Overheating at high rpms

[jobofly]

Well I was hoping it was because of a full load but it happened again today. Idling around the temp is perfect even cruising at 2000 rpms is fine no overheating. As soon as I try to get on plane I start to overheat. So I back down on the throttle and the temp goes back to normal.

what do you guys think?

 

[Air O'Nautical]

Classic problem that most have dealt with before..
Problem is it can be several things.

Its best to start with the beginning of the cooling system and go through it.

Intake and hose coming in..
impeller..
engine hoses..
Heat exchanger if any..
manifolds
risers
exhaust flappers or other exhaust restrictions.

Most of the time it's just an impeller bad or broken.

Happy hunting,

Mark.

 

[Mike Blake]

Had that happen one time. It was the raw water impeller.

 

[jobofly]

Thanks guys, I'm going to pull the boat out of the water and change the impeller and thermostat. I'll double check the drive for blockages too. Is the impeller hard to change? Do I have to pull the out drive off?

 

[Summer Fun II]

Also check the water pump on the engine for any wobble.

Grab the pulley and try to move it side to side. If it moves its on its way out.

 

[magstang1]

Or a slipping belt on the engine. Do you have any black dust in the engine compartment around the pulleys?

Yes the impeller is pretty easy. No, the drive doesn't have to come off. You just separate the upper and lower half. The impeller is in a housing on top of the lower unit. The drive should be pulled to grease the u joints however. You're already in there.

Edit: Nevermind the part about the impeller. I missed that you are OMC.

 

[Bt Doctur]

On the 400/800 series ,yes. for Cobra`s no.dont even have to pull a boat to change the cobra impeller.

 

[jobofly]

Ok so it's almost impossible for me to change the impeller myself. I called the local boat shop and they will do it for $300. They are an omc expert and know the stringer drive really well. I think it's worth it to take it to the shop, thoughts?

 

[jobofly]

just to follow up, i finally had a chance to do some troubleshooting. i took the hoses off and they looked fine. i took the thermostat housing and found that the thermostat was very rusty and i had a hell of a time getting it out of the housing. im thinking i found the problem but my local boat shop was closed today so ill pick one up this week and let you know. thanks for all of your tips and advice

 

[CliffA]

Also check the water pump on the engine for any wobble.

Grab the pulley and try to move it side to side. If it moves its on its way out.

i had a similar problem when i first got my boat.... i did the usual things....checked belt tension, new thermostat, back flushed hoses and water passages, new water hoses, new water pump impeller, new temp sending unit but the engine still ran too hot under load and on plane....finally decided to change out my engine mounted water circulation pump and that solved the problem...there was no play in the pulley and no obvious visual signs the original pump was defective....could be the vanes in the pump just wore down enough over time to the point the pump could not move enough water at higher engine rpm's.....cost of a new pump wasn't too bad....around $100 and an easy job to replace....as was stated before be sure to tension the drive belt properly to not have any belt slippage on the pulley.....

cliff

 

[jobofly]

Ok new thermostat in and it did not overheat. I did not take it out on the lake, just tested it in the slip. I let it idle, then let it run at 3000 rpms and keep going back and forth on the throttle. The temp cam up to where it was suppose to and did not move any further. With the new thermostat it actually runs a little cooler, before it would,always stay at the last line on the gauge now it goes just past the half way point on the gauge so I'm hoping that has fixed it. Over the winter I'm going to have a new impeller put in just to be safe for next summer.

 

[jobofly]

To be clear, it was the last line on the gauge before the red on the guage

 

[Rixram]

Ok new thermostat in and it did not overheat. I did not take it out on the lake, just tested it in the slip. I let it idle, then let it run at 3000 rpms and keep going back and forth on the throttle. The temp cam up to where it was suppose to and did not move any further. With the new thermostat it actually runs a little cooler, before it would,always stay at the last line on the gauge now it goes just past the half way point on the gauge so I'm hoping that has fixed it. Over the winter I'm going to have a new impeller put in just to be safe for next summer.

Not to be a kill-joy, but although it is very likely you fixed it with replacing the T-stat, your test was inconclusive. You spun the engine up, but there was no load on the engine. You will not have a clear answer until you are underway and actually pushing the boat.

Please tell me you used a marine T-stat, and not a 180 degree automotive one. On your boat (check to be sure, but...) you should be using a 140 degree T-stat.

I just went through this same issue on my newly acquired boat. PO replaced the T-stat with a 180 degree truck unit. Said he'd knock $1,000 off the price if I took it as-is, with the temp issue. Well, a few hours after I bought it and $10 later, temp holds at 140 degrees.

 

[jobofly]

I bought a marine 160* thermostat from my local marine shop. I haven't had it out of the marina since I replaced the thermostat. We will find out this weekend.

 

[jobofly]

So I had the boat out today and it didn't overheat....whew I was nervous that it would. It did get to just a hair past the last line on the temp gauge but no further. I am going to replace the impeller over the winter just in case.