Perkins 4-108 Corrosion on new heat exchanger

[MikeNZ]

Hi Guys

I rebuilt my 4-108 about 3 seasons ago and all has been well up till recently (apart from a little white smoke - but that's another story and it's been there and unchanged since the rebuild). I've done about 700 hours since the rebuild.

First recent problem was the fresh water pump, which started leaking on a 4 day passage. Started as fine mist then progressed to all-out spay. Replaced the water pump a week back. All seems OK with that and this may not be related at all to the main problem.

However, while I was installing the new pump I noticed a small drip under the elbow hose that goes from the engine head to the heat exchanger (leaking my green 50:50 fresh water/anti-freeze mix)......but when I removed the hose to clean it and gunk it back up I found pretty extensive corrosion on the alloy hose fitting on the heat exchanger. No corrosion on the steel hose fitting on the head, though.

To make sure I did a thorough job of the rebuild I replaced the original heat exchanger with new when I did the rebuild - it too was extensively corroded! I'm shocked that the new one is corroding as it cost a small fortune to get one down here in NZ ($3,000 odd) so I need to find out why.

Only thing I have changed of any significance that I can think of is the hot water cylinder which tees off the fresh water side of the cooling system. The previous hot water cylinder lasted 25 years which I thought was OK.

Maybe the original heat exchanger corroded for the same reason and I have just bought myself more time?

As many of you know there is no anode in the heat exchanger and no place for one I can see. There are no anodes on the engine either as far as I know. The engine is a 1986 4-108 NEW model.

I think it must be electrolytic corrosion (a "battery" effect that requires a current and water). What do you think?

Anyone got any ideas?

Thanks

Mike

[S/V_Surya]

My 4107 does have an anode in the heat exchanger. I change it every year cause it needs it, and I am in mostly fresh water. There is a bolt on the underside back of the engine of the exchanger. I looked at my parts manual for the 4107 and 4108 and it shows it also. Also, be careful. I believe the ones you purchase need to be cut back a half an inch or so because they are to long.

[Bermuda Forte]

My '86 4-108 has no zincs on the heat exchanger. I understand that the rubber end cap/mounts prevent dissimilar metal corrosion that might occur otherwise. I'm not too sure.

[onestepcsy37]

my 1979 4-108 low line model has a zinc in the heat exchanger. it's a bolthead on the side of the exchanger. back it out and replace it. i also had to cut down the zinc because the only one i could find with the correct thread was a bit too long. when i bought a new heat exchanger as a spare it also had the zinc in it.

not sure about the rubber hose story; all four hoses connecting to my heat exchanger are rubber.

one possiblity is the two straps that hold the heat exchanger to the engine. i don't know an awful lot about corrosion but i kinow the engine block is cast iron, the straps are stainless steel, and the heat exchanger is brass...

[bobconnie]

Every Perkins heat exchanger Ive ever seen has a zink on or in it! either as said above on the back, a screw in zink or somewhere else on it ! ya need a shop manual for your model number engine and it will show ya where it's at !! Im sure somebody on here has one or knows where to find one on line ! this will help ya a bunch with this and other problems you will have in the future ! Just a thought

[Bermuda Forte]

http://www.cruisersforum.com/forums/...d-60015-2.html

Here's the thread I read about rubber & zincs.

[MikeNZ]

Thanks for the ideas. My 4-108 is the 4-108 "NEW" variant, and it has a cast aluminium Bowman unit which is a combined heat exchanger, exhaust manifold, and header tank. It has rubber end caps like you described, Bermuda Forte.

I can't see any tapped anode holders anywhere on it and I've read on several sites that this unit doesn't have them. My hunch is that this a design fault and it needs some anodic protection. Has anyone tapped a zinc holder into their Bowman heat exchanger?

The alloy casting is about 6mm thick so I think it's a pretty easy job to add one on the fresh water side, and one on the salt water side. What do you think?

[DeepFrz]

If I were doing it I would identify the heat exchanger and then contact Bowman for help.

[mdvick]

Originally Posted by DeepFrz

If I were doing it I would identify the heat exchanger and then contact Bowman for help.

+1. I agree.

[lancelot9898]

FWIW, I have a 1985 "New" model 4-108 and years ago I asked a Perkins mechanic about the location of that elusive zinc. He told me there isn't one nor is there a need for one due to the isolation from the rest of the engine with those rubber end caps. I've had no problems over the years with electrolis although knock on wood. It's been at least 5 years since I've removed that heat exchanger element to clean out with a muratic acid bath.

[NCboatrx]

The Bowman heat exchanger with the rubber end caps as used on the later model 4.108 has no zinc anodes and is usually a very long lived piece of equipment.

Your issue must be caused by some electrical problem. If you have multiple ground (earth) connections on the engine, join them together before the engine and have only one negative cable at the starter. Each ground connection is a potential corrosion spot.

Check very carefully for any electrical leaks from items such as the VHF and other 12V equipment.

Do not connect the engine to the boat's bonding system, isolate it and if you have space, fit a flexible coupling (like the R&D ones) to break the electrical connection to the shaft. Carefully check all engine wiring.

Stanley

[zeehag]

so the model with heat exchanger built into reservoir for fresh water and antifreeze is on newer engines?? mine is like that-- what years did they start making them this way?

[MikeNZ]

Thanks again, good people....

I've contacted Bowman. They have confirmed there are no zincs needed. They have asked for photos.

I'm chasing down all possibilities of stray currents, and the ideas you have suggested NCboatrx are top on my list. I have a flexible coupling already, BTW, but there's a good chance I have a grounding issue to sort out.

Cheers, and thanks to you all.
What an amazing resource we have here on Cruisers!

Mike

[btrayfors]

Mike,

FWIW, my 1981 4-108 has a pencil zinc in the heat exchanger, and when you replace it you have to cut off the new one to fit -- as noted above.

However, aside from the zinc problem, the heat exchangers on the 4-108 are undersized, especially if you operate in tropic waters. I found this out a long time ago, and I replaced mine with a new larger heat exchanger from a 4-236. Only had to do minor modifications to make it fit. Result: NO overheating problems since I did that some 15 years and 15,000 miles ago.

I had struggled with overheating problems for 7 years before that modification, replacing the standard 4-108 exchanger, doing the muriatic acid thingy, etc., etc.

Bill

[MikeNZ]

Bill

The whole issue of the 4-108 is VERY interesting and I have studied hard from forums all around the place.

My conclusions from my own experience:

* The 4-108 is only a 40HP engine, at best, in typical marine use (some say 35HP)
* A useful continuous max RPM is 3000 x 90% = 2,700RPM
* They are prone to overheating at higher loads even with well-maintained cooling systems
* Mine likes to run at around 2,100 - 2,300 rpm which is well below recommended RPM - anything more and she's just too noisy and rough
* I burn 2/3 gal (4.5 litres = 1 gal here in NZ) per hour at 2,200rpm
* At 2,200 rpm and a clean bottom and prop the engine temp sits at 80 Degrees C
* Anything over 2,500 and the temp rises rapidly to over 90 Degrees C

Mike