boat overheats on muffs running at high RPM's

[302mustang]

I rebuilt my motor, everything looks tip top, but if I spin it up to high RPM's for a period of time the motor overheats. I have it on muffs and if i run it at idle the temp does not seem to increase.

Is this normal? It was approaching 250 and I turned if off. I didn't hear anything strange or see any signs of smoke or overheating.

Should i not run the boat that long on muffs? could it be the impeller.

And does the boat have a thermostat?

96 glastron
GS30 vovlo penta 3.0 4 cyl.

 

[HT32BSX115]

Re: boat overheats on muffs running at high RPM's

There's a couple of things you should check.

1. failed raw water pump. If it has not been replaced in a long time, inspect or replace it. If it's destroyed, you may have old pump remnants stuck somewhere downstream of the pump.

2. plugged/restricted water pickup or not enough flow from the hose.

3. Stuck thermostat..... or in backwards.

4. plugged/restricted (from rust) exhaust manifolds/risers....cooling water comes in but can't get out! (you did check/replace the manifold/riser?)

It should have a 140-160 degree t-stat (don't know exactly but someone here will chime in) temps above 170 or so should not be happening. 250 is absolutely no-no! (make sure your temp gage is working though!!)

By the way, my Merc manual suggests an upper limit rpm when running on "muffs" Volvo may suggest that too.



Cheers,

Rick

 

[QC]

Re: boat overheats on muffs running at high RPM's

I spin it up to high RPM's for a period of time the motor overheats. I have it on muffs

Don't do 'dat no mo . . . Your hose cannot keep up with the water supply requirements above about 1500 RPM. Your impeller could be toast, but either way you cannot draw any conclusions from what you've seen . . .

Yes, boats have a thermostat, if it runs at the correct temp at idle, the 'stat is probably doing it's job.

 

[302mustang]

Re: boat overheats on muffs running at high RPM's

So in another words, if my temp hovers 150 at idle (which it does) were good to go on the t-stat.

And were saying that since the hose can't keep up with water requirements about 1500 rpms, we can safley assume i didn't have enough flow to keep her cool.

I know she'll stay at 150 at idle in my driveway all night long.

I'm just worried of opening her up on the big water and having an overheating problem.

 

[Gary H NC]

Re: boat overheats on muffs running at high RPM's

Before you toast the new motor i would advise replacement of the impeller.
Just for piece of mind anyway....

 

[QC]

Re: boat overheats on muffs running at high RPM's

So in another words, if my temp hovers 150 at idle (which it does) were good to go on the t-stat.

Probably. We can't absolutely confirm that it is opening all of the way, but stats only control minimum temp, not max, so it appears to be doing it's job . . .

And were saying that since the hose can't keep up with water requirements about 1500 rpms, we can safley assume i didn't have enough flow to keep her cool.

No, we can't safely assume that, but it may be the case. It is also possible that the combo of high RPM and restricted flow has had a negative impact on your impeller, so I agree with Gary ^^^^^

I'm just worried of opening her up on the big water and having an overheating problem.

Well you need to test her on the big water before you can conclude anything for sure. HT32BSX115's list can all be in play too

 

[Limited-Time]

Re: boat overheats on muffs running at high RPM's

I rebuilt my motor........................

And does the boat have a thermostat?

96 glastron
GS30 vovlo penta 3.0 4 cyl.

Not to be a smart azz......but just what type of rebuild did you complete that did not include an evaluation or replacement of the thermostat??

As for the big water test verify the temp gage is correct and monitor it closely while under way. Additionally there are specific guide lines with respect to operating RPM when breaking in a new or rebuilt engine. You may want to do a search for some guide lines.

 

[Bondo]

Re: boat overheats on muffs running at high RPM's

but just what type of rebuild did you complete that did not include an evaluation or replacement of the thermostat??

Ayuh,.......... Define Rebuild.....................

And,...........

but if I spin it up to high RPM's for a period of time the motor overheats. I have it on muffs

Define High RPMs,............

 

[Maclin]

Re: boat overheats on muffs running at high RPM's

Find a tub or a large tote tall enough to reach your water intake, put it under the unit and fill it up and run it that way, leave the hose on in the tub to be sure the water level stays above the intake. That way you can run the engine at any rpm.

 

[Limited-Time]

Re: boat overheats on muffs running at high RPM's

Find a tub or a large tote tall enough to reach your water intake, put it under the unit and fill it up and run it that way, leave the hose on in the tub to be sure the water level stays above the intake. That way you can run the engine at any rpm.

Thats no where near the same as running the boat in open water. And will not supply the engine with enough water flow to cool it at elevated RPM.

 

[QC]

Re: boat overheats on muffs running at high RPM's

I agree ^^^^, ultimately you have no more total flow to cool it than you do with the hose on muffs . . . same total cool water flow

 

[happycamper]

Re: boat overheats on muffs running at high RPM's

302Mustnag,
As someone already suggested, it's likely that the garden hose can't supply enough water through the muffs at high rpm. I've noticed the same thing.

See the attached pic which shows how I test my VP 3.0 using a big plastic storage container.

I almost never use the muffs anymore because:
a) it doesn't really test the supply pump, and,
b) it takes 40+ minutes for the thermostat to open when supplied with cold water from a garden hose.

The test tank recirculates the water, which warms up in about 15 minutes at 1500prm.

Good luck.

 

[Maclin]

Re: boat overheats on muffs running at high RPM's

QC and Limited Time...

Running in a tub is the same as sitting in the water at the dock as far as water available goes. Assuming the water level is more than one inch above the intake at all times then total water flow is available with no air intrusion breaking up the suction. Think of it this way: Squirt yourself in the face with a hose for 30 seconds then go dunk your head in the tub for 30 seconds and compare the feelings.

My VolvoPenta exhaust is thru the cav plate and blows most of it outside the tub so for tub-running that is an advantage, just have to keep the hose on. For thru-the-prop exhaust you can't run it forever without renewing the water in the tub somehow, this is true. But just keep water soming in from the hose, let it overflow the tub, that will keep it cool enough to run however long you need it to. It takes a lot more water but you will know for sure how it will behave out in the big water.

 

[Maclin]

Re: boat overheats on muffs running at high RPM's

Hey Happycamper,

That looks like the same tote I found at Walmart for my use...... My 1991 boat has a VP 290 DuoProp drive so I had to get the extra length for it all to fit in. I use an old rubber bunji type cord across the top to keep the weight of the water from spreading it out. My drive can't go down all the way while the boat is on the trailer so it is at a slight angle, but that lets the exhausted water blow out the back and miss the tub. Also lets me see the true water flow out the exhaust. That can be very reassuring.

Mine also has a water intake drain hole at the front of the "torpedo", well below the intake grates. That can interfere with suction also when running on the muffs, and I believe Volvo says to plug it when running just muffs and not in a test tank.

 

[WillyBWright]

Re: boat overheats on muffs running at high RPM's

All engine rebuilds should have an accompanying water pump rebuild. Always!

 

[QC]

Re: boat overheats on muffs running at high RPM's

Maclin, I agree with one exception. Although it will not cavitate the pump, remember there is hot water being put back into the tub if it is thru prop exhaust. Waaaaaaay better than the muffs but the total cooling available is still only the flow from the hose. All the other water in the tub has been subsequently heated by the engine. Again, better, but it is important to note that it is not the same as having a bazillion gallon source of cool water, also known as a lake . . .

 

[thunderroad]

Re: boat overheats on muffs running at high RPM's

40+ min to get up to normal temperature??? Mine gets to normal in 3-5 minutes at idle on the muffs. You sure your thermostat isn't stuck open? Sounds like the entire engine is being flushed with a continuous stream of fresh, cold water.

 

[Maclin]

Re: boat overheats on muffs running at high RPM's

QC,

The bolded part of your answer here "ultimately you have no more total flow to cool it than you do with the hose on muffs" is what led me to redress the total water flow being the same between muffs and having the drive submerged.

The tub method protects the impeller better as water flow from the muffs is bubbly at best. I addressed the recirculated hot water issue.

I do apologize for my editorializing which elicited your response that compares a tub of water to a lake, that was my bad.

 

[QC]

Re: boat overheats on muffs running at high RPM's

OK, I missed that, but also you state "you will know for sure how it will behave out in the big water", that is not true. Unless you can put the same load that WOT, in gear, does, then you have not duplicated the total heat that the engine can generate . . . max fuel rate = max heat . . . You cannot duplicate max fuel rate without one of the following:

1) Dyno

2) Lake

3) Ocean

4) A massive pair of vice grips clamped onto the propshaft with the precise grip equal to the load of a propeller at WOT RPM . . .

 

[Haut Medoc]

Re: boat overheats on muffs running at high RPM's

Sorry to interrupt all the quibblin', but it should not overheat either way- hose or tub at any rpm with no load, the exception being the hose collapses from vacuum......​