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Old 02-26-2006
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Over heating Diesel

I have a friend who is cruising and recently experienced some overheating issues with his Perkins 4-108 while heading from Sawyer Key to Key West. He indicated he'd checked the impeller, the raw water strainer, exhaust and even cleaned his heat exchanger. The problem seemed to go away, but he was curious as to what else he could have checked. I indicated I would post his question, so thanks to anyone that can provide some insight. He also said that he had on a couple of occassions found crab pot lines tanled around his keel and prop.
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Old 02-26-2006
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other items to check re overheating on Perkins 4.108

I too have a Perkins 4.108 on my Little Harbor 38. The other two items to consider checking would be the thermostat to make sure it opens at the rated temperature (do this on a stove, placing the removed thermostat in a pot of water with a cooking thermometer) and check and / or replace the temperature sender. What you listed as done probably fixed the problem, but if not, these are to two remaining items I can think of to look into.


Rob Proctor
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Old 02-27-2006
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I concur with Rob on his advice. The thermostat is often the culprit - especially on boats that sit for long periods in salt water. On engines that do not get flushed, barnacles have a tendency to grow inside the housing around the thermostat. Here's a picture of barnacles growing inside a thermostat housing cover. The barnacles can impede the opening of the thermostat, thus causing the engine to run hotter that it should. "Salt never sleeps"
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Over heating Diesel-barnacle.jpg  

Last edited by administrator; 02-28-2006 at 05:14 PM.
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Old 02-27-2006
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I have a Westerbeke 4-107 which is a Perkins "marinized" by Westerbeke. I don't know if this applies to the 4-108, but I had an overheat problem last summer. It turned out to be the oil cooler heat exchanger which had clogged with old disintegrated zincs. This engine has 3 heat exchangers, the main fresh water cooling, a transmission fluid cooler, and the direct oil cooler. If any one of them restricts water flow the engine overheats. Normally I would notice this in the amount of water in the exhaust (my boat exhausts out the side, above the water) but it had been bad since I bought the boat and I thought it was normal (until it was fixed). I guess this is a tribute to the safety factor that was built into the heat disposal design.
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Old 02-28-2006
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We just had a great discussion on the Morgan board reguarding this subject. One new thing that came out for most was that the 4-108 needs to be blead. Not many of the participants were awear that after bleeeding the system properly another 1/2 gallon of anti freeze mixture will be necessary. Check out the archives for spacifics.

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Old 02-28-2006
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Thanks to all for replying. As of now it appears that cleaning the heat exchanger solved the problem although he is going to check the thermostat.
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Old 03-01-2006
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Alot of very respectable answers here to which I would like to add cavatation,and possible head and or gasket problems but you would probly notice more loss of cooling fluid or cooling in with the oil resulting in a creamy color on your engine oil dipstick, none of which should be taken lightly,find the problem!
good luck Brad
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Old 03-02-2006
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Check the mixing elbow, that's the piece where the raw water enters the engine exhaust flow. Usually the exhaust is a rubber hose after the mixing elbow. Mixing elbows have a rough and relatively short life and if they get plugged up, it doesn't matter how clean your heat exchanger is, there will not be sufficient raw water to exchange the heat.
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Old 03-20-2006
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Back Pressure!!!

Excessive back pressure can cause loss of power and overheating.

Following up on the previous post by 'P32'. Back pressure due to corrosion or carbon in the exhaust outlet/mixing elbow is often a culprit. The elbow should be checked as part of annual or bi-annual maintenance. Another cause of excessive back pressure can be the exhaust hose or the exhaust outlet. Assuming that things are properly constucted initally something like an overheating episode can cause the exhaust hose to partailly collapse and raise back pressure. Changing trim of the vessel can bury the exhaust port on certain points of sail and increase back pressure.
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