8.1Gi inboards with closed cooling problem(s)

Hipnautical

Recruit
Joined
Sep 2, 2015
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3
I can't figure out what to do to remedy this. Here's what I know:

--Starboard engine seems to run at the right temp(just over 160).
--Port engine runs about the same, but will creep up approximately 20 degrees while running slow(under 1000 rpm). Never 'overheats', whether slow or on plane.
--Port engine cools to same indicated temp as starboard when run above 1100-1200 rpm.
--Both exhausts appear steamy/smoky(hot). Port exhaust is hotter than starboard, no matter what the gages indicate.
--Both engines have brand new impellers.
--When I am running I have taken temp readings with a laser thermometer, and almost every surface is 20 degrees hotter on port engine.
--Planning to change thermostats on both motors today, but hard to believe they will change anything based on what I can tell.
--Both engines run very well, with only 240 hours on each.
--I am wondering if I have bad exhaust risers? Trying to rule everything out before tackling them...

Hoping someone can shed some light on these issues so I can make the right changes. I am sure the exhaust should not be steamy when it's Summertime and the air and water is warm. Looking forward to your input!
Thanks in advance.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
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Jul 23, 2011
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47,306
welcome aboard.

first question, do you boat in salt water or fresh water? If you boat in salt water, pull the manifolds and risers and inspect them. clean as required. if they are older than 5 years old, replace them

second question, when you replaced the impeller, did you back-flush?

when was the last time you cleaned out the heat exchanger?

steamy exhaust generally indicates low water flow.

other places to check the hose between the transom shield and the outdrive. the older VP drives had/have a problematic casting that should be replaced every 2 years along with the bellows. The SX drives has a fitting, however my direct experience with that drive is limited.
 

Hipnautical

Recruit
Joined
Sep 2, 2015
Messages
3
Thanks Scott;
The boat is Inboard, not sterndrive.
Going to change the port thermostat tonight, see if anything changes.
The raw water side is clear in every spot I've checked, but you have me thinking the external strainers(screens where water enters the hull bottom) may not be clean, so the flow could be choked.
What's puzzling is that both engines have same symptoms, and usually one engine will have an issue, not both, in my past experiences.
I'm thinking if I had a bad riser or manifold the problem would be more isolated???
Anyhow, thanks for your input.
If the screens(strainers) are clogged up, it would make sense because they would be alike, in other words both motors would have the same symptoms.
At least it would make sense to me.
Anybody else have an idea???
Pulling the boat today, wish me luck:)
 
Last edited:

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,306
if your in salt water, the risers and manifolds rust crazily fast. hence the reason to inspect them often (pull them off, clean them up, re-gasket), and the reason why if they are older than 5 years, replace them. if you dont, you may end up replacing the motors when they fail.

your inlet strainer could also be plugged, and no, they would not be plugged the same, steam is a sign of slow water flow, overheating is a sight of slower water flow.
 

ericga

Petty Officer 1st Class
Joined
Mar 4, 2012
Messages
213
After cleaning the strainer do a water pump output test to see if you have sufficient output from the pump. Also one thing people often overlook is the belt, pulleys, tensioner, alternator and water pump bearings. You may have a sticking bearing causing the belt to slip a little at low RPM.
 

Hipnautical

Recruit
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Sep 2, 2015
Messages
3
Over the years, and with my experience owning, maintaining and operating TWIN inboard motors, I have never had a failure or problem arise on BOTH motors simultaneouly. If a motor fouls a spark plug for instance, that motor runs rough, but the other one is just fine. If you get bad fuel, etc, usually both motors suffer. I have an issue here that is affecting both, so I doubt there's a clog in the raw water side of one motor, such as debris, chunks of an old impeller, etc. I also believe if a riser or manifold were bad, it likely would make a problem for just one motor, at least initially. My boat has external sea strainers, which are basically perforated plates over the intakes. I unfortunately didn't check or service these before launch(and it's my first year with this boat), and I suspect they're corroded, partially plugged, or both.
Having the marina haul the boat today, then I'll check, remove, and clean the strainers. Of course the experts at my marina think I'm wrong, so time will tell. Gotta start with easy/cheap and go from there!
Wish me luck. If I'm right, we should have a great holiday weekend. Been living with and trying to figure out this issue since launch......My hunch has always been weak water flow, but the exhaust outlets are half submerged, so can't really tell how strong or weak it's flowing----Scott Danforth, your statement "steamy exhaust generally indicates low water flow" is what got me thinking this way---THANKS!------Stay tuned.....
Thanks again for your input. I'm keeping my fingers crossed.
 

ericga

Petty Officer 1st Class
Joined
Mar 4, 2012
Messages
213
If your manifolds/risers have the same age on both engines they likely have a similar level of deterioration. When they deteriorate they often flake inside and cause water flow restrictions. One side could be worse than the other and both exhibit the same symptoms to a different degree. (see picture on the link below)

http://www.rexmarine.com/blog/post/inspecting-exhaust-manifolds-and-risers

Cleaning the strainers would obviously be the first thing to do. I would still do a simple water pump output test while the boat is out of the water. You should be able to collect at least 7-8 gallons of water from the exhaust running the engine at 1000RPM for 15sec.
 
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