Twin 4.3L Mercruiser AlphaOne XL's - Starboard Overheating only on Plane

[beezer2010]

My cousing has a '90 Carver Santego 27 with Twin 4.3L Mercruiser AlphaOne XL's. The Starboard engine is overheating above 3000 RPM, typically on plane.
He had new Stainless props installed over the winter and on our maiden voyage this weekend we found the Starboard overheating problem when we got above a certain RPM. We checked thermostat and seemed OK. We were also directed to check the impellar on the respective lower unit. So, we had the boat pulled and dropped the lower unit and foud the impellar to be in apparent great shape. We did find large amounts of zebra mussles in the lower units fresh water inlets. Cleaned both lower units and took out for a test. Well, still overheating. Odd thing... We can take either engine to max RPM's and the other engine in neutral and idling and both temps are perfect. When we bring the other engin to max RPM, the Starboard Engine will spike the temp and the OT alarm will sound. Drop the RPM's and the motor immediately cools down. Please help... :nonod:

Brian

[sschefer]

I'm not 100% familiar with that engine but some open cooling systems have a secondary waterpump on the engine. Not all but some do. You may also have a mussel problem in the engine but the cooling passages are pretty large and there is a chemical (I don't know what it is) that is used for decontamination that will get them out of there. Not even sure that's your problem but it's worth a look.

[beezer2010]

There is a circulation pump on this engine. Based on the amount of zebras in the intake, I was worried about further infiltration! In hindsight, I wish I had looked over a couple other things while I had the lower unit off, etc. We may have to take the lower unit off again and go from there.

Brian

[CharlieB53]

The Starboard engine is overheating above 3000 RPM, typically on plane.

We checked thermostat and seemed OK. We were also directed to check the impellar on the respective lower unit. So, we had the boat pulled and dropped the lower unit and foud the impellar to be in apparent great shape. We did find large amounts of zebra mussles in the lower units fresh water inlets. Cleaned both lower units and took out for a test. Well, still overheating.

Odd thing... We can take either engine to max RPM's and the other engine in neutral and idling and both temps are perfect.

When we bring the other engin to max RPM, the Starboard Engine will spike the temp and the OT alarm will sound. Drop the RPM's and the motor immediately cools down. Please help... :nonod:

Brian

Very curious condition.

EITHER engine alone is capable of WOT running without over heat, yet when BOTH engines are WOT the Starboard engine heats.

What is the difference in BOAT SPEED with ONE engine vs BOTH engines?

What growth is on the hull ahead of the Starboard engine?

Clean the hull.

Another line of attack could be poor engine grounds, have you verified engine temp using melt sticks or a temp gun?

[beezer2010]

With one of the engines WOT, the boat plows along pushing quite a bit of water. When I bring the other engine up in RPM's the boat planes out and the temp of the starboard engine rises to OT.
The bottom of the boat looks fantastic, my cousin had it sanded and painted with bottom paint this winter. We did continue to run teh motors after the OT alarm sounded (only about 10 seconds) and then immediately checked the engines and the engine in question was definitely HOT.

When dropping the lower unit to check the water pump impellar, I checked the bottom of the boat and there were no apparent devices, etc. that should have caused an issue. There are three transducers, two with speed wheels, that are all located off the end of the transom, between the two engines. They were there before. The only other difference from this year to last, when there was supposedly no issue, was the fact that NEW stainless props were installed. Could the new props be causing a water flow issue at higher speeds. We are trying to track down the old props.

Brian

[flabum1017]

There is a water restriction somewhere still. If you found a lot of growth on the outside of the drives, there may be growth on the inside of the water passages as well. This can be checked by pulling the hoses comming from the drive off at the thermostat housings. Run the motors and compare the two for water flow.
Run them up to abot 2000 rpm each and watch how much water pumps (make sure your bilge pumps work) Don't run the motors more than 30 - 45 seconds doing this. If the starboard water flow is considerably less, then you have a restricion in the drive or in the hose between the drive and the gimball housing. You can also check the inlet side of the power steering cooler as well. The hose between the drive and gimball could also be cut.

If the waterflow is the same, then you have a restriction in the engine such as a thermostat not opening all the way ot clogged/rusted manifolds and risers.

[beezer2010]

The more feedback I get the more frustrated I get in that I didn't have the idea to pull that hose off and check the flow visibly. I was worried about the lack of flow into the engine damaging something. I also wish I would have checked the power steering cooler, etc. At least I could have back flushed the system from the thermostat to the upper unit, when I had the lower unit off.

This forum is fantastic! I wish I had checked on this site this past weekend.