Mercruiser Inboard Overheat

[b0mbtrack]

Mercruiser gm inboard v-8 1980 freshwater cooled.

The engine slowly overheats after running at a higher rpm. The engine will get up to a little under 200. The sensor itself is not in the thermostat housing it is conected to the intake manifold so I'm not sure how hot they usually get but the engine usually ran just under 160 with a 140 thermostat in it.

Below is a list of things that we did to fix it but had no luck. They are not in order.

-Changed thermostat and cleaned out small passage in thermostat housing.
-Replaced entire raw water pump, not just the impeller (jabsco pump).
-Replaced belts as they were constantly getting loose.
-Hauled boat and cleaned intake strainer as had growth and barnacles growing. Cleaned entire bottom while it was out.
-Bypassed the raw water filter and checked intake hoses for blistering.
-Removed and inspected risers as they are the only part of the engine that is salt water cooled but they were clean. While they were off we could see the manifold passages and they were clean as well. At the same time we checked exhaust hoses and they were fine.
-Removed the heat exchanger and clean it out.
-Through all this we must have added at least 4 gals of anti-freeze.
-Banged severely on top of engine with biggest sledge hammer we could find.

 

[Don S]

Re: Mercruiser Inboard Overheat

Are you sure the engne is actually running that hot? You could have a bad gauge or sender. I find it hard to believe that a cooling system with the manifolds in it used to run between 140? and 160?. 180? is normal for that system. Even with a 140? thermostat.

 

[b0mbtrack]

Re: Mercruiser Inboard Overheat

I checked the temp with my temp gun to verify that the gauge was ok but I checked it at the intake manifold where the sensor is and not at the thermostat housing where I would normaly check it. The gauge was accurate but now I'm curious if I should have checked it at the thermostat instead of where the sensor is.
As far as it usually running at a cooler temperature in the past, I'll have to take it for another test run to verify what it is running at now.

 

[PitchFork]

Re: Mercruiser Inboard Overheat

I have a 1994 GM5.0L that has a 143? TS in it and it runs b/w 150? and 180?. A constant 160? when running on plane. Look at the manual to make sure all parts are in the TS housing. Mine was missing the check balls when I first bought the boat and running 20?+ warmer.

Check idle speed low idle not enough flow

 

[Don S]

Re: Mercruiser Inboard Overheat

If you check it at the sender, then you are reading what the sender is telling the gauge. IF they are the same (or close to it) then it's running a bit hot.
Did you check inside the water pickup? I have found them full of barnacles and other marine growth before.
An air leak around the strainer lid will also cut down the water flow. How dirty was the bottom, that will slow you down, and cause the engine to work harder.

 

[b0mbtrack]

Re: Mercruiser Inboard Overheat

Hey Don, Yeah, the sender was accurate to my heat gun. I did not take off the intake screen to look inside there but I'll probably do that next. (I might have different terms for some things so to make sure I'm talking about that same thing as you are I'm talking about the sea strainer on the outside of the boat).

As far as the intake strainer lid I bypassed the entire filter and went straight to the pump with the hose. (I'm talking about the raw water filter that is inside the boat before the pump.)

The bottom was very dirty but I powerwashed for that reason, I was afraid it would either slow down the boat or creat air bubbles.

Tomorrow I'll check out that water pickup, both the inside of the screen on the outside of the boat and the thru-hull leading into the boat.

 

[Don S]

Re: Mercruiser Inboard Overheat

I take it that you haven't run the boat since you cleaned the bottom?

The seawater strainer I was referring to (with the lid) is what you are calling the raw water filter.

 

[b0mbtrack]

Re: Mercruiser Inboard Overheat

Hey Don, I did run the boat again after I power washed it, this was also while I bypassed the intake filter and went straight to the thru hull fitting with the hose going to the raw water pump.

So tomorrow I am going to try and take off that strainer and check the inside of it for barnacles.

I dont know if the anti-freeze system (Block) has ever been cleaned out (other than the heat exchanger) so I might try and pressurize it to flush it even though I put a good amount of new anti-freeze in there. The bypass in the thermostat housing was clogged which I thought was strange for a freshwater system so maybe something else is clogged.

Last thing I might do is drill a few holes in the thermostat itself and see if it runs cooler.

 

[Apollo75]

Re: Mercruiser Inboard Overheat

Mercruiser gm inboard v-8 1980 freshwater cooled.

The engine slowly overheats after running at a higher rpm. The engine will get up to a little under 200. The sensor itself is not in the thermostat housing it is conected to the intake manifold so I'm not sure how hot they usually get but the engine usually ran just under 160 with a 140 thermostat in it.

Below is a list of things that we did to fix it but had no luck. They are not in order.

-Changed thermostat and cleaned out small passage in thermostat housing.
-Replaced entire raw water pump, not just the impeller (jabsco pump).
-Replaced belts as they were constantly getting loose.
-Hauled boat and cleaned intake strainer as had growth and barnacles growing. Cleaned entire bottom while it was out.
-Bypassed the raw water filter and checked intake hoses for blistering.
-Removed and inspected risers as they are the only part of the engine that is salt water cooled but they were clean. While they were off we could see the manifold passages and they were clean as well. At the same time we checked exhaust hoses and they were fine.
-Removed the heat exchanger and clean it out.
-Through all this we must have added at least 4 gals of anti-freeze.
-Banged severely on top of engine with biggest sledge hammer we could find.

----------------------------------------------------------------------------------------------------

"Banged severely on top of engine with biggest sledge hammer we could find."

Love that part

"Through all this we must have added at least 4 gals of anti-freeze."

Please consider the following while checking coolant protection level. The more antifreeze to water ratio the more pronounced the effect.

"Note: Ethylene glycol, which makes up 96 percent of the antifreeze/coolant, has about half the heat transfer capability of plain water. So when you mix antifreeze and water in the recommended 50-50 mix, you give up 25% of your system's cooling capacity."

This depends on the type of antifreeze used.

Article : http://www.bullittarchive.com/5010.htm

The original heat exchanger was engineered for this but a heavy mix may raise temp.

Best of luck

OFM

 

[Thumbtack]

Re: Mercruiser Inboard Overheat

Hey, just letting you know we did use mixed anti-freeze and not just straight anti-freeze.

 

[b0mbtrack]

Re: Mercruiser Inboard Overheat

I think I stumbled upon it and it was a stupid mistake on my part. I took a look at the old thermostat and sure enough there were three little holes drilled into it that I didn't notice when I took it out. I went ahead and drilled 3 little holes into the new thermostat this morning.

So two things must have happened, we fixed whatever was originally causing the over heat but at the same time put in a thermostat that would cause another overheat. I'll test run it most likely tomorrow when it isn't pouring out to make sure.

 

[Don S]

Re: Mercruiser Inboard Overheat

Those little holes are not the cure, or the problem. All they are is vent holes to allow air out when filling with antifreeze so it doesn't build an air lock.
You stated you cleaned the heat exchanger. How did you clean it, with what? Did you do the raw water side as well as the antifreeze side?

 

[b0mbtrack]

Re: Mercruiser Inboard Overheat

I did have the heat exchanger off because there was a small hole on an elbow that was leaking anti-freeze. I had the hole welded thinking that was my overheat problem (this was also the first thing I did in the very begining of the summer).

While the heat exchanger was off all I did was take the end caps off of each side and blow out the passages with air pressure and stick a rod down as many of those passages as I could. I held a flash light on the other end to see if they were clear and I didn't see any blockage. As far as the anti-freeze side, I simply looked in to the nipples on the anti-freeze side and didn't see anything out of the ordinary, I didn't do anything to that side as I just assumed that side being anti-freeze would be clean.

Just out of curiosity, if there was an air blockage in the top of the thermostat would that cause antifreeze to get blocked or slowed down?