Recovering from overheat -

[JeremyL]

Hello!

I am repairing my boat after it overheated, and have it to the point that the engine runs at normal temperature when running under 3000 RPM, but it heated back up to 225+ after 2 minutes when run at higher RPM.

This was my first maintenance outing after the first set of repairs, so I only did this once before coming back in. The engine cooled quickly when running at low speed.

Disclaimer: I'm new to boats, but now have enough knowledge to fully understand how silly I was to get myself into this position in the first place. Thanks for any replies!

Engine is a 1985 Mercruiser 120hp 2.5L.

History-
-Boat overheated and broke a belt.
-Overheat was caused by old impellar and water pump, belt broke becasue the water recirculating pump was corroded and dragging.
-Overheat destroyed said water pump and impellar, and the flapper in the exhaust elbow. The rubber exhaust bellows was also cooked enough to create a slow water leak.
-When working on engine, found timing gear cover corroded through, and t-stat housing fairly corroded as well.

Work performed
-Replaced water recirculating pump, belt, and timing gear cover.
-Cleaned corrosion off of t-stat housing.
-Replaced Water Pump and Impellar. (This was professionally done, as I was unable to separate the upper and lower gear house covers in the sterndrive due to corrosion.)
-changed the oil/filter

Work still to be done
-Replace the exhaust bellows.
-Replace the flapper in water exhaust elbow.

I am waiting for parts to arrive to complete the work to be done, but I am concerned that I still have an undiagnosed overheating problem after yesterday's maintenance run.

I've read posts that indicate cracked header or blown head gasket that only presents at higher RPMs, or the possibility that the water pump was not correctly sealed when replaced.

Can the small leak in the water rubber exhaust bellows cause an overheat at higher RPM? the lack of pressure, maybe?

The water exits through the propeller, there could be debris inside of the lower gear case, but I'd expect a water flow issue to present an overheat at any running speed, not just higher RPMs... right?

Thanks for any help!

-

 

[jtsailjt]

Re: Recovering from overheat -

I had a 225 hp outboard that badly overheated (plastic bag in water combined with failed overheat sensor) but since it didn't seize up I had all the necessary parts replaced and it ran fine below midrange rpm. But my brand new overheat sensor was being triggered soon after I accelerated close to WOT and would stop sounding if I slowed down. Took it back to same mechanic and he told me that cylinders had slightly warped during original overheat and at higher rpm's were causing too much friction, causing engine to overheat. I wasn't happy.

I hope that's not your problem but just thought you'd want to be aware of it as a possibility.

 

[Faztbullet]

Re: Recovering from overheat -

Sounds like you have passage ways in manifold or head that as stopped up... if corrosion is salt/calcium you can try to run a delimng agent thru engine.

 

[CharlieB]

Re: Recovering from overheat -

When you replaced the exhaust 'flapper' was the old one there and you took it out?

If the flapper was MISSING, it may be laying in the bottom of the exhaust pipe at the transom plate, once at high throttle it can restrict exhaust flow causing the engine to overheat.

You could get lucky and remove the drive, use very long nose pliers to break the flapper into pieces and remove them.

 

[JeremyL]

Re: Recovering from overheat -

the original overheat was pretty severe, so I'll hope its not warped cylinders too... any idea how the mechanic determined that to be the the problem?

I'll look into running a De-Liming agent through the engine. I suspect the exhaust manifold is fairly rusty on the inside and I'll find that it needs to be replaced if I pull it off. When draining the exhaust manifold, the water came out very rust colored after having been in there for under a minute. (garden hose water via earmuffs).

The "flapper" {aka Exhaust Shutter : )} remants were indeed found and removed. The debris was found by the mechanic when he replaced the water pump, who said that other than the need to replace the Shutter and upper exhaust bellows, the engine runs well.

He also said that to replace those things, he'd need to remove the Exhaust Elbow, which, due to corrosion, wouldn't likely go back on and seal properly, and should be replaced.

The old upper exhaust bellows was cooked enough to remove on its own, which allowed me to remove the tube that houses the exhaust shutter.

The interior of the exhaust elbow feels clear to my fingers as far as I can reach, without any amount of buildup to be concerned about. I do plan on replacing the entire exhaust manifold/elbow once I am able, but need to determine the root cause of the overheat-

Thanks for your suggestions!
.

 

[CharlieB]

Re: Recovering from overheat -

It is also possible that your old exhaust risers are rusted and restricting the water flow OUT of the engine into the exhaust stream.

Restricting flow here can allow the motor to remain cool under litght loads (slow speeds) yet fail to allow enough flow that higher loads and speeds Boils wter in the exhaust, further restricting flow out of the engine, thus, temps begin climbing.

Often in this case you should be able to feel the manifold and riser temp while underway and sense the rising temp here before the motor temp comes up.

The hose at the riser would be burning on the inside, pull the riser, look at the inside of the hose and at the size of the water outlets where it joins the exhaust stream.

If corroded almost closed, this is a pretty sure sign you will need to replace at least the riser, if not the whole exhaust manifold.