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Old 08-09-2016, 15:20   #1
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Perkins 4-108

I have a 1962 Creekmore 36 with the original Perkins 4-108. Engine was rebuilt 15 years ago but I don't know what was done. I just replaced the water pump">raw water pump and a few small things. at 1800 rpm and 2000rpm, the engine temp seems to stay between 190 and 200 degrees. Is this an acceptable operating temp and if not, will installing a lower temp thermostat make a difference? I can cruise at 1600 to 1800 rpm but if the chop is up and I need to punch thru it, 2000 to 2100 works better. 3 blade fixed prop.
Many thanks
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Old 08-09-2016, 15:42   #2
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Re: Perkins 4-108

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at 1800 rpm and 2000rpm, the engine temp seems to stay between 190 and 200 degrees. Is this an acceptable operating temp
I think this is a little too hot. Mine stays at 170F even when running 2100 for hours. 180F would be acceptable. Above? Not so much. But there is so many things to consider - first would be to crosscheck your temperature gauge readings with an infrared thermometer to see if the gauge is correct. Do you have any steam, even a little, coming out your exhaust when the engine is up to temp?

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will installing a lower temp thermostat make a difference?
I installed a 160F thermostat just recently and just because. But I also replaced the heat exchanger and the sea water pump.

My hint after the usual suspects (partially clogged sea water strainer, scaled hoses, partially broken impeller, scaled/sooted up mixing elbow) would be to clean the heat exchanger in diluted muriatric acid and flush the whole fresh water side with a good descaling agent.
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Old 08-09-2016, 15:44   #3
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Re: Perkins 4-108

I think spec is 180 but if yours "never" gets over 200 it sounds OK . You should have a 4psi cap so boiling is going to be way up over 212. A Thermostat won't cool it down only warm it up.
That said it sounds too hot. Maybe clean your heat exchanger and elbow
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Old 08-09-2016, 16:02   #4
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Re: Perkins 4-108

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I think spec is 180 but if yours "never" gets over 200 it sounds OK . You should have a 4psi cap so boiling is going to be way up over 212. A Thermostat won't cool it down only warm it up.
That said it sounds too hot. Maybe clean your heat exchanger and elbow
I am pretty sure I had a 7 psi cap.
1979 4-108.

I changed thermostat for cooler running: It would slowly over-heat above 2400 RPM. The new thermostat actually made it worse.
Tried a bunch of different stuff so as to be able to run all day @ 2600 or 2800 RPM if needed, but was never able to fix the problem.
Concluded that the heat exchanger or raw water intake was too small.
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Old 08-09-2016, 17:04   #5
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Re: Perkins 4-108

[QUOTE=CSY Man;2208977]I am pretty sure I had a 7 psi cap.
1979 4-108.

Thats funny so do I but I thought it's supposed to be a 4psi cap. When my 4-107 runs that hot something is wrong. I run 165 at low power but at high power 181 and no higher unless something is not right. I can actually watch the thermostat open and close on the temp gage, it's so steady.
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Old 08-09-2016, 17:08   #6
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Re: Perkins 4-108

My 4.108 runs at about 180F @ around 1900-2000 RPM, which is our normal motor-cruising speed.
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Old 08-09-2016, 22:44   #7
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Re: Perkins 4-108

Running over temp over time will shorten the life of the rings and sleeves.

You either have a water flow problem or a heat transfer problem. An old engine could have a partial blockage in the exhaust elbow. A boat in salt water can have substantial marine growth anywhere in the raw water path. I have found mussels in engines that sit seasonally.
Heat transfer problems occur in the heat exchanger, the coolant side of the cylinders and head passages. I would descale the raw water side and use a cleaner on the coolant side that removes rust.
Don't forget the oil cooler. Oil removes engine heat, too.
Too much anti-freeze in the coolant mix can cause higher temps. I once used straight anti-freeze when much younger and got to do an unplanned engine overhaul.
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Old 09-09-2016, 04:00   #8
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Re: Perkins 4-108

4 lb caps used on wet liner engines ie. 4.99 & 4.107 --- 7lb used on 4.108. [Dry Liner]. Need to know max in fwd gear and high idle Rpm also hull and prop condition other wise otherwise its a Dart Board. Std thermos fitted to 4.108 fw cooled 80c or 180f. Is it a well known conversion ?
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Old 09-09-2016, 04:23   #9
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Re: Perkins 4-108

Turns out the temp gauge for my 4.108 has ther sender mounted to the back corner of the block. It reads 8-10 hotter than the infrared thermometer shows.
Is there a better place to get a more accurate reading. Bob
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Old 09-09-2016, 07:16   #10
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Re: Perkins 4-108

my newly rebuild 108 runs at 160-180 f. no matter the rpm. i use a 10 psi cap.
all my heat exchangers are freshly rebuilt and cleaned. my exhaust hosing is newly replaced, as were all the other hoses on engine.
i have approx 180 hours on mine from rebuild after runaway.
if you have doubts about your engine, it is better to have it overhauled than to have it die on a lee shore in a bad wind.
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