1986 Mercruiser V-8 5.7L - overheating issue??

[Blueghost924]

Hi, I have a 1986 Mercruiser V8 5.7L that I have an overheating issue, even at idle. When I first start up the engine, it will come up to about 150. When I then bring it up to about 1100 RPM, the temp will rise to about 180, and when I bring it back down to idle it continues to climb until I shut it down. This is on the trailer on the muffs. The thermostat, retaining ring and gaskets are all brand new.

Last year, I ran it in the ocean in July where it ran hot (around 190) if the engine ran over 2000 RPM but would cool back down to 170 steady at lower RPMs. Hence the reason why I decided to replace the thermostat this year.

Some other items to note:

1. I do see water coming out the exhaust ports on both sides of the stern drive.
2. The sterndrive impeller was replaced by a shop back in summer 2019. I always run the sterndrive with sufficient water on from the muffs before starting.
3. I'm wondering if I installed the thermostat and retaining ring incorrectly? The retaining ring is brass and is a solid piece.
4. I've never replaced the engine seawater pump, and I'm wondering if that could be the culprit.

I want to start troubleshooting to see what the issue is, including if it's the sterndrive impeller, engine seawater pump or the thermostat? And, what hoses do I remove to check for proper flow.

On a side note, the darn thermostat cover is leaking - and I can't seem to get the gasket from leaking. I did note that the retaining ring seem to stick up a tad bit higher that flush from the cover. It's a brand new gasket.

Help?

 

[Scott Danforth]

Start with replacing the impeller

 

[dubs283]

1. that's good but does not mean sufficient flow
2. did they replace just the impeller or whole housing/kit AND the base assy?
3. possible but if it's operating most likely you got it right, the retainer should fit into the housing receptacle it's mounted to, also feel free to use gasket sealer (recommended)
4. possible

 

[Meames1]

On the gasket leak. I'm unsure, but are the bolt holes for the thermostat housing in the intake manifold blind or drilled through? Because if they are through, that gasket you show in the second picture has a path for coolant to flow right out of.

 

[Lou C]

Those holes are blind on GM cast iron intakes.
What I’d do is put the boat in the water and bring a 5 gallon bucket. Look up Mercs test for the volume of water that impeller should pump in a set amount of time. That will tell you if the impeller is supplying enough water. However for a complete test you need to add a section of clear hose to the impeller to stat housing hose so you can check for air bubbles in the raw water flow once the boat is on plane.

That’s the water intake side; you still have to find out if water passages in your exhaust manifolds and elbows are clogging with rust; this will tend to cause overheats at higher speeds and the manifolds and elbows will get very hot.
Last thing which is not that common but can happen on old engines with a history of hot running or even one bad overheat is blown or leaky head gaskets. This can allow exhaust gas in the cooling water which will cause hot temps & overheating even if the water volume is adequate. The way to check for this is to temporarily replace the feed hoses to the exhaust manifolds with clear hose. Run the boat in the water till the thermostat opens up. If you see air bubbles in those hoses then the HGs are suspect; could also be a cracked cyl head.

 

[kenny nunez]

As Lou stated about manifold blockage another thing that happens is the plastic water pocket cover in the upper housing will melt because there is not enough water to put out the exhaust flame. With the lower dropped look to see if the cover is melted. When they melt they leak. Put a length of 5/8” hose over the pipe and connect that to a garden hose. If there is a leak anywhere you will see it. It could also be the hose between the bell housing and transom plate. if there is a blockage the copper pipe will blow out of the pocket.

 

[Blueghost924]

Thanks for the replies. Quick question: Is there a hose I can disconnect from the motor or thermo housing that comes from the stern drive that I can disconnect to see if I'm getting adequate flow from the impeller? I'd like to see if the stern drive impeller is truly the cause (start the engine and note the flow of water from the impeller).

 

[Scott06]

Thanks for the replies. Quick question: Is there a hose I can disconnect from the motor or thermo housing that comes from the stern drive that I can disconnect to see if I'm getting adequate flow from the impeller? I'd like to see if the stern drive impeller is truly the cause (start the engine and note the flow of water from the impeller).

Yes the one that comes up from the outdrive to the thermostat housing.

 

[Lou C]

Yep I said that up above, Merc has a spec for how much it should pump in X amount of time.

 

[Bt Doctur]

Rule is water in=water out. Shadetree test is to run the engine at idle, feel the hardness of the circ hose. Run at 2500 and feel it again.If now hard as a rock your restricted from manifolds or riser/elbow

 

[Lou C]

Another not real common cause is leaking head gaskets putting combustion gas into the cooling water. Worth checking for this if other causes don’t pan out.

 

[Scott Danforth]

Another not real common cause is leaking head gaskets putting combustion gas into the cooling water. Worth checking for this if other causes don’t pan out.

compression and leakdown test should confirm if this is the issue or not.

 

[zellerj]

When the impellor was changed in 2019, why was it changed? Was the old one in pieces? If so, a piece of old impellor may be blocking the passage. Usually this blockage is at the power steering oil cooler located at the bottom port side of the engine. The hose on the PS oil cooler is also the hose coming from the outdrive.

 

[Scott Danforth]

the power steering cooler does make an effective filter. .210 inner diameter tubes are much smaller than the .31" diameter strainer holes in the outdrive.