454 Running Hot???

2ifbyc

Seaman
Joined
Apr 14, 2009
Messages
73
I have a 1990 Mercruiser 454 Bravo 3. I got it last year. It has been running great all summer. Just this week I had an alarm go off in the dash. I noticed that the temp gauge was much higher than usual. I let it cool off before restarting it and it dropped back down again. It has been doing the same thing since. Seems to run fine at idle and lower speeds but when i open it up the gauge begins to rise after 10 mins or so. Normally it dosent move from the 100-125 degree area but lately its been going up over 150 and 175 so i shut it down and let it cool off before starting it again. Any ideas? Never had any problems before. What should I check first? Im assuming its an obvious thing but I'm new at this. Thanks Guys!!!
 

BAproject

Petty Officer 1st Class
Joined
Sep 2, 2009
Messages
276
Re: 454 Running Hot???

Well, those running temperatures don't seem very high, But I could take a guess and say that the impeller in the outdrive is starting to go
 

2ifbyc

Seaman
Joined
Apr 14, 2009
Messages
73
Re: 454 Running Hot???

Just had a Brand new 2010 bravo 3 outdrive installed this season. Could it be the raw water pump on the engine?
 

Philster

Captain
Joined
Sep 15, 2009
Messages
3,342
Re: 454 Running Hot???

It doesn't seem to have a thermostat in it if she is running below 140 (bad news for the engine internals). I would think 160-175 is the most normal operating range, with 180 after a hard run.

First suspect when running hot is the impeller. Especially if you don't know the age/condition.
 

2ifbyc

Seaman
Joined
Apr 14, 2009
Messages
73
Re: 454 Running Hot???

How would I check if it has a thermostat? And if it doesnt should I put one in? Either way its running hotter than usual so should i replace the water pump? Arent there a few different types of water pumps? Which one needs fixin?
 

Philster

Captain
Joined
Sep 15, 2009
Messages
3,342
Re: 454 Running Hot???

Well... thermostat replacement is sort of like Engines 101. We usually don't have to explain this one. If you don't know, get someone to help you and buy and get an engine manual. It's an easy check/fix, but you need someone to help. Sort of like changing a tire. All you need to do is one and you're set for life, but someone has to walk you through it and you need tools.

When talking boat engines, the rubber/plastic impeller and housing are the issues at hand. These draw water into the engine. When you do not know the age, you MUST replace it.

So, your list now includes: Pull/check/replace t-stat. Pull/check/replace impeller/kit. Overheating can be many things, but you have to work down the list in order of most likely things first.
 

Lou C

Supreme Mariner
Joined
Nov 10, 2002
Messages
11,802
Re: 454 Running Hot???

If you're in Long Island you're a salt water guy like me and firstly that thermo has to be replaced, 125 is too cold, older Mercs used a 142 themostat and newer ones use a 160 stat. Your temps are not sky high and for it to run at 175 would not be abnormal, the 125 is though. But if the alarm went off I bet it was pushing 200 which is too hot if raw water cooled. If you replace the stat, and it cools well at lower speeds, but overheats at higher speeds you want to check the exhaust sytem. You could have corrosion and clogging in the manifolds/risers and if they have not bee replaced in the last 7 years or so then they are due to at least be inspected. If they are really hot when running (can't even put your hand on them briefly) then that's a clue to clogging. And if they rust through they will ruin your engine so do check them. Impeller should also be looked at. Replacing impellers every other year, and thermos as well (thermostats if raw water cooled get sticky in salt water and start sticking open then you have a cold running motor). Manifolds/risers, check running temps yearly and inspect at 5 years.
 
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