​Mercruiser 3.0 Running Hot/Overheating - Need Help and Ideas

[enginesilo]

My engine has always seemed to run hot on the gauge (175 F degrees) with no other symptoms or issues, but this year it has me worried as it seems to have climbed to around 190 degrees while I was getting things ready for spring on the hose.

I shot the engine in a few spots with an IR thermometer and read around 190+ degrees on the block below the exhaust manifold, and the manifold and riser were at least 170 degrees. However, the thermostat housing never seems to read very high, maybe 135 or so. It also seems like the boat didn't start spitting out water from the back for a good while, and then it still seemed minimal.

Last year I thought it could have been the impeller, so I dropped the lower unit on the drive and replaced it. The old one was still in perfect shape so no parts sheared off to get stuck or anything like that.

Today I decided to go ahead and try to change the thermostat (160 Degree) incase that was a possible easy fix, it seemed to run cooler for a little while, but then it was right back to around 185+ degrees.

A bunch of people told me I should be able to hold my hand on the riser without it feeling too hot, my riser is definitely too hot to keep your hand on for more than a second or two tops.

Engine is a 2005 Mercruiser 3.0L with a Serial starting with 0W0, here is a link to my engine parts catalog:
http://www.marinepartsplus.com/catalog/mercruiser/serial/3.0L_GM_181_I-L4/804862+99

Has under 100 hours on the engine. It is a salt water boat, and I've had it in the water for the past 4+ years or so.

Engine starts right up, idles fine, and seems fine other than the temperature is too high.

Can anyone out there help me think of what the check next?
-Possibility a tube is clogged? If so, which and where, and how do I get to it?
-Is there another secondary pump of some sort, like a circulating pump or something on the engine that could also need changing? Or is that only the larger engines?
-Any way to detach the tube that comes in the transom from the outdrive where it connects to the engine to see if water is gushing it as it should?
-If the thermostat housing seems cool but the riser and exhaust manifold are hot, could it be a clog in the manifold?
-I'd hate to get back to the impeller since I just changed it, but could there be anything there to look at again?


All feedback appreciated!

 

[havoc_squad]

If the engine block is getting overall in overheat temperature and a new thermostat put in place, it sounds like your raw water engine circulating pump has failed or your exhaust/intake manifold/riser is failing or clogged.

Salt water boats have to have the manifolds and possibly risers replaced every 5 years due to advanced corrosion. Hours typically isn't a factor as much, because its the sitting with the salt water in the engines not running that wears on the cooling channels on the manifolds.

If the engine block is overheating faster than the exhaust manifold, I would probably guess you have raw circulating pump that has failed. If it is the manifold/riser, you'll be having hot water being forced back (stopped up) and this will start to overheat the manifold faster than the engine block (most likely).

 

[enginesilo]

If the engine block is getting overall in overheat temperature and a new thermostat put in place, it sounds like your raw water engine circulating pump has failed or your exhaust/intake manifold/riser is failing or clogged.

Salt water boats have to have the manifolds and possibly risers replaced every 5 years due to advanced corrosion. Hours typically isn't a factor as much, because its the sitting with the salt water in the engines not running that wears on the cooling channels on the manifolds.

If the engine block is overheating faster than the exhaust manifold, I would probably guess you have raw circulating pump that has failed. If it is the manifold/riser, you'll be having hot water being forced back (stopped up) and this will start to overheat the manifold faster than the engine block (most likely).

Thanks for the feedback. So are there any tests I can do to see if it is one or the other? Is the raw water engine circulating pump the one that sits below the thermostat housing bolted directly to the front of the engine with a pully on it? Just want to be sure you aren't talking about the impeller in the lower unit outdrive. I haven't worked on this engine a ton so I'm still trying to figure out what is what.

As I mentioned I don't have a ton of experience wrenching on I/O style engines, so here are some guesses at possible troubleshooting which may be good ideas, or horrible. Share if you think they are common and worth trying, or not.

​Is it possible to disconnect the water inlet where the water hose connects to the thermostat and put a garden hose directly to the thermostat area to troubleshoot and eliminate a possible clog from the outdrive area? I realize I would need to get creative since i'll still have the water inlet from the impeller in the engine bay and will need to still put a hose on the muffs so the impeller doesn't burn up. I guess if possible that would eliminate an impeller or tubing issue to the thermostat area.

Any way to test a the circulating water pump, and do these commonly blow or is it rare? If it was this how difficult are these to install?

I realize it can easily be the exhaust manifold/risers, but I want to try to troubleshoot elsewhere before I pony up both the money and time to replace them right now if it isn't the issue right now. Since its always run a bit hotter even before it went into salt that has me wondering if it is something else causing it. If I want to do some inspecting here would it make sense to start by removing just the exhaust riser elbow and inspect that first before pulling the entire manifold? That would be much easier with only 2 bolts, 1 gasket, and the rubber tube clamps. This engine has the carb mounted to the exhaust manifold and is definitely way more DIY than I want to do right now, but if the easier tests don't work i'll obviously have to take this task on now.

All feedback appreciated.

 

[havoc_squad]

All conventional inboard based engines are no different in cooling than a gas or diesel engine. They require a circulating pump to move the cooling water in the engine and work with a thermostat. It doesn't matter if it has an outdrive water pump/impeller pickup system or not.

You could connect a temporary rubber hose on the thermostat outlet and cover the hole that will go to the intake/exhaust manifold, and leave the hose open in the bilge to see how much water you get coming out. (Like 5 seconds or less, enough time for water flow to reach the thermostat and come out.)

If you're not getting a strong flow of water after 7 seconds at mid idle speed, your circulating pump is toast.

 

[dubs283]

thermostat outlet and cover the hole that will go to the intake/exhaust manifold

the t-stat housing outlet and the outlet that goes to the exhaust/intake manifold are the same, that is how hot engine water exits

the op needs to remove the riser and check for blockages in the manifold and riser - noting little water exiting from the exhaust with a freah sea water pump is a classic symptom of a blockage

 

[Bt Doctur]

A quick test: with the motor off, squeeze the large hose from the circ pump to the therm housing to get a feel for it. Now start motor and set rpm around 1500 no more, squeeze large hose. There should be no difference with the motor running or not. If it`s a lot harder to squeeze, you need a riser or a manifold and riser.
The hard hose tells you your restricted getting the water out.

 

[havoc_squad]

the t-stat housing outlet and the outlet that goes to the exhaust/intake manifold are the same, that is how hot engine water exits

the op needs to remove the riser and check for blockages in the manifold and riser - noting little water exiting from the exhaust with a freah sea water pump is a classic symptom of a blockage

I was referring to putting a CAP on the inlet for the water flow to the manifold while checking the water flow out of the hose. I don't think leaving that open while the engine is running would be a good idea due to exhaust pressure/noise.

However, I agree with Bt Doctur that squeezing the hose should be an easy first step check.

 

[enginesilo]

Thanks for the suggestions. I'm trying to find the time tomorrow or later this week to do some diagnostics. I didn't want to pull the riser for inspection but after I do some flow tests with the various tubes, the riser will be next up.

 

[saltybutwet]

great info! and something i could try too
so, any updates on your overheating issue?