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Old 10-27-2013, 10:49 PM   #1
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Default Mercruiser 5.0 overheats on high RPM

Please help! I purchased a 1999 SCR3000 with twin 5.0s. After a short time the starboard engine overheated and "locked" up internally. A long story short the new "Bobtail" engine that was installed still does the same thing (overheats on high RPMs)(closed cooling)
My mechanic stands there scratching his head not really understanding why it does this.
The heat exchanger is super clean and a new raw water pump was installed. Bobtail engine came complete with new water pump.
I am not sure if the hoses are hooked up correctly. The heat exchanger is a Orca.
When the engine was replaced both outdrives were gone through and passed pressure testing.

Your help and suggestions will be greatly appreciated.
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Old 10-28-2013, 02:13 AM   #2
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SailingH, what outdrives do you have? The Alpha units have a raw water pump in them and the Bravo units havethe raw water pump mounted on the engine and belt driven. There is also a circulating pump on the engine (same as car pump). Is this a partial closed system (engine only) or full (engine and manifolds)?

Raw water side: check raw water pump, manifolds, risers, and heat exchanger for flow. Is the outdrive intake clogged? Pull raw water output hose on exchanger and verify flow.

Closed water side: check circulating water pump, t-stat, engine for flow. Verify coolant is full.

Use a handheld IR thermal gun and check temps. Open side should be no more than 160 F. Closed side should not exceed t-stat rating.
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Old 10-28-2013, 02:33 AM   #3
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ok, lets make this simple.......

take the garden or dock hose and attached it directly to the top hose of the heat exchanger, by passing all the outdrive and raw water pump. let the raw water line pump over board and check for a strong flow.

( uses a couple of hose clamps and a pieces of garden hose.)

if you run cool, then its not your engine, and your half solved.

working back wards, from the heat exchanger use the garden hose too see if the water will run thru...... , a kinked hose, plastic bag, or clogged pickup, your thermostat could be installed backwards....

if all raw water lines check out, then we know its your t stat, engine water pump, or bad hose.

if so by pass the heat exchanging and run the raw water directly to the t -stat and check results.

since you have twins, obvious one is working correctly,

my guess, is your have the raw water hoses on back wards. your engine is a left turning, standard, whichs counter clock wise turning.... looking at it from the back of the engine. the water from the sea, goes into the bottom of the raw water, turns counter clock wise, and pushes out the top hose to goto the engine.

that should pin it down.
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Old 10-28-2013, 09:13 PM   #4
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The outdrives are Bravos.
Output on the "new" raw water pump was pulled and flow was verified. The engine will not overheat while in idle. But does overheat when cruising. This is a brand new Mercruiser engine which came complete with new engine mounted water pump, new risers, manifolds and new thermostat.
I am wondering if the Heat Exchanger is plumbed correctly?

Thanks so much for your help. Keep the thoughts coming. I've missed a whole summer of boating because of this.
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Old 10-29-2013, 03:43 AM   #5
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You have a lot invested here in both $'s and time, I would concider finding another mechanic.

You are using an aftermarket closed cooling system, is it sized for this engine? Other engine is fine so I am guessing so.
Does the new engine have dry joint exhaust? You never answered if the closed cooling if partial (engine only) or full (engine and maifolds, risers are still raw water) I donn't believe dry joint can be in a closed loop system and only the engine can be cooled. If if is a full design system and now only being used to cool the engine what did the mechanic do with the close cooling hoses that would have gone to the exhaust. If not dry joint but wet joint and it is a full system di te mechanic chang he gaskets between the manifolds and risers to the closed cooling ones?

I don't believe merc uses a counter rotating engine but handles this in the bravo drive but chaging the shift lever. This changes the clutch operation so the reverse gear becomes forward and the forward gear becomes reverse. Therefore the 2 engines should both be standard and the cooling system should be plumber the same. Are they plumbed the same? Pictures of each may help.

You stated verified water flow with hose off the pump but is it enough flow? Should be a flow similar to a garden hose.

Going back to square 1, why do you think the engine is overheating? Helm guage reading high? Alarm going off? How high?

It may be a bad guage or sending unit. Try using a handhelp IR thermal gun and take reading of the t-stat housing, manifolds, heat exchanger and see if it really is hot.
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Old 10-30-2013, 03:15 PM   #6
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Mike,
Thanks for taking the time to help...
1. Orca is the after market heat exchanger
2. According to Orca it is sized correctly and is clean (visually inspected)
3. Not sure if it's wet or dry. Hoses are attached to the bottom of the manifolds if that helps.
4. Gaskets were changed to "full flow"
5. Flow was similar to a garden hose (good flow)
6. Gauge indicated approx. 175oF at idle for over 45 minutes. Upon cruise it went above 250oF and blew the cap off the overflow bottle
I believe my Mechanic used Mercruiser's drawing for plumbing of the hoses and Orca says it can't be down that way. Still working on that solution as I type this.

Any other advice of help will be greatly appreciated at this time.
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Old 10-30-2013, 06:43 PM   #7
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Here is a picture of how a full closed cooling system should be set up

closed cooling.jpg

Did you compare hose routing to the other engine? Should be the same.

Can you post pictures of each engine showing how each are routed?
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Old 10-30-2013, 09:35 PM   #8
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Also what part number is the orca sytem? That way I can go to their site and see what they say.
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Old 10-31-2013, 12:34 AM   #9
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The hose routing is identical. Orca is working with me at this time to be sure that the heat exchanger (cooler) is connected correctly.
A couple of things that they have suggested are that the thermostat needs to have 1/8" hole in it for weep by and relocated above the sensors instead of below them. Also the gaskets need to be double checked. (full flow not restricted flow)

One other suggestion from mechanic "B" was the fitting (that looks similar to the thermostat housing you would see on a small block chevy) behind the engine where the raw water hose protrudes thru the hull to the outdrive. He said that this could be partially clogged restricting flow.

Pictures to come....
Thanks
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Old 10-31-2013, 01:22 AM   #10
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Not sure what you are refering to here "Also the gaskets need to be double checked. (full flow not restricted flow)". What/where is this gasket?

You have a Bravo so he raw water pump is inboard and you state the flow is good therefore I would think any restriction would be after that and not back at the transom which is where the raw waters come to the pump. However if a higher rpms the flow is not enough I would start with the water pick up on the outdrive as it could have marine growth inside. You could swap drive to see if that move it from one engine to he other than you would know it's a clog in the drive.

A clog could also so be down stream restricting the hot raw water from being exited.

Once again pictures would help a lot here.
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