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454 gauge temp fluctuating issue

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Velocity280

New member
I'm having some issues with my new engine and hope someone has experienced this before and can help me out . Gen 6 454. I'm getting a temp of 150 under idle, which seems normal (140 thermostat, no bypass holes drilled). This is a raw water setup with a sea strainer and a bypass kit instead of a circulating pump. When I run it anywhere over 3000 rpm, it'll climb slowly to 180 on the gauge, then climb rapidly to almost peg the gauge at 250. I've checked the block with an infrared temp gun and the engine stays around 175. The oil lines read around 200. The gauge will drop if I raise the Rpms in neutral. Back down to 150 or so. Where should I start looking? Does this sound like a bad temp sending unit?
 
You MIGHT have an air leak into the raw water system. A bit of air in the water can cause all kinds of hell.

Jeff
 
What outdrive do you have? What year is this boat.

You guys have to include more information we are not psychics...........

When you say BYPASS KIT what do you mean?

And no it does not sound like a bad sender.............its sending exactly what it should...........its getting hot!! at least for now as much the internet and keyboard can tell..............
 
The boat is a 1995. Bravo 1 outdrive. I drilled 2 1/8" holes in the thermostat last night and warmed it up in the driveway and it seemed to settle the gauge down. Hopefully that solves my problem!
 
Again....ugggg...

So is the water being sucked in thru the drive or thru the hull.

Why would you drill a hole in the thermostat? That is NOT how to repair an issue as far as I know....if it is i am unaware of it.
 
I'm running a transom pickup tube, thru a seastrainer . I THINK, I'm picking up air pockets when the water level in the seastrainer gets below the outlet hose for a split second. The bypass holes seemed to fix the problem . It was recommended to me by the engine builder as well as a few other marine techs to bleed off any trapped air pockets.
 
Maybe look into a larger sea strainer where the outlet hose is at the bottom (if such exists) so the only time you run out of water is when you are not drawing any water.

The Impeller should be able to draw enough water I would think so this should not occur.

What you describe is a classic loss of impeller function at higher rpm.

Of course there may be other contributors. But if the impeller is weak (due to many reasons) it will not have enough suction to pull enough water into the engine especially as rpm increases.


Remember, if this is a OEM system and all parts are OEM also, it worked before so it should work again.

Finding the root cause is what is needed instead of a work around.

I personally dont have enough experience on this type of cooling system so I will back out and let some others chime in with more direct experience.
 
kghost said:
Maybe look into a larger sea strainer where the outlet hose is at the bottom (if such exists) so the only time you run out of water is when you are not drawing any water.

The Impeller should be able to draw enough water I would think so this should not occur.

What you describe is a classic loss of impeller function at higher rpm.

Of course there may be other contributors. But if the impeller is weak (due to many reasons) it will not have enough suction to pull enough water into the engine especially as rpm increases.


Remember, if this is a OEM system and all parts are OEM also, it worked before so it should work again.

Finding the root cause is what is needed instead of a work around.

I personally dont have enough experience on this type of cooling system so I will back out and let some others chime in with more direct experience.
Click to expand...

as far as I know, the system itself Is oem. Although the engine build is far from it. The impeller is new, and i pulled it apart yesterday to check it all out. I'm not running a circulation pump, in its place is a crossover kit, which seems standard for the high performance side of these setups. I could imagine that may be contributing to this problem, but I will keep testing and see if this thermostat mod actually fixed my problem.
 
Docksidemarineservices said:
Take the POS sea strainer out. That is definitely not OEM
Click to expand...

Looking at your screen name, I have to assume that you have a "go fast" boat. The sea strainer would be OEM by the boat manufacturer not by Mercruiser.

You should check the lids of your strainer, if you choose to keep it, for cracks.

BTW testing your engine on the garden hose for anything except to flush and see if the engine starts is a waste of time.
 
I would think, on a "go fast boat", that you would want the water pickup at a point that it won't come out of the water "at speed"... and if the pickup in the drive isn't big enough an you use the "transom pickup". then sucking air and getting variations would be just part of the game.

BTW... for some engines, there are intentionally small holes drilled in the T'stat to avoid air pockets... at least my 225HP LM318 (Chrysler) came from the factory that way.
 
if you have 1 3/4" hose between the tstat and the manifolds, there's likely no restriction in the water jacket......you need a little restriction to ensure it cools correctly
 
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