runs fine on the muffs but over heats within minutes on the water. What else can I do? I'm ready to sink this boat in the intra coastal.

 

[Howard Sterndrive]

Re: 3.0l over heats in the water but runs like a champ on the muffs. What the hell?

what year/drive?

 

[RyanKernen]

Re: 3.0l over heats in the water but runs like a champ on the muffs. What the hell?

what year/drive?

its a 91 cobra behind a 3.0L OMC.

 

[Lou C]

Re: 3.0l over heats in the water but runs like a champ on the muffs. What the hell?

When you put it in the water disconnect the hose that goes from the transom mount to the thermo housing. Hold the hose up and have someone start the engine (briefly). You should see a 2-4" head of water coming out of that hose. If not either there is a restriction or an air leak in the pump or hoses. What I do when I replace the impeller is after I remove the housing, I hook up the muffs and hold them tight to the lower unit and have someone turn on the water. Water should spurt forcefully out of the opening on the right side of the impeller mounting surface. If not you have an obstruction between the water intake holes and the upper unit water tube, could be debris in the passage between the holes and the tube (we get marine growth there so it has to get cleaned out every so often) or bad water tube gaskets.
Next I disconnect that same hose transom to thermo housing. Then I stick a garden hose in the end of that hose, go to the back of the boat and turn on the water. Water should spurt out of the left side hole on the impeller mount. If not you might have old impeller pieces clogging the hose, P/S cooler, etc.
Last the little nipple on the impeller housing must allow water out, it helps the pump to prime. When you run the engine leave the plastic cover on the end of the drive off, make sure there is no water leaking around the impeller housing and that water spurts out of that little nipple.
You may need to replace the impeller wear plate too, because they also get worn.

 

[Gilded_Splinters]

Re: 3.0l over heats in the water but runs like a champ on the muffs. What the hell?

The water pump in your drive is not terribly powerful. It produces maybe 7 or 8 pounds of pressure, whereas your garden hose produces far more than that...10 times that at my house.

The muff/hose arrangement is forcing far more water up there than a failing pump will draw out of the lake on it's own and therefore give you a false sense of what's going on.

Lou's advice above is excellent to spot possible obstructions and really ensure that water is coming and going where it needs to.

I use the following arrangement for winterizing, but also for my initial start ups of the season:

I have a 7 gallon pail with a simple garden hose faucet in a hole in the lower side. This is all glued in with 5200 sealant. Attached to this is about 3 feet of garden hose with the muffs attached. I place this pail on top of another one that is upside down, which puts the water source close to the height of the water intake on my drives. After the muffs are in position, I open the valve, start the engine and let the pump draw the water out of the pail in much the same way it does naturally with no additional pressure from the garden hose.

To winterize, I drain blocks and manifolds of all fresh water, blow a shop vac into all hoses to evacuate any water still standing in any hose bends, etc..

The above bucket rig is used (although from the swim platform this time) to circulate 5 gallons of RV antifreeze throughout the empty system.

The garden hose is fine for flushing junk out, but don't rely on cooling system performance based on the incorrect amount of water that it will deliver to your pump. A bad pump will frequently seem fine on the hose.