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Old 06-11-2010, 21:42   #1
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Perkins 4108 Overheating

Two problems which may be related
1. The engine overheated some years back on a final cruise before being hauled for a major retrofit which lasted several years. When relaunched some years later, the overheating issue was of concern and trials showed that the temp would rise gradually until the coolant vented past the filler cap pressure release. The fan belt was noted to be slipping, eventually diagnosed as a result of the alternator tensioner backing off while the engine was running. The tensioner bolt had stripped threads. Thought all was cured, but have not done much motor cruising this year.

2. For winter lay-up, we typically purge the raw water in the heat exchanger and muffler by displacing it with antifreeze, drawing it from a pail above the engine with the engine running until the pail empties and antifreeze is noted coming from the exhaust. This time around, the pail would not drain! Inspection of the water pump">raw water pump revealed a badly scored impellor. While replacing the impellor, I tried to turn the pump shaft and noted that it would not turn. (I believe this is normal?) With the pump reassembled I tried to prime the muffler and heat exchanger with antifreeze agin, but again the pail would not draw antifreeze. The pump was dismantled again and the shaft rotation determined by having a friend role the engine over with the starter motor. Pump rotation was clockwise. Tried this a second time, and the impellor would not rotate at all. Pulled the impellor and looked for shaft rotation without the impellor installed. It was intermittent. Then tried turning it by hand and noted it free wheeling then locking up. At that point, I pulled the pump to examine the drive end for damage. Found none on the pump or on the split shaft sticking out of the timing cover. That shaft could be turned by hand with no resistance in either direction. With the starter motor, the engine was turned over again noting the pump adaptor plate rotated clockwise, but the shaft at it's center did not rotate. The manual says not to remove the pump adaptor plate, since a special tool is needed for re-alignment the plate. Also some comment in another part of the manual that states the linkage is cenrifugally activated, but graphics are poor and no details found related to a clutch mechanism. Not sure if there is a clutch behind the adaptor plate, or if the pump shaft is supposed to be gear driven by the timing gear which also drives the fuel pump on the other side of the housing. If the latter, then some significant work ahead to repair.

Anyone have a similar problem before or know of a marine engine mechanic who knows for sure?

Appreciate all comments.


Thanks, rM
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Old 06-11-2010, 22:26   #2
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Numerous threads on 4-108s over-heating, do a search.

The engine is rated for 51 HP at 4000 RPM

Max continous RPM is 3000.

My 1979 4-108 will slowly but surely over heat if I run it over 2400 RPM.

Some folks may be over-propped, or running too high RPM to get the "goods".

First thing to check is impeller. Second is heat exchanger. (Clean in acid bath)

Third is to flush and overhaul cooling system including sea water strainer.

These are old engines in old boats and lots of deferred maintenance is going on.

Recommended RPM for cruise is 70% of max continous, or 2100.

If your 4-108 still over heat heats after you checked the cooling system including impeller/heat exchanger and running 2100 RPM, then you are possibly over-propped or the boat is too big for the engine...Or the engine too small for the boat.
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Old 07-11-2010, 07:08   #3
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Dear CSY man,

Thanks for the insight.

As idicated, I found the impellor badly worn during winter lay-up (while trying to prime the heat-exchanger and exhaust with anti-freeze). The engine while running would not draw the anti-freeze from the bucket suspended above raw water pump. Hence the inspection and replacement of impellor. Regretably, installed the replacement impellor backwards first time around. Same failure to prime the raw water system with anti-freeze. Corrected the impellor installation - still no priming.

The question from a mechanics perspective... should the drive shaft in the pump adapter plate turn freely if the engine is not running? That is, is that shaft engaged centrifically by the adaptor plate at idle or higher engine speed, but disengaged when the engine is not running? Adaptor plate turns as a result of starter cranking the engine over, but pump drive shaft does not rotate. What is the mechanical connection between the Pump drive adaptor plate and the pump drive shaft? If it is a geared/splined connection, then I probably have a worn out adaptor plate or pump drive shaft? Either alternative will require removal of the adaptor plate or timing cover? Ift he pump drive shaft is engaged by a centrifically operated clutch, how docan you prove, determin the clutch is not slipping at normal engine RPMs? Anyone with a similar situation or problem?

Thanks again for input.

rM
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Old 07-11-2010, 07:49   #4
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I sold my 4108 manual with the engine, but you are confusing me with the

"Adaptor plate turns as a result of starter cranking the engine over, but pump drive shaft does not rotate."

The adaptor plate I know does not rotate at all--its bolted onto the front of the engine. There is a shaft visible inside it that rotates; it is gear driven with no clutches or any other nonsense. As far as I remember, there is a slot in that rotating shaft, and the shaft of the raw water pump has a projecting flat which mates with the slot. I have seen a Perkins where the adaptor plate was not properly aligned, and the misalignment had worn off the projecting flat on the pump shaft so that the pump was no longer being turned by the engine--instead of the flat, all that was remaining was a round projection.

Does this start to make sense to you?? If so, you will need a new pump shaft, and you will need to align the adaptor plate so it doesn't happen again.


The Perkins WORKSHOP manual states, verbatim:

"Should it be necessary to remove the sea water pump adaptor plate, it must be alighned on refitting by means of the tool shown in Fig. M.4.

To line up the adaptor plate, loosen the holding nuts and insert the tool in the water pump drive position to centralize the adaptor plate to the sea water pump driving shaft. Once the tool is located, the adaptor plate securing nuts should be tightened. The tool can then be removed and the sea water pump fitted.

To cover donditions where a centralizing tools is not available, the following interim measure can be taken.

Loosen the adaptor plate securing nuts so that they just support he weight of the sea water pump.

Rotate the engine two or three turns by hand: this will ensure that the adaptor plate/sea water pump will centralize to the fuel pump gear shaft. Then retighten the aaptor plate securing nuts.

It should be remembered that the latter procedure should only be used as an interim measure as it is not as accurate an alignment when using the tool previously described."
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Old 07-11-2010, 10:07   #5
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There is a sticky thread on CF for ON LINE SHOP MANUALS that you can down load Perkin manuals.

You should not be able to turn the seawater pump by hand as it's gear driven. I checed nine as follows,

1. Sea water strainer and hose going to engine making sure it's stiff and doesn't collapse, it is a suction line

2. Heat exchanger and clean it

3. Fresh water pump, thermostat and make sure fresh water jacket is bled on the engine is bled, so you have all water in the system and no air.

Having a dual cooling system can be a pain, but it beats running salt water thru the like the old style was done.
You also might want to invest in a bigger heat exchanger..
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Old 03-09-2011, 16:31   #6
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Re: Perkins 4108 Overheating

I have the same problem. My raw water pump was not turning. I removed the pump and the shaft that drives the pump spins freely inside of the gear (same gear that drives fuel pump). What is the force that drives this shaft? Was it pressed into the gear and came loose? Is it a centrifugal thing?

I am on the boat now and need help. Please reply if you know the answer.
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Old 03-09-2011, 17:05   #7
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Re: Perkins 4108 Overheating

Read my description above. The gear has a slot, the pump shaft has a flat projection which fits into the slot. Misalignment can wear the flat projection so the gear no longer drives the pump shaft.
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Old 03-09-2011, 17:18   #8
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Re: Perkins 4108 Overheating

Mine is machined round with no slot in either the gear or the shaft. Pressed on???
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Old 03-09-2011, 17:19   #9
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Re: Perkins 4108 Overheating

So in other words; not the pump side of the shaft but rather the engine side. The shaft pulls out from the gear.
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Old 03-09-2011, 17:27   #10
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Re: Perkins 4108 Overheating

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Old 03-09-2011, 17:28   #11
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Re: Perkins 4108 Overheating

It just spins inside of the gear hole and does not drive the pump. No flat surface side or key. How did this ever work?
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Old 03-09-2011, 20:08   #12
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Re: Perkins 4108 Overheating

I guess I will plumb in a 10 psi 12v water pump and keep heading into the wind.

SW please come back!
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Old 08-09-2011, 12:56   #13
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Re: Perkins 4108 Overheating

Is there no answer to #10 and #11 on this post? It must be a mechanics secret.
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Old 08-09-2011, 13:20   #14
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Re: Perkins 4108 Overheating

I had a similar problem with my Perkins Prima 50. The spplined adaptor on both the male (engine side) and the female (pump side) became worn and eventually lead to an overheated condition. Replaced both but the alignment tool was no longer manufactured so mechanic did their best but it is always a concern now.
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Old 08-09-2011, 13:31   #15
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Re: Perkins 4108 Overheating

Could I just put a weld bead on each side of the shaft on the gear and shaft?
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