Help! Overheating only when underway.

[rmurray]

I need help, I am stumped. The boat ran great last weekend. I launched it today and it overheats when underway. It goes from 120 to over 200 very fast and goes into limp mode. A few revs in neutral kicks it back to 120 fairly quickly. Back underway and it heats up again. In trying to get it back on the water, we replaced the thermostat and impeller. I also checked the hoses running to and from the thermostat housing, and they look OK. The impeller that I removed looked Really good also. The old thermostat was pretty rusted and corroded, but the new one went in nicely. I can run it in neutral and rev it up in neutral, and it does not heat up a bit. It only heats up when underway. I really don’t know what’s happening. Can anyone make any suggestions that might help me out. We’re at the lake for two more days and would really like to use the boat. 2003 GXI/SX.

 

[rmurray]

Could it be my water pump? It’s not leaking or anything, just seems so “all of a sudden”. I figured it could be the water pump since I think that’s the only thing left in the chain. Should I check the risers or anything else? Could it possibly be the water temp sensor acting up? I really need to figure this out or the weekend will be a total bust. 4 hour drive to the river.

 

[Tyger23]

Engine is under a lot more stress when underway then when running on a trailer. The fact that it only happens when underway makes it seem to me like you've lost a seal between the upper and lower units of your outdrive. Basically, if you're sitting still, the outdrive can still generate enough suction to supply water, but if you're underway, there's not enough suction to pull water in while it's rushing by the outdrive.

This thread seems similar to what you might be experiencing:
https://forums.iboats.com/forum/eng...465-found-a-piece-of-rubber-behind-the-prop-🧐

 

[Lou C]

If you can see if you can find a document called "Volvo Penta Overheat Diagnosis" there is a PDF floating around the 'net that details all the procedures that Volvo advises to diagnose overheat problems. If the thermostat housing was rusted up inside, did you make sure the passages were open, the ones that feed the exhaust manifolds and the bypass passage inside? They should all be open. Also if the thermostat housing was rusted up inside the exhaust elbows could be as well, are they original to the boat? If you go to check them, then drain the manifolds before cracking the exhaust elbows loose first. If you find the exhaust water passages clogged up then replacement is in order.
I'd also put the boat in the water and do the raw water volume test that Volvo specified in their manual, this will tell you if the impeller is pumping enough water. These open systems depend on a lot of cool water in (impeller, water tube gaskets in the outdrive) and water out (exhaust water passages in the manifolds & elbows) anything that restricts water in or out will cause overheating.

 

[bruceb58]

How old is your impeller? I replace mine every 2 years on both my Volvo Penta and my mercury outboard.

 

[rmurray]

How old is your impeller? I replace mine every 2 years on both my Volvo Penta and my mercury outboard.

Impeller is brand new. The one I took out was one season old. Boat has 220 hours on it.

 

[rmurray]

Engine is under a lot more stress when underway then when running on a trailer. The fact that it only happens when underway makes it seem to me like you've lost a seal between the upper and lower units of your outdrive. Basically, if you're sitting still, the outdrive can still generate enough suction to supply water, but if you're underway, there's not enough suction to pull water in while it's rushing by the outdrive.

This thread seems similar to what you might be experiencing:
https://forums.iboats.com/forum/eng...465-found-a-piece-of-rubber-behind-the-prop-🧐

I appreciate the link, but not sure this is anything like my issue. I’m still suspicious of the recirc pump. I will be looking at it today. I’m getting a little noise from it, like loose bolts, but everything is tight. It might have failed, but would the motor still cool if it failed?

 

[Lou C]

as long as a previous owner did not replace the marine circ pump with an automotive one (where the impeller blades could have rusted off!) failure is marked by a noisy bearing that will start to leak, it will still pump though.
One last thing I forgot to mention, on both the OMC Cobra and the Volvo SX there is a plastic nipple in the pivot housing that can crack and allow the cooling system to lose water pressure and suck air in, it is not easy to replace but should be looked at. Tilt the drive up, look under the left side, it is where the transom water hose goes into the pivot housing.
If the boat was not properly winterized sometimes these can crack (ie drive left up so water can't drain).

 

[kenny nunez]

As Lou said the pivot housing nipple or even the hose that connects the nipple to the transom could be cracked.
If you want to really “bullet proof” that portion do this.
Get a through the hull pickup installed and connected to the raw water inlet Then cap off the supply hose fitting and add a marine water strainer as an option if you beach the boat often.
I realize you probably cannot do this right now but give it some thought.

 

[Lou C]

http://www.volvopentastore.com/Gimb...on_id.864076792--store_id.366--view_id.783173

Here it is, part # 24. They usually don't cause a problem but if you don't put the drive down so the raw water intake hose can drain water then it can crack. The OMC Cobra has a drain plug there that I take out when I winterize it, and a good bit of water drains, I am not sure if Volvo actually used a drain there, but the designs while similar are different because of the differences in where the raw water impellers are located (Cobra in upper gear housing, Volvo on engine). A good practice if you get freezing temps...
After you drain the engine, manifolds and impeller hoses, put the drive down, take the hose that comes from the transom to the impeller housing off at the impeller (if volvo) hold it up and fill it with -100 marine AF till it runs out the drive water intakes. This will push water out of the hoses and PS cooler. I also fill the engine & manifolds with AF. Helps reduce corrosion by keeping oxygen away from the cast iron.

 

[bruceb58]

Put a clear piece of hose on the output of the water pump where it connects to the thermostat housing. Then you can actually see if you are picking up air from the transom housing fittings or o-ring.

When was the last time you took of the drive? When you did, did you replace the oblong o-ring?

 

[rmurray]

Ok, so here’s more info. First of all Lou, Kenny, and Bruce, I think you guys are correct. I recently pulled the drive and did some work. I talked to a mechanic and he said the same thing. The water hose or nipple is most likely damaged. When the boat is in the water, it runs cool. As soon as the boat gets up on plane, it overheats. Basically, when the hoses out of the water, it’s probably sucking in air. I’m not sure what happened between last weekend and this weekend, but it definitely hampered our trip. I am going to pull the drive off when I get home and inspect the hose. I will definitely consider lose suggestion to add a better through hole fitting there.

 

[rmurray]

Put a clear piece of hose on the output of the water pump where it connects to the thermostat housing. Then you can actually see if you are picking up air from the transom housing fittings or o-ring.

When was the last time you took of the drive? When you did, did you replace the oblong o-ring?

I pulled the drive a couple weeks ago and replaced all the o-rings. I think the water hose or end fitting might be cracked.

 

[rmurray]

as long as a previous owner did not replace the marine circ pump with an automotive one (where the impeller blades could have rusted off!) failure is marked by a noisy bearing that will start to leak, it will still pump though.
One last thing I forgot to mention, on both the OMC Cobra and the Volvo SX there is a plastic nipple in the pivot housing that can crack and allow the cooling system to lose water pressure and suck air in, it is not easy to replace but should be looked at. Tilt the drive up, look under the left side, it is where the transom water hose goes into the pivot housing.
If the boat was not properly winterized sometimes these can crack (ie drive left up so water can't drain).

Lou, I’m guessing you are right. I’m in so cal so no winterizing, but I did mess with that hose quite a bit when re-installing the drive. It ran great right after though, so I’ll have to take it off and inspect. Thanks.

 

[rmurray]

So, I think I figured it out. Apparently I never removed the old o-ring from the water inlet nipple, and when adding the new o-ring, it ended up in the wrong channel, where the drain hole is. At any rate, the new o-ring is destroyed and that was probably causing an air leak into the system. Damn, I’m pissed at myself right now. The nipple looks fine and I plan to use it again. Thanks everyone.