Odd Overheating Issue in Exhaust of one of my 5.7L 350s

[sawickipedia]

I am thoroughly stumped and could use some help but also this whole situation makes me wonder if it's just inherent to these engines.



Situation: At idle, the exhaust hose section right after the inside riser (meaning in the middle of the boat see diagram) on my port engine runs hot getting up to/over 200F. I've ruled out all the usual suspects as I replaced the impeller, the riser & flushed the raw water hoses and no change. Also at idle - the other exhaust line on the port engine (so the same engine) runs at a much lower temp (100F) and the exhaust bellow doesn't run hot either (so water is definitely getting into the system) so there's no steam coming out of the exhaust. And when ramping up the RPMs - the exhaust temp drops way down (obviously because more water is being pushed into the exhaust). Oh and the new riser never gets above 80F so plenty of cold raw water is getting inside of it. And the engine water temp at the thermostat was never hot - the engine stayed a constant 145F (via the sender/gauge and the FLIR gun) so the engine wasn't overheating and was perfectly fine.

I have a FLIR infrared temp gun so I can check the engine and other temps on the boat. So I decided to also compare the starboard engine. And when I ran the starboard engine - the inside line (the one closest to the port engine) also ran warm at idle around 175-180F. So maybe these engines just can run warm exhaust at idle?

I also checked the manifold for that riser/exhaust line and it was running 175-185F so it doesn't seem to run that hot. Not sure what temps manifolds should show. I would like to say that the aft side of the manifold where it connects to the engine was getting hot topping out at 265+F. Looking at the other manifolds on the boat they all seem to have hot spots where the manifold connects to the engine but 280F was definitely the hottest one - the others were all more like 240F.

Pics from the FLIR gun:
From 2 weeks ago -
What started this whole inquiry was feeling a hot exhaust hose and so pulled out the FLIR gun and popped a 325F!!!!!! Yeah the hose is rated for 250F so I shut the engine down ASAP before the hose melted and started replacing elbow and impeller:


From this weekend:
This is at 2k RPM and where the manifold connects to the engine on the port engine inside side and is reading at 265F - the bluish section at the top of this pic is the exhaust hose section that runs hot at idle but runs plenty cool at higher RPMs:


Top of the Starboard engine's inside (closest to the port engine) manifold showing 211.6F where it meets the Riser (the yellow is the manifold and the purple rectangular ojbect rising from the manifold is the elbow):


Here's the Starboard engine inside exhaust also running hot at 197F (the sun was out and raising the temp of this section - in the shade it runs 175 to 185F) making me wonder maybe these inside section runs hot at idle. That's the hose there running down from the riser and you can see the edge of the manifold which is underneath the riser sticking out a bit to the left of the riser and exhaust hose:


This is starboard engine's outside manifold where it meets the engine showing 271F (so in terms of the port engine it's the same side as the one I was looking at) so probably a normal temp for where a manifold meets the engine but I'm not sure:

 

[Bondo]

Ayuh,..... Welcome Aboard,...... Is this on the hard, or in the water, runnin' across the pond,..??

The point where the head meets the manifold is Always hot, as that point is solid cast iron, no water jacket,.....

 

[sawickipedia]

Ayuh,..... Welcome Aboard,...... Is this on the hard, or in the water, runnin' across the pond,..??

The point where the head meets the manifold is Always hot, as that point is solid cast iron, no water jacket,.....

In the water in my marina. Good to know that point between the manifold and the engine is always the hot point. Part of the fun of the infrared temp gun is you get to poke around see exactly what's going on. Still not sure what to make of the exhaust section running hot - again maybe normal for the 5.7 350s but it seems like it could be something.

 

[Scott Danforth]

Ok, we know you are posting in the Mercruiser section, so we assume twin Mercruisers

You have twins with fiberglass mufflers, do going to assume inboards

Beyond that, the inboard SBCs for the past 65 years have had multiple different cooling systems.

High temps is low water flow

What year, or serial numbers?

When was the last time the raw water pumps were serviced? How do the seacock strainers look?

 

[sawickipedia]

Ok, we know you are posting in the Mercruiser section, so we assume twin Mercruisers

You have twins with fiberglass mufflers, do going to assume inboards

Beyond that, the inboard SBCs for the past 65 years have had multiple different cooling systems.

High temps is low water flow

What year, or serial numbers?

When was the last time the raw water pumps were serviced? How do the seacock strainers look?

Yes twin inboards - they are this version - 5.7L MIE GM 350 V-8 1988-1993 - 0B788197 THRU 0F024999.

Yep higher temps mean low water flows it's why I replaced the riser and impellers and flushed the raw system w/ shore water.

What I am beginning to wonder is this idling issue might just be inherent to idling on these models. The reason I wonder is that the outside exhaust on both engines is fine temp wise and the exhaust on this model flows straight from the impeller via the heat exchanger so it's not the water pump for the engine block.

And yep I checked and cleaned the sea strainers - they were fine. And running shore water hose (via hose connector attached to the hose that's the outflow hose from the impeller) showed plenty of flow to the riser hose so I know the heat exchanger isn't restricted. Hence stumped.