Yanma cooling / impellor issues

[Rob_Webb]

Yanma 3GM, approx 15 yrs old, reckon approx 1500 hrs total time. Salt water cooled. Saildrive.

I replace my impellor every year but I also tend to lose it once mid-season requiring an emergency replace - which I've got kind of used to.

So last week's replacemetn was kind of expected (first for the year) but then yesterday same thing happened i.e. CW temp alarm went off, checked exhaust, yup no cooling water coming through, shut down, replace impellor and restart - all OK again.

But both last week and this week the 'faulty' impellor I removed had no visible signs of damage. It's a saildrive hence cooling water drawn through the leg with (I am told) a filter system in the leg but no other filter in the line. Sio I wondered if some debris had made it through the saildrive leg filter system and buggered the impellor. But as I said, both replacements in the last week had no visible signs of wear or damage.

So at the point that the engine overheats and no cooling water is coming through the exhaust, the impellor would apparently still be spinning fine i.e. trying to pump water through the engine.

Anyone else had any experience of this?

I am now thinking that maybe the engine cooling galleries have scaled up with 15yrs of hot salt water running through them and maybe some chunks of scale are rattling around, intermittently blocking the water flow channel resulting in the overheat. But then when I shut down and change the impellor (which is a red herring and not the actual cause) things kind of 'settle' and the blockage temporarily clears until the next time - possibly only a week later now.

So do I need some kind of acid flush through to clear the internal cooling channels?

Or am I way off track and is there another possibility?

Cheers

 

[savageseadog]

There is no built in filter in the SD20 leg, if yours is the same as ours there is a brass valve out the side of the leg from that a tube running directly to the raw water pump.

We have had a small eel about 6" long and a very similar diameter to the pipe sucked up and trapped in the impeller. The shock of finding it lead to be being dropped into the engine bilge. Lucky this happened while in the berth... Needless to say no cooling water came through, but it was just before the start of a long offshore race.. Eel was still alive and lively so realised back into the marina (right next to the competition!).

Never had any impeller issues but we carry a whole new spare raw water pump which in my reckoning could be swapped over in less than a min if pushed! As long as the tool kits are where they should be!

I have herd that impellers can look ok but the bonding between the impeller and the centre drive can go so check that. Other than that check the thermostat some people don't realise raw water cooled engines have these, if in doubt throw it out its pretty cheap, have a look at the scaling in the head by extracting the head and block anodes.

We have had issues with scaling up within the rubber tubes, if you squeeze them and hear cracking noises take them off and de-scale them separately.

The raw water cooled GM's are a really simple cooling system. I believe the cooling channels in the GM are in fact made bigger because of its raw water use and its a specifically designed marine engine.

If you want a workshop diagram for the 3GM/3gm30c cooling systems PM with your email addy

 

[Burnham_Bob]

Just a thought - you're not carrying anything round on the saildrive leg are you? My yanmar overheated when we got a plastic bag stuck over the inlet. Turn off and relieve the suction and it fell off. Just a chance you are carrying something flapping around on the leg.

And if i remember rightly a few years ago there was a dud batch of yanmar impellors that were loose on the brass boss. The boss went round but the blades didn't. Have you had a close look at the replaced impellors? And of course if one of the impellors broke there could be a rubber bit in the system.

Good luck finding the fault. If you are in a marina why not try using a hose and running fresh water - not salt- through the water intake.

 

[pvb]

If the intake pipe to the pump is rubber, one possibility - vague but still worth checking - is that the pipe may have delaminated internally. If this has happened, it's possible for the inner skin to collapse under suction and block the water flow. It could happen randomly. After 15 years, might be worth fitting a new bit of pipe and seeing whether the problem goes away.

 

[Tranona]

Suggest you systematically work through the cooling system from the pump onwards to detect if there is a blockage, including the thermostat.

The impeller failures (very much in the past) were with the 1GM which has a very different pump from yours.

 

[ParaHandy]

I had exactly the same problem. I knew something was wrong a few years back when, after changing the impellor, there was little or no cooling water flow. When I put the old impellor back, the flow was restored. The reason was the difference in flank width of the impellor; the old one was 0.35mm bigger. I raised a stink and was told that the impellors were "in spec" ... its obvious that a low width will cause the pump to stop working.

Anyway, a few years later, a perfectly good impellor (I change it every year) failed. The flank width was also, by my thinking, low. Very shortly afterwards and having measured the groove in the waterpump front cover, I decided that too much wear had occurred so I polished the outer face with fine emery and re-fitted the pump with the cover reversed. I also replaced the shoe which is easily and cheaply done.

So far no problems .... hope that helps

 

[charles_reed]

Re: Yanmar cooling / impellor issues

I'm afraid it's not uncommon, the bronze centre bush has come unbonded from the impeller material.

Check you've got the paper gasket between body of pump and the faceplate. If its not there the impeller will be jamming.

 

[ianwoods]

Hello, There is a possibility that there has the growth of little molluscs inside the leg which handicapped water flow. On my saildrive I noticed small mussels inside the grid on the leg - just too big to dig out so had to smash them up with a thin rod. A cooling problem occurred last year plastic bag on the leg/ loose mollusc? Found no problem with the Globe impeller I was using but replaced it anyway. Check your leg on liftout.

 

[Rob_Webb]

I manage to check the saildrive leg every weekend when I swim off the boat (see pic below showing the state of our warm and clear water right now!) and whilst I always manage to scrape away a few wisps of growth I can't get up into the depths of the saildrive without dismantling it, so that would have to be on liftout. But the natural flow through the intake pipe from the saildrive leg appears to be OK when I disconnect it and drop it below sea level, if not exactly a torrent.

I'm going to start with the delaminating/collapsing hose theory and then work towards an acid flush of the engine itself. Also double-check the (new) impellor size and gasket seal.

Then onto a replacement (larger) water pump which I might need to gear down to run slower so as not to overcool the engine. But that's stage 2 of the investigation.

Thanks. Keep you posted.

 

[Rob_Webb]

Update.

Engineers been crawling all over it the last 24hrs.

Water pump and impellers (including the 'faulty' ones I removed) all seem OK. But they left me with 3 new ones just in case.

THey opened up the engine cooling pipes and the exhaust elbow and found a lot of build up "a potential blockage". Scraped out all the [--word removed--], flushed through with acid rinse and reassembled.

Running on the berth all seems fine (but then it always does).

The water flow out of the exhaust certainly seems a bit stronger (I'd guess 25% more flow purely on observation) and the flow seems different, a bit 'chunkier' rather than thin spray.

And the hoses all seem fine, they ran out of time to replace them today but have left me with spares for the weekend (just in case) and will replace hoses next week.

So provisionally I've got to take this as progress of sorts and give it another go.

But over the winter I will (1) lift out, dismantle the saildrive leg and have good cleanout and (2) replace the water pump with a newer generation version that uses a bigger impeller and a more robust drive mechanism than that horrible little pin thingy.

A friend with the same engine said he went 7yrs with the same impeller!

Let you know how it goes.