454 overheating

Stu

Stu

New Member
Oct 14, 2006
63
Morro Bay, Cailf
Boat Info
1987 268 Sundancer with Bravo OD
Engines
454 Mercruiser w/ Bravo OD
My engine usually runs at around 145 degrees but every now and then it crawls up and unless I notice it I have even found it pegged :-( I immediately put it in neutral and rev the engine and she cools back down to standard temp. Would that be a bad thermostat or is the pump going out? Thanks all
 
Sounds like a bad impeller to me, raw water or fresh water side. It should increase slightly when coming off plane. After a minute or two it should return to normal. If not, usually a bad impeller.

While on plane water is moving thru the thru hull at a much higher rate. Once off plane you are counting more on the impeller to move water.
 
Sure, your hull is not moving as fast thru the water thus water flow has slowed down at the sea cock thru hull fitting. Water flow/speed at that point is dependent more on the impeller than it was while on plane when the water was moving into the thru hull fitting at a faster rate. Perhaps a better way to think about this is when your engines are running with trans in neutral, boat is not moving. What is pulling water thru the cooling system then? Your raw water pump impeller is. Put the boat in motion, forward of course...what's moving water thru the system then? The forward motion and the sea water pump.

This is not from my head, it's from my mechanics, and many other mechancis. Easy enought to test too. Put your boat up on plane, heat up the engines, drop her down, watch the temps go up with a gun or your guages. Next time do the same thing except this time when you come off plane do not slow down as much.

On our gas engines this is a way to test if it's a bad impeller or something else. I have done this test twice, each time it pointed to a bad raw water pump impeller, each time I replaced it, each time problem resolved.
 
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Unless you have a forward facing scoop on the thru hull, water does not get "pushed up" the hole the faster you go. In fact, just the opposite happens. The faster you go, the pressure at the thru hull drops and the water pump has to work harder to pull water from the bottom. The impeller is always in charge of how much water goes into the raw water circuit... there is no "bypass".

I'm not sure what kind of engine/drive the OP has so not even sure he has a thru hull.
 
Stu said:
My engine usually runs at around 145 degrees but every now and then it crawls up and unless I notice it I have even found it pegged :-( I immediately put it in neutral and rev the engine and she cools back down to standard temp. Would that be a bad thermostat or is the pump going out? Thanks all
Click to expand...

Same kind of problem I had. Turned out to be manifolds were shot.

Do you have raw or fresh water cooling?
 
I am not quite sure this explanation is on target.

First, what engine do you have? Most mercruiser's have TWO temperature sensors, one wired to the temperature gauge and the other wired to the engine temperature alarm. If you peg the gauge, but the alarm doesn't go off. . .then there is a good chance it is a sensor.

But I would not rule out a bad sensor AND an overheat problem.

I would think the first things to check are the sensors and thermostat -> just because they are easy. Impellers would be the next thing to check. I forget where the BIII impellers are located. They can be replaced with the boat in the water right? Only the Alpha drives have the impellers in the "drive" and require the boat out of the water.

On my boat, once the engines warm up I NEVER see the temperature gauges move. The thermostats control the engine temperatures throught out the operating range, and control them well.

If you are seeing the gauges move during some part of the operation of the boat, then I would suggest that either the thermostats are being slow to respond OR the cooling system is not adequate in some way. Regardless. . .if the temps stay under 180-190F, I would not loose sleep over it.
 
4Suns posted while I was typing. I agree with what he said.

Water flow through the engine is a function of the impellers in both water pumps. . . and that in turn is a direct function of engine RPM. Regardless, in a boat that is working well, there should ALWAYS be plenty of excess cooling capacity.

- - - - -

Manifold and riser problems are where you look AFTER you exhaust the "easy" options. When were they last replaced? What kind of water does this system see?
 
Four Suns said:
Unless you have a forward facing scoop on the thru hull, water does not get "pushed up" the hole the faster you go. In fact, just the opposite happens. The faster you go, the pressure at the thru hull drops and the water pump has to work harder to pull water from the bottom. The impeller is always in charge of how much water goes into the raw water circuit... there is no "bypass".

I'm not sure what kind of engine/drive the OP has so not even sure he has a thru hull.
Click to expand...

Gary like I said, this info came from 2 local mechanics, and both times it has been our problem and their 'check this' comments did exactly as they told me it would. Your reply does not explain why are engines cool down faster when moving faster thru the water. In both cases we had a bad impeller, and while running on plane the temps were normal, but when off plane the temps increased and would not return to normal. If I understand your comment correctly we should have seen the oppositie of this.
 
comsnark said:
4Suns posted while I was typing. I agree with what he said.

Water flow through the engine is a function of the impellers in both water pumps. . . and that in turn is a direct function of engine RPM. Regardless, in a boat that is working well, there should ALWAYS be plenty of excess cooling capacity.

- - - - -

Manifold and riser problems are where you look AFTER you exhaust the "easy" options. When were they last replaced? What kind of water does this system see?
Click to expand...

OK, increased RPMs makes more sense...even with a bad impeller, with increased RPMs one would expected increased water flow.
 
comsnark said:
I am not quite sure this explanation is on target.

First, what engine do you have? Most mercruiser's have TWO temperature sensors, one wired to the temperature gauge and the other wired to the engine temperature alarm. If you peg the gauge, but the alarm doesn't go off. . .then there is a good chance it is a sensor.

But I would not rule out a bad sensor AND an overheat problem.

I would think the first things to check are the sensors and thermostat -> just because they are easy. Impellers would be the next thing to check. I forget where the BIII impellers are located. They can be replaced with the boat in the water right? Only the Alpha drives have the impellers in the "drive" and require the boat out of the water.

On my boat, once the engines warm up I NEVER see the temperature gauges move. The thermostats control the engine temperatures throught out the operating range, and control them well.

If you are seeing the gauges move during some part of the operation of the boat, then I would suggest that either the thermostats are being slow to respond OR the cooling system is not adequate in some way. Regardless. . .if the temps stay under 180-190F, I would not loose sleep over it.
Click to expand...

"On my boat, once the engines warm up I NEVER see the temperature gauges move. The thermostats control the engine temperatures throught out the operating range, and control them well."

Per the comments from 2 local mechanics and the folks we purchased our boat from (service shop) you should see your engines heat up slightly when coming off plane, and then quickly back to normal. We see this on ours. It makes sense to me based on your RPMs comment.

Maybe it's an engine make/model thing.
 
mrsrobinson said:
Gary like I said, this info came from 2 local mechanics, and both times it has been our problem and their 'check this' comments did exactly as they told me it would. Your reply does not explain why are engines cool down faster when moving faster thru the water. In both cases we had a bad impeller, and while running on plane the temps were normal, but when off plane the temps increased and would not return to normal. If I understand your comment correctly we should have seen the oppositie of this.
Click to expand...

Your engines are cooling down faster because the impeller, all banged up as it is, is spinning faster and giving you more cooling water. When you slow down a little the beat up impeller slows down and, missing a fin or two, can't pump water fast enough to cool the engines. AND... when you slow down a little, the boat is probably increasing total drag by sinking in the water and so lots-o-BTU's are still being dumped in the engine with a bad water pump working substantially less than it was at a higher RPM.

Your mechanics came up with a hair-brained idea if they explained it as water is being pushed up a hole in the bottom that is horizontal to the direction of motion. If there is a scoop (?), water will get pushed... if it is a horizontal thru hull, the pressure drops the faster you go. I ain't making this up either... It's called Bernoulli's law and is the same reason airplanes fly.
 
"Your mechanics came up with a hair-brained idea if they explained it as water is being pushed up a hole in the bottom"

When I shared our symptons 2 years ago with a local shop he said try this:

Next time you come off plane watch the temp gauge. If it rises and does not return to normal temp within a few minutes, bump up the RPMs. If bumping up the RPMs returns the engine to normal operating temp you probably have a bad impeller. I replaced the impeller, problem resolved.

Same sympton 4 weeks ago after grounding the boat, called a different mechanic, same suggestion, same result, impeller replaced, all good now.

I swore their reasoning why was what I originally stated. Maybe his reasoning is hair-brained, but his test and solution resoilved our problem.

Hopefully the original poster can take some value from my comment/suggestion.
 
Four Suns said:
OH! So YOU came up with the hair-brained explanation!
Click to expand...

"I swore their reasoning why "

I think it was their explanation, this is not something I could come up with on my own..I will suggest they both study Bernoulli's law so they are not misinforming their clients.
 
Sorta like telling people not to go out too far in the water... They may fall off the earth.

Actually I lied...

You know why airplanes fly? Because people think they can.
You know why airplanes crash? Someone on the flight goes "wait a minute..."
 
I believe a good mechanic does know about "Bernoulli" in one form or another... pretty important concept in carburetors and other types of air flow and sensor problems.

And... understanding the pressure drop at a through hull and it's effect on a bad impeller can also explain why a generator works while not moving but gives a raw water warning when up on plane.
 
I tried to explain that concept to a guy who was missing his lower radiator shield/under carriage tray. He could never figure out why his BMW M3 overheated at speed but it idled fine. I guess BMW put it there for kicks...

Doug
 
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