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Old 04-07-2007, 06:18   #1
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Erratic Temperature Reading in Perkins 4.108

I noticed an erratic temperature reading in my 4.108 yesterday while running at the dock. The temp guage, which usually has been steady around 180-185, seemed to be reading a little higher (190-200). Thinking it was the slow idle speed perhaps, I reved the engine a bit and the temperature dropped immediately, down to about 170, the jumped back and forth between 170-190...no pattern, just erratic, quick movements. I will clean contact and spray electrical cleaning compound, but what I am wondering is if this is a failure mode for the temperature sensor on the 4.108 Perkins...if so, what is a good source for temp sensor, TAD?, or can I pick something up at NAPA? Any ideas would be appreciated.
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Old 04-07-2007, 06:49   #2
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I replaced my gauge and sensor with stuff from WEMA.

WEMA USA, Inc. Home Page - Boat fuel, water and holding tank level sensors and related instruments

Works good.
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Old 04-07-2007, 10:42   #3
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Most engine senders are actually rock solid and simple to test. They are typically a 0-240 ohm variable resistor, in the case of heat senders they generally are about 220-240 ohms at 70F and as they heat up they drop to around 40 ohms at 200-210F. As their resistance drops, the voltage flowing through them goes up and the "temp gauge" (which is just a volt meter) reads "hotter" meaning more voltage.

So you unscrew the temp sender, connect an ohmmeter (any $20 digital voltmeter with a 200 ohm or 2000 ohm scale will do) with aligator clips, etc. so that one lead is firmly bounded to the outer shell (the threads) and the other lead connects to where the sender transmitter wire was clipped on.

Drop the sender in a pot on the burner, heat the water up, and watch the resistance change from 240 ohms to 40 ohms as the water starts to bubble. (No need to boil it.)

Some equipment manuals actually give you specific ohm readings versus temperatures, this is factory test procedure.

The senders RARELY fail internally. More often, the wire attached to them corrodes or there is a hair of wire that breaks off and shorts something. If the wiring is old and brittle...time to replace it. Especially if the engine wiring wasn't originally "heat and oil resistant" engine compartment grade, in a wire loom or other cover.
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Old 04-07-2007, 11:00   #4
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Checking wire and connections are the logical first step. The sender does not fail often but they do. The next logical step and the second in line to fail is the guage. Then finally the sender, but if you want to be assured that the entire loop is working and will continue to do so for quite some time, replace the guage. wires and sender. I don't go that far on paying customers boats but that is what I do on mine.
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Old 04-07-2007, 11:57   #5
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I replaced all 3, gauge, wire and sender.
No need to trouble shoot then and the stuff was almost 25 years old anyway.

That being said, the erratic readings went away, but the 4-108 still overheats when ran at constant power at or above 2500 RPM for more than 10 minuttes. The temp slowly but surley creeps up into the red, then comes back down to the green when I ease off power.

Have not found the problem...(No, not over-propped, yes the cooling system and heat exchanger is cleaned and serviced)

Now I just live with it as I never "need" high RPM anyway.
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Old 04-07-2007, 13:18   #6
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Sounds like the classic blockage someplace. A bit of impeller blade in a cooling passage? Some growth in the intake?

Overheat means either there's more heat coming out of the engine (exhaust elbow due for replacement from plugging up?) or the coolant is being restricted.

Then again, I've met brand new thermostats that were DOA out of the box, sometimes it is that simple.
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Old 04-07-2007, 13:35   #7
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Wel, the elbow has been serviced as well. 2 thermostats giving the same result.

I suspect a desgin flaw, too small of a raw water intake system perhaps.
Have no reason to suspect a bit of impeller as this engine has never eaten impelles.
Have used 2 different heat exchangers, both boiled out before installation...
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Old 04-07-2007, 14:08   #8
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1: Check cooling water level. (when engine is cool)
2: Check for lose connection on sender and on gauge.
3: Check for lose connection on 12VDC feed to gauge.
4: Check cold water impellor.
5: Check cold water strainer

Please note: none of the above are in any neccessary order, excpet for No.1
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Old 05-07-2007, 22:24   #9
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Thanks for the suggestions

I'll post when I get it solved. Thanks for all the suggestions.
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Old 05-07-2007, 23:25   #10
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Quote:
I'll post when I get it solved. Thanks for all the suggestions.
Post as you go along instead.
We all learn from each other and this temp thing in the 4-108s seems to be a common thing, but not sure what the solution is...?

Is the cooling system too small/restricted for the engine/power output?
Probably not, because if that was the case 100% of us would have the same problem.

Guys running too hard with the 4-108 or is it installed in too big boats with too big props, hence over-heating in problems...?

Perhaps: I have seen and heard complaints from folks in the 43-45' range about the engine being "too small", hence they run max power all the time.

Personally I have run the 4-108 in a 42' sloop without problems, but that was a "racing" type boat with low weight and low drag.

My own boat, the CSY 33 should be perfect for the engine, but uh, it still over-heats with constant power above 2500 RPM or so.
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Old 06-07-2007, 01:22   #11
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It's a hard issue to nail down CSY. Although boat size is not the issue, prop/Hp is. If the engine has the right load, it should be able to work at max cont. RPM and not over heat. But it maybe that the work required is on the point of too much for the enigine. It maybe able to pull the revs, but the load is too high.
However, in this case, it seems it is a different issue of temp fluctuation, so we shouldn't confuse the issue with the other known complaints just yet.
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Old 06-07-2007, 07:53   #12
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With regard to the engine being "too small", I agree with Alan Wheeler about it likely is a mismatch between prop and marine gear. In the case of the Gulfstar 37, they were delivered with a 4.108 and a 2.1 Borg Warner transmission...and a 16x12 prop, which in my case has been replaced by a 3 blade of 15x10...the previous owner simply ran the boat at around 1700-1800 rpm, and didn't worry about it, as have many Gulfstar owners. I have a background with diesel engines, and of course this seems wrong to me for an engine that is rated at 3000 rpm. It should be happiest cruising at around 2100-2400 rpm...but not with the prop as delivered or fitted. At the higher rpm, you clearly get overloading..smoke, etc, although no overheating unless you push it that way for a long time.

I do not appear to have overheating, there is no steam from the exhaust, water flow is fine, etc., just an erratic temperature reading on the guage. I'll post when I get a chance to work on it. A little reading on boatdiesel.com and use of their prop calculator will illustrate the points I made in this post.
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Old 22-01-2008, 06:22   #13
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Perkins 4108 Heat Exchanger Cap Gasket source?

Does anyone have a U.S. source for the gaskets on the round heat exchanger mounted transversly at the back of some 4108 Perkins? I pulled the heat exchanger, acid washed the tubes (really a lot of blockage, clean now) and reinstalled, but it worried me that the old gasket leaked a little when I reinstalled. I would like to put fresh gaskets, but I need a source. Actually, looks like you could make your own out of an inner tube or something like that...any comments on gasket sources would be appreciated. This is the HX with only one removable end cap, the other one is brazed solid.

If you have been following this thread, I was having the common problem with overpropped loading. The engine workes fine this time of year, water temp in the 50's and 60's...but when it gets to be summer, and water temp goes up to 80-90 degree F, then it overheats quickly at anything like normal cruise (1700 - 1900 rpm). Seems to work fine now, but it is not hot yet, so we will see.

Thanks in advance for comments on gaskets for the Heat Exchanger...it appears to be the common standard model, looks like it might have been made by Sendur, but I am not sure of that..
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Old 22-01-2008, 07:36   #14
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We had a 4108 turning an 18 inch prop through a 2:1 gearbox. It wouldn't rev much over 2000 rpm so never really delivered a lot of power. Glad it's gone. Anyway........ Sounds like low coolant level or an airlock. If the coolant level isn't enough to cover the temp sender it will read low although the engine is too hot. You then rev the engine and some coolant swills around the sender and the gauge quickly gives the correct reading which is higher than normal. You'll see this effect in your car if you let the water get low. It's not a sea water impellor problem as the temp gauge reading would increase quickly and decrease slowly. Have you got a coolant pump problem so it's not circulating too well and pushing the air out? Is the belt tight? When the engine is cold take off the coolant filler cap and run the engine for a couple of minutes. You should see a surge of coolant in the filler neck when you rev the engine. Don't try this when the engines hot! Might be worth taking the head off to check the water passages are clear. It's not a difficult job on this engine but try everything else first.
Moored up in the river Dart one night I was aware of the bilge pump going every so often. We'd just had a wet passage from Falmouth so I had visions of water dribbling down to the bilge and I wasn't too concerned. In the morning I lifted the covers to have a look in the bilge before starting the engine. Sea water was dribbling out of the old 4108's air inlet and down into the bilge!!! The stern gland is water lubricated from the engine exhaust. Water was flowing from the stern gland back into the exhaust, through the open valves, into the cylinders and out of the air inlet. Horrors!!! I spent the day taking out the injectors and drying out the engine. I also replumbed the stern gland feed with a big antisyphon loop in it. Checked that there was no water in the oil and started up with no problems. If I'd not looked in the morning and just tried to started the engine..............

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Old 22-01-2008, 08:21   #15
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We had a 4108 turning an 18 inch prop through a 2:1 gearbox. It wouldn't rev much over 2000 rpm so never really delivered a lot of power. Glad it's gone.
Poor engine...

Sounds like you were seriously "over-propped". The 4-108 should be able to turn 4000 RPM, with 3000 being the max continous as recommended by Perkins.
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