Over-heats at 3/4 throttle

[Stickman21x]

First of, thank you for any input in advance. I'm first time boat owner. I picked up a 94 sea ray with 5.0 alpha.

When cruising 1/2 throttle, 20 mph or so (sorry forgot what the rpm was) it runs great. Open just over 3/4 or full throttle or 25 or so mph, it overheats in 10 mins.

It had not run for a year and was told the impeller was bad. I dropped the drive and changed the impeller, base plate and all o-rings. Went to check thermo and housing was heavily corroded. To the point the top ports were half the size. Replaced full housing including thermo.

Went out amd only had issue in upper throttle. Fastest way to cool it was open seat tops and what made a big difference was running engine only, at 1k rpm.

What would cause high throttle over heating?

Side question: what is a good cruising speed for 18 foot bow rider with 5.0 alpha? Didnt seem to be able to go over 30 mph on a flat lake.

Thank you again for any input!

 

[havoc_squad]

Approaching WOT RPM in overheating can likely be two factors:

1. The engine block and/or exhaust manifolds are not getting enough water flow to offset the heat generated. Highly likely the cause.

That could be because of corrosion buildup in the block or manifolds, or a weak engine water pump.

2. Engine is overheating due to a lean fuel/air mixture.

This can happen if fuel supply does not keep up with the amount of air allowed when throttle opens the butterflies wider.

First thing I would do is buy an IR temp gun.

If the exhaust manifold(s) starts overheating first and the block is at operating temp, you know you have cooling issues in the exhaust manifold(s).

If the block starts overheating before the exhaust manifolds, then you have a lean issue or a cooling flow issue possibly in the engine heads.

If both engine and exhaust manifolds are overheating, then you have a failing engine water pump or a large cooling obstruction.

Lean conditions when they are not drastic might present themselves like hitting a wall on RPM where the throttle past a certain point stops matching the expected increase in RPM the farther it is moved.

This should be accurate, but if not hopefully the top experts here will call out anything that needs to be corrected.

 

[tank1949]

IR gun will or should pinpoint clogging. If saltwater cooled boat, get ready to replace manifolds, usually risers first. Same thing can happen with FW cooled boat but it takes a lot longer to clog. Pull risers first and see if clogged up. If manifolds/risers are clean, maybe thermostat not fully opening. At least you will have peace of mind knowing risers are clear. Trust me.

 

[Rick Stephens]

It can tell you a lot to pull the block drain plugs and see how much scale is inside them. Then purchase the correct coated metal manifold to riser gaskets and pull inspect your risers and manifold water jackets. Post some pictures. Being a new to you boat, these are essential to determine what needs doing to manage cooling.

If not already done, a new to you boat should have all scheduled maintenance done right now. There is no telling how the previous owner treated the boat and the only way to be certain you won't eat a big bill due to missed maintenance items is do them all.

 

[H20Rat]

what is your full throttle RPM?

 

[Stickman21x]

Thank you everyone for the input. I just left town but trying to get it back on the water next weekend. I will be sure to take my check list of the input and report back. I will also make sure I check the rpm as well.

It has always been FW cooled but do know there is a good amount of corrosion from when I pulled the thermostat housing. I will put the plug and see what the block looks like.

Thank you again!

 

[Stickman21x]

So out on the water and I'm sure it is the riser issue. I can hold the throttle and accelerate with no issues so not thinking it is a lean issue.

my ir gun shows about 10 to 15 degrees less then guage.


Block was 204 and riser was 217. Took it out of gear and throttled up motor to cool off. Crazy thing is the gauge slowly went down to 170 or bit lower and then jumped up to 220.

WFO throttle rpm was 4500, right were the tag on engine says it should be.

I drained the block and there were fine sand and such. Tried flushing but not sure how much that did. Tried to get riser plug out but it was frozen in.

I'm going to pull the riser and take a look as the someone stated earlier.

Tried to post image but they were to big and done know how to downsize on a cell phone in the middle of a lake.

 

[Scott Danforth]

anything over 200 is a severe overheat in a boat

you should be about 160F max

since you did a raw water pump replacment, check the flow. at just above idle, it should fill a 5 gallon bucket in about 20 seconds.

next, when was the last time you back-flushed the PS cooler? that is the first thing to plug in a system from debris coming in the drive

third, unless you have a heat exchanger, you are raw water cooled. Fresh water cooling is with a heat exchanger.

since you had a lot of scale, most likely the boat has not been maintained. rust and scale in the motor will cause issues.

 

[wahlejim]

If the PO knew the boat needed a new impeller, it overheated on him. Check your exhaust shutters. They are probably toast by now and may be causing some blockage as well

 

[Stickman21x]

So I got jazzy and tore into the risers before doing a 5 gal bucket test. But I flushed cooler when I changed the impeller. Very little came out. I took the circulation pump off and there was little corrosion in the block in that area. Took back of pump off and looks good. What didnt look good was the spring in the hose from pump to thermo housing. Looks like the end broke apart in 5 or so pieces. I'm sure causing a large obstruction. Heavy scale in the risers.

I will grab my book and look for exhaust flaps to check those.

I'm also working to shrink pics of what I found last night.

Nothing great thing thing happened, I made a huge mistake and didnt lift the drive up. Got lucky, stopped before it got the prop. Learning curve are awesome...

Thank you again everyone!

 

[Stickman21x]

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[Stickman21x]

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[Stickman21x]

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[Stickman21x]

Flaps seemed good. They were both free and did not seem warped.

 

[Bondo]

Ayuh,..... It might clean up, but,.... the manifold is beginin' to fail in the upper right corner, 'n the left side,.....

Ya gotta remove the gasket to see the risers surfaces,......

 

[Stickman21x]

Thank you everyone for the input. I cleaned the risers real good and removed the broken spring parts.

Went out this morning and worked it hard. 3/4 to full throttle for at least 15 mins. If anyone here knows lake meade, went from calville through the narrows no issues.

I will be ordering new risers.

Thank again!