overheating at higher rpm

[Ringerd]

1994 4.3L Mercrusier, counter rotating if that makes any difference, around 417 hours on the engine.

Impeller replaced this spring, about 5 hrs ago, outdrives serviced at same time( new gimbel bearings, bellows, etc.) Has been running good.

Took it out the other day all was fine for 30 minutes or so running at about 2000rpm. Opened it up to get on plane and the temperature shot up causing the alarm to go off. Shut it down immediately and went back on the other engine. After it cooled down, the engine restarted easily and temperature ran around 150 which is what that engine always showed on that gauge.
Replaced the thermostat on the engine thinking it may have stuck. Let it idle for 30 minutes or so and it never got above the 150 mark. Didn't take it out then because of time issues.
Took it out today, ran fine at 150 degrees for 30 minutes or so with an rpm of around 2000. Opened it up and immediately the temperature shot up to about 200. I also noticed at this time that this engine only got to about 4000rpm. It usually makes it to about 4400. Not sure what the other engine got to, in the rush to shut that engine down I didn't look, but the sycroniser showed the other engine was putting out more rpm.
I shut it down and slowed back down. Restarted the engine and at idle the temperature came back down to 150 quickly.
Engine restarted easily and no water in the oil.

Where do I go from here?

 

[Bondo]

Re: overheating at higher rpm

Ayuh,... Welcome Aboard,.... Is this a raw water cooled saltwater boat,..??

If the impeller is pushin' water In,... It's gotta be able to get Out....
A Clogged/ Restricted riser maybe,..??

 

[Ringerd]

Re: overheating at higher rpm

It is a raw water boat but has never been in salt water.

Is there a way to check the riser without removing it?

 

[Ringerd]

Re: overheating at higher rpm

Still having the overheating problem.

Removed risers. Wide open.

Shutters fine.

Timing was a little low. Reset it and readjusted the idle. Does run a little cooler but still runs hot at idle, about 150, temperature increases as rpm's increase. Will not speed up enough now to get on plane.

Used a gun to check the heat and compared both engines. At almost every place we checked the port engine was about 20 degrees hotter while idleing. We shot the heads, thermostat housing, risers, exhaust elbows and a few other places.
The only 2 places where the port engine was about the same as the starboard were on the raw water hose going into the thermostat housing and on the front center of the housing right past where the raw water comes in. After that, the one engine is hotter everywhere.
As rpm's and load increase the port engine starts getting hotter everywhere while the starboard does not increase nearly as much.

On the hot, port, engine there was only a few degrees difference in the readings we got from one side to the other, so there was no particular "hot spot".

Checked the water flow from the water pump. 15 seconds at 1000 rpm gave us over 7 quarts which was twice what the chart said minimum should be. Did not appear to have any air bubbles in it at 1000 rpm.

Did the clear hose on the starboard exhaust manifold test. Here, we have an almost full hose at idle after the thermostat opens, but the amount of water in the hose decreases as the rpm's increase. As suspected the temperature goes up.

The engine starts easily and idles smoothly.
Sunday, both engines ran fine( except for the port engine being hotter as usual) but I noticed that at every rpm, my speed was slower on the GPS, than previously. When opening both up, neither would get over about 3800 rpm and the boat would not plane off. The last time I tried opening it up before coming in the rpm's started decreasing at WOT so I pulled them back to 2000 rpm, everything cooled to about 170 on the hot engine and 140 on the cool one and stayed there until we got back.

After adjusting the timing it takes longer for the temperature to start rising but it still gets there.

I haven't let the temperature get above 180 on the dash gauge except for the one time about 2 months ago when it first overheated. That time it hit 200 but when I decreased throttle it immediately started back down.

The port engine has always run, by the temperature gauge on the dash,about 20 degrees hotter that the starboard but until this spring it never overheated. I always thought it was just the sensor or maybe it just ran hotter.

I have some thoughts on what may be the problem but want to hear from the pros.

 

[Todd157k]

Re: overheating at higher rpm

I had this problem when I first bought my boat. Turned out, the thermostat was in upside-down. When I'd go over about 2500 rpm, the water would push the thermostat closed. I took it to a marine shop, they took it apart, put it back together and still had the problem. They put it back in the way it was. doh!
Anyway, once inverted correctly, the problem ceased.

 

[Lyle29464]

Re: overheating at higher rpm

I had one do all of this. turns out it was a head gasket that would leak at higher RPM.

I would look at plugs and do some pressure tests.

 

[Ringerd]

Re: overheating at higher rpm

Thanks for the replies.

Not thermostat upside down. We replaced that anyway.

I was suspecting head gasket or maybe water circulation pump.

Lyle,
I will look at the plugs.
What pressure tests are you refering to?

 

[Lyle29464]

Re: overheating at higher rpm

I think a regular dry compression test will show what side is leaking. The plugs might help. If it has run a while at normal temp you may not notice a change on the plug.May need to do leak down test

 

[Ringerd]

Re: overheating at higher rpm

Thanks,
I'll try that.