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Old 17-01-2019, 09:26   #1
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Perkins 4-108

Will a bad thermostat cause the engin to overheat
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Old 17-01-2019, 09:31   #2
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Re: Perkins 4-108

If its like a car, yes, if the spring sticks closed, although, the most probable solution is a bad impeller, or clogged heat exchanger. I'm by no means a perkins expert, but do work on allot of other engines
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Old 17-01-2019, 19:42   #3
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Re: Perkins 4-108

Yes, it can be stuck closed and cause the engine to overheat. In my 4-108 the thermostat got stuck open and I had the opposite problem (running too cold at idle).

Lots of threads on overheating Perkins, it has an undersized heat exchanger and will overheat if you look at it funny.
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Old 17-01-2019, 19:48   #4
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Re: Perkins 4-108

Hi,

We have a 4108 which overheats if you run it for too long above 2600. We have a checked the heat exchanger and cleaned it, so my next job is to change the thermostat. Runs all day at 85 deg C at 2500.

Sounds similar.
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Old 17-01-2019, 20:10   #5
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Re: Perkins 4-108

Lots of threads on overheating 4-108s.
I tried everything and finally concluded the heat exchanger was one size too small.
Usually cruise was 2100 RPM, no problem.
High cruise was 2400 RPM, would run a bit warmer.
2600 and above the temp would slowly but surely creep into the red.
Perhaps it would be a non-issue in cold waters but all my cruising was Florida and the Bahamas with the same result regardless what I did with thermostats and impellers. ��
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Old 18-01-2019, 05:43   #6
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Re: Perkins 4-108

Thanks I have checked all impellers and raw water filter. Can I run with no thermostat?
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Old 18-01-2019, 05:56   #7
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Re: Perkins 4-108

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Originally Posted by Rbrucek View Post
Thanks I have checked all impellers and raw water filter. Can I run with no thermostat?
It depends on how cold the water is. If you’re in Florida or the Caribbean it should work fine. Unfortunately, I found that the Chesapeake was a little too cold to run with no thermostat and anywhere farther north would be worse. The issue is at low power (near-idle) when you’re docking or charging batteries.

In my case, it still overheated at full power with no thermostat - after trying all the fixes I decided to not run my 4-108 at full power. Easy solution. However, regular cleaning of the heat exchanger does make a difference, so I would add that to your list.
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Old 18-01-2019, 06:08   #8
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Re: Perkins 4-108

My sailboat is fitted with a cruising prop and for the better part of 30 years I resigned myself to running the engine at no more than 18oo rpm to keep the temp below 185'. The consensus was that with the bigger prop and an undersized heat exchanger, that caused the problem. Finally I bit the bullet and bought a larger heat exchanger (14" long) from Sendor. It took a little bit of modifcation of the cooling pipes, but it fit-and did it make a difference. I can cruise all day at 2000 rpm and the temp gauge doesnt get above 175'. I have run it as high as 2400 rpm and the temp remains steady at 175-speed doesnt increase very much.
Cost-somewhere over $400 for the new Heat exchanger. I wish I'd done it 30 years ago.

You could change props and maybe get the same affect but then you would have to run the engine at a higher rpm for the same speed-and you would consume more fuel. That didnt make much sense to me. Bob
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Old 18-01-2019, 16:37   #9
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Re: Perkins 4-108

There are many heat exchanger replacement makers. Most will make a custom one, longer than yours for more surface area. Or you can find one for a 6 cylinder. It may have excessive cooling, but you can restrict the water flow.
I've fixed several overheat problems for others by doing a proper cleaning on the coolant side. Old coolant looses it's anti rust properties and allows rust on the surfaces in the block and head. Old coolant also no longer has lube for the water pump causing early failure of the main pump.
Many 4108 owners have installed an oversize exchanger.
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Old 22-01-2019, 05:52   #10
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Re: Perkins 4-108

Thanks everyone I'm going to clean the heat exchanger now
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Old 22-01-2019, 06:13   #11
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Re: Perkins 4-108

If you have a HW heater in the coolant line, check the plumbing. There is a reduction in hose sizes. On my boat, all the cooling water passed thru the HW heater, and the PO had a PVC reducer installed. When it leaked, he just kept tightening the hose clamps, and the ID of this fitting was down to pencil size.
Brass fittings solved the problem.
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Old 23-01-2019, 00:45   #12
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Re: Perkins 4-108

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Originally Posted by Rbrucek View Post
Thanks everyone I'm going to clean the heat exchanger now


I'm sure this has been raised before on other 4-108 overheating posts but I'll mention it again. The cylindrical core stacks used on many engines are enclosed in a bronze/ brass tube with a hole through it at each end to allow the flow of coolant through and over the tube bundle. Raw water goes through the small tubes longitudinally. If the coolant flow is able to just travel down the outside of the bronze tube the heat transfer is greatly diminished and you get a slow coolant temp increase and eventually an overheat. To direct the coolant flow there was a flange inside the heat exchanger housing to stop the coolant from taking the shortcut outside rather than through the more restrictive tube stack. Perkins used several different methods to direct this flow including simply aligning the hole to the tubes with the circulating pump inlet or , on the perkins Prima , the inclusion of a plastic sealing ring around the core outer tube that had to be pushed into a casting sealing face in the housing.
The trick is to get a good flow of coolant through and around the tubes, not straight past em.
Hope this might help if you're having the slow overheat.
Pete
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