Weird overheat issue on a 5.0 OMC with Cobra Drive

[mgibb112]

I had a weird overheat issue today and was hoping someone might be ble to lend some advise/ technical assistance. I took my 28ft Chris Craft out today for the first time this season. Prior o putting it in the water, I replaced both impellers ( with housings and plates) and had both drives serviced. The boat ran perfect for 2-3 hours with no problem whatsoever... at high and low speed.

On my way home, I noticed a slight change in the sound of the starboard engine and what I felt to be a slight vibration. The noise almost sounded as if the exhaust got loader....no hesitation whatsoever in power. When I began to push the RPMs a little (4800rpm) and then I started to watch the temp gauge climb so I shut the motor down. When I got back to the dock, I fired up the motor to help put it in its slip and the overheating was gone. I let the boat idle for several minutes with no problem. Any thoughts before i start tearing into it?
At

 

[HT32BSX115]

Re: Weird overheat issue on a 5.0 OMC with Cobra Drive

Howdy,


Welcome Aboard!


Well, a little more info might be needed.

In general, an engine that overheats at high power settings has some sort of obstruction in the raw water circuit either TO the engine or leaving the engine (exhaust manifolds and/or risers)

The obstruction can be due to debris that has been ingested at the water pickup or it can be impeller fragments from a previous failure that made it into the oil cooler (if installed), power steering oil cooler or t-stat housing.....you might also have sucked something into the raw water circuit and your "new" impeller might have thrown a few vanes.....(anything can happen!!)


It can also be a defective t-stat but they don't fail all that often.

If you operate in salt water and you haven't replaced your manifolds and/or risers in the last several years they can be blocked with corrosion that will prevent water from exiting through them (reducing over all water flow through the engine)

The engine block, heads and even the circulating pump can also have blockage depending on how they are.........

You can also have a defective gage. It might be useful to get a simple infrared temp "gun" like the one that Harbor Freight has (for something like $30) and verify that you indeed have an over heat problem......


Regards,


Rick

 

[mgibb112]

Re: Weird overheat issue on a 5.0 OMC with Cobra Drive

first thing I did when I got back was to pull the intake line at the thermostat housing....I had perfect flow. I only changed the impeller out of preventative maintenance and the old impeller was completely intact. I'm going to change the t- stat today and see what happens. I'll keep you posted from there. I'm leaning towards some obstruction or maybe a manifold riser problem. I really hope something just jammed the thermostat and not some sort of head gasket issue.

 

[mgibb112]

Re: Weird overheat issue on a 5.0 OMC with Cobra Drive

BTW.. the gauge is right. The risers were pretty hot. The weird thing is that it ran great all day. I probably cruised 40-50 miles with no problem. Ill do some testing and keep you posted. Thanks for the response.

 

[mgibb112]

Re: Weird overheat issue on a 5.0 OMC with Cobra Drive

I took the thermostat housing off and replaced the t-stat ( the old one looked a little rusty). I checked water flow to both risers ( everything looked good). I then took the boat out for a cruise ( about 20 min) and everything seemed ok. I can't really say if the problem is fixed entirely but everything seems good so far.

 

[HT32BSX115]

Re: Weird overheat issue on a 5.0 OMC with Cobra Drive

BTW.. the gauge is right. The risers were pretty hot. The weird thing is that it ran great all day. I probably cruised 40-50 miles with no problem. Ill do some testing and keep you posted. Thanks for the response.

HOT risers are usually due to poor flow either from the pump or clogged risers/manifolds. The thermostat should NOT affect the riser temp at all......Testing your t-stat in hot water is a good way to determine if it's the culprit.... You should have something like a 160* stat.....if you're running in salt, I believe it should be a 140.

 

[Four Winns Erie]

Re: Weird overheat issue on a 5.0 OMC with Cobra Drive

I had the same overheat problem you described on my boat last week. I had changed the impeller last spring and had no problems all last year with overheating. I checked the impeller after overheating last week, and the metal cage the impeller spins in had rotated in the housing a little, so half the water passages were blocked. You might want to check your impellers just to be sure, it's quick and easy as you know.

 

[ACausey]

Re: Weird overheat issue on a 5.0 OMC with Cobra Drive

I just spent several week diagnosing a similar problem on my 4.3. It would cool just fine below 2000 RPM but would start to heat as soon as I pushed the throttle up. I replaced all the pumps, cleaned the manifolds and every other water passage on the engine, and was about to give up when I decided to build a little diverter system out of PVC pipe, ball valves and heater hose to see how much water flow I actually had. Turned out to be a crack in the exahust manifold allowing exhaust gas to pressurize the cooling system and prevent water flow at higher RPM.

I would say a blockage would be my first guess on yours, but if you can't find anything, it could be a crack.

Good Luck!
Andy