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Old 25-09-2015, 19:24   #1
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Perkins 4-108 cooling water circulation

I am troubleshooting a Perkins 4-108 that has had strange overheating problems. Can anyone explain how the water flows through the block and head? This motor has a header tank at the front of the motor. It appears that water flows from the right side of the header tank down to the right side of the block. The water pump (circulation pump) is on the front of the block (belt driven) but a passage from the pump directs water back into the block and where it goes from there I am not sure. The thermostat, located at the front of the head, lets water flow up into the left side of the header tank. I assume the header tank has a wall that divides the tank into two separate parts.
To complete the circulation, the water leaves the left side of the header tank, goes through the exhaust manifold, into the heat exchanger (mounted to the rear of the motor) and then back to the right side of the header tank.
So am I correct that the pump must be taking water from the block and sending it to the head where it flows to the thermostat?
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Old 25-09-2015, 19:33   #2
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Re: Perkins 4-108 cooling water circulation

Air in the system is possible with over heating to follow.
In many cases the 4-108 runs hot @ high power due to insufficient size of the
Heat exchanger.
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Old 25-09-2015, 20:21   #3
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Re: Perkins 4-108 cooling water circulation

Starting at the circ pump, the coolant flows from the circ pump into the block, from the block up into the head. It flows forward in the head into the thermostat housing, up thru the open thermostat into the expansion tank. Out of the expansion tank on the exhaust manifold side and into one tube on the front of the exhaust manifold. Out of the front of the exhaust manifold in the other tube and flows aft in the steel pipe into the heat exchanger at the rear of the engine. Out of the HE and forward into the oil cooler and gear cooler ( if installed) and back to the circ pump via a passage in the side of the block just behind the alternator.

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Old 25-09-2015, 21:02   #4
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Re: Perkins 4-108 cooling water circulation

Can you explain "strange overheating problems?" Page 54-56 of my 4108 manual clearly outlines the cooling system and the troubleshooting guide on p 67 lists 15 possible reasons for overheating from air cleaner restriction to low coolant. Are you getting plenty of water out the exhaust? It seems to me that overheating is probably diagnosed in similar ways on any engine beginning with the most simple possible causes: Please tell us more about what you have learned and what you have done.
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Old 26-09-2015, 04:44   #5
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Re: Perkins 4-108 cooling water circulation

I too have the 4-108, but with the Bowman H/E which is a combination of header tank, exhaust manifold and Heat Exchanger. Recently I've experienced some heating "problems" and this is may or may not a benefit to your problem. I do have a water heater taking fluid from the engine which is a source of possible vapor lock. For years(25 years) my temp gage read a constant 180 degree when operating at 2500 rpm. From what I can determine the normal operating temp should be between 150 and 200. When I changed out the anti freeze this past June, the temp gage started reading 190 at 2500 rpm. Not really sure as to why there was a change other than some sort of vapor lock yet the bowman should be self bleeding. Anyhow last week, I replaced the heater hoses going to and from the water heater which involved refilling that part of the cooling circuit with new antifreeze. I now show a temp reading of 165 at 2500 rpm and 150 when backing the rpm down to 2000. It has never read that low in the 30 years I've had the boat. (1985 Perkins)
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Old 26-09-2015, 09:06   #6
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Re: Perkins 4-108 cooling water circulation

Overheating was making me nuts and putting a strain on my Boat US towing unlimited pkg. Did everything to fix it; impeller, gauge, sending unit, hoses.,etc. It's always the last thing you try that fixes the problem. I didn't notice that the intake through hull was still 1/2" for the old Atomic 4 , and hadn't been changed when the the Kubota diesel was installed. Changed to a 3/4" with new strainer. Kubota now running cool as a fridge. Doh!
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Old 26-09-2015, 15:29   #7
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Re: Perkins 4-108 cooling water circulation

Don't know what you've tried, but had same prooblem with my 4108. Temp kept creeping up slowly. Water came out exhaust but it looked like not a full flow. Took hoe off exhaust elbow and got great flow. Found remnants of old zincs from heat exchanger lodged in exhaust elbow. Cleaned it out and voila, cool again.
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Old 26-09-2015, 15:41   #8
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Re: Perkins 4-108 cooling water circulation

Read this thread. Perkins 4-108 0verheating - Page 3 - Boat Design Forums So many questions still being posed have been addressed all over cyberspace and this is another example.
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Old 26-09-2015, 19:15   #9
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Re: Perkins 4-108 cooling water circulation

Try changing your thermostat. I have 4108 and have experienced this issue. It worked for me.
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Old 26-09-2015, 19:17   #10
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Re: Perkins 4-108 cooling water circulation

Also don't exceed 3000 rpm.
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Old 28-09-2015, 13:20   #11
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Re: Perkins 4-108 cooling water circulation

I had a 4-108 in my Tanton 44. I spent thousands of dollars on trying to solve the overheating problem - everything in the cooling system was replaced since I have owned the boat and I babied that engine. God forbid I needed an extra burst of speed or horsepower to get through tough seas or to try to make a bridge opening.

The BEST day I ever had with my boat was the day the blue POS Perkins was craned off the boat. The second best day was when the bright shiny red Beta engine was craned aboard.

That was my ultimate solution to the overheating problem.
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Old 28-09-2015, 15:15   #12
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Re: Perkins 4-108 cooling water circulation

My 4-108 would slowly overheat if I ran continously above 2500 RPM.
Tried a bunch of things but concluded that the heat exchanger was too small or the raw-water intake was too small.
Either way, no big deal as my normal cruise power was 2100 RPM.
I could run @ 2400 all day long.
Also 5-10 minutes at 3000 if needed to make a bridge and 4000 Available for emergencies.
No big deal in my case, never tried to spend money buying a bigger heat exchanger.
Tried the easy stuff, acid boiling heat exchanger and oil cooler, new thermostat, new impeller, flush and replace coolant, new hoses, new cap, etc. Probably $200 total.
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Old 28-09-2015, 18:07   #13
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Re: Perkins 4-108 cooling water circulation

Often overheating is caused on the fresh water side. When I run a cleaner, I do both fresh and salt sides. Before doing the cleaning this way, I seemed to chase heat problems that turned out to be related to head/block or fresh heat exchanger side.
Also if you drill a small hole, 1/16" is enough, thru your thermostat, it will allow the new cooling solution to completely fill the engine w/o an air block.
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