Yanmar 3YM20 Undersized water pump belt and loss of power

[msfeldt]

My 3YM20 overheated yesterday and today but only happens when at higher RPM's for several minutes. When I slow down to idle it pumps raw water again and the overheat light goes off. At higher RPM's the water flow from the exhaust slows to a trickle. I don't hear any squealing from the belt and there's not much belt dust but I suspect the belt is slipping. The belt was replaced two months ago but my mechanic told me that I may need to go to an undersized water pump belt because it was tensioned as far as it would go even though it was brand new. Yes the thru-hull is open, filter is clear, checked the impeller and it's in great shape, water comes up to the impeller very well, coolant changed recently and level is normal, oil level normal. Has anyone ever had to use an undersized water pump belt?

The other thing during the 1st overheating episode I had to give it a little gas to avoid a hazard and the RPM's wouldn't come up above 1500 in forward or reverse. The same thing happened the following day but this time the engine wasn't overheating and when I put it in reverse I was able to get the RPM's to 2500. There is no smoke from the engine. Fuel/water seperator is good and checked before leaving.

Any thoughts would be greatly appreciated.

 

[dock goody]

Unfortunately I'm not on the boat at this time so I can't look up the number of the belt, but last year I had my heat exchanger serviced by a Yanmar dealer (same engine) and they replaced this belt . I had the same issue (overheating etc.) however I didn't leave the dock because of the overheating warning. I called the Yanmar service mechanic who replaced the belt with a smaller belt and told me that the belt number in the Yanmar catalog (larger belt) was incorrect for this engine and that the smaller belt was the correct one.

 

[petersea]

Re: Yanmar 3YM20 Undersized water pump belt and loss of powe

Hate to be the bearer of bad news but if you are getting low RPM's you have other problems besides the water pump belt. 1500 rpm and even the 2500 the next day are way to low... If the engine is still under warrenty get the Yanmar people out there to fix it..

 

[msfeldt]

Thanks for the input. Glad to hear I'm not the only one with the belt issue.

 

[Steve Dion]

Re: Yanmar 3YM20 Undersized water pump belt and loss of powe

I believe you have an issue with the heat exchanger. Contact Yanmar to see if your engine has the smaller heat exchanger. If it does you may need to see if they will do anything for you. If not the new heat exchanger is about $1500-1800.

 

[Bill Roosa]

Re: Yanmar 3YM20 Undersized water pump belt and loss of powe

Also check the suction side of the raw water circuit. Collapsing hoses, plugged strainers and thru-hulls not open all the way etc.
I agree with Petersea you have something else besides an overheat problem.

Did the engine overheat warning come on or did you notice steam coming from the engine? The reason I ask is if the head gasket is blown it can pump air into the engine cooling passages in the block/head and that can over pressure the cooling water fill cap pressure relief. That lets very hot water leak out and it appears like the engine is overheating when in fact it is not. Also explains the lack of power/RPMs.
More explanation on what you are seeing when it over heats would be helpful.

 

[kloudie1]

Re: Yanmar 3YM20 Undersized water pump belt and loss of powe

Be aware also that there have been problems with the rubber impeller coming loose from the drive hub.. everything looks fine but they don't pump at higher RPM.. Nice&Easy on this forum a year or so back reported on this .. The power problem is something else.. how many barnacles on the prop?

 

[sailaway78]

I think you have another problem

My thought is that you may have other issure. First the overheating. Couple of things. The impeller issue Kloudie mentioned is correct. Last year I had a brand new impeller, with approx. 10 hrs or so on it that spun the center hub. This was not visible even when pulling the impeller. The impeller looked brand new, but only the center part was turning. As for the belt, if it's at the outer limits of it's adjustment, it's too short. A friend of mine who purchased a new 2004 Hunter 36 had problem exactly like yours, on a 3YM. The belt was too long on the new boat, and it wouldn't tighten enough. At lower RPM's it was fine, but would heat up if the engine was ran at over 1500 RPM. The loss of power, lower RPM is a completely different problem, but it is possible something causing the loss of RPM is also causing the overheating. Will this engine turn up full RPM in neutral. How many hours on the engine. Did this start gradually or was it immediate. Keep us all informed, but check the center hub of the impeller, and get the correct belt before you go other ways.

 

[msfeldt]

Excellent suggestions

Following up on your questions. The first indication of a problem was the overheat light came on 1st and I looked over the port side and saw just a trickle pumping out. When I eased back on the throttle the raw water output improved and the overheat light / buzzer turned back off.
In regards to the impeller it physically looks perfect, no visible damage and after I took the water pump cover off I put a socket wrench on the crackshaft pulley to turn the engine over and the impeller rotated normally.

It funny that "barnacles on the prop" was brought up. I went under the boat with my mask and I was blown away by how much growth I got in the last month. I went from virtually no growth to lots of small barnacles and alot of green "vegetation". I figured it would cause some resistance but you would think it would have happened right off the bat. When I left the mooring ball I was around 2500 RPM's. I didn't think about trying to get to full RPM's in neutral - excellent idea.

600 hours on the engine. It has been trouble free up until now. Was being used weekly but sat for 1 month unused until Monday.

I'd say the problem came on suddenly. I let the engine idle for 10-15 minutes, made sure raw water was pumping out good then headed out of the inlet at around 2500 RPM's, after getting 3/4 of the way out all heck broke loose.

I really think it has to be that the belt starts to slip at higher RPM's. I'm going to try to get a shorter belt tomorrow and try it out. As far as the loss of power I'm a bit more concerned. I hope the overheating (as short as it was) didn't cause any permanent damage. I'll scrape the prop clean, check for anything wrapped around the shaft, and check the throttle cable. If I'm still having the problem I'll get a mechanic on board to get things straightened out.

I can't say thanks enough for all your help.

 

[petersea]

Re: Yanmar 3YM20 Undersized water pump belt and loss of powe

My suggestion is to replace both the impeller (even if it looks "good") and belt (with correct size). This should cure the water pump problem, if not you will need to do further diagnosis.
As far as permanent damage to the engine due to overheating it is unlikely based on your description and the buzzer is designed to come on at 205 degrees. 240 or more is when you will start to warp heads etc etc. Keep us updated on what you find and what the"fix" was..............

 

[msfeldt]

thanks

Sounds like a good plan.

 

[sea_casper]

Re: Yanmar 3YM20 Undersized water pump belt and loss of powe

First clean the strainer on the inlet near the through hull valve, next check for growth near or on the inlet screen on the bottom of the boat, haul out or diver required for that.

I once had an overheat at low power, and after checking everything, gave the nearest hose a squeeze, and "burp" the problem was solved, must have been airlocked, it has never happened again.

What is the correct belt? I have put on a new belt, and after the first adjustment after the initial install, it is at the end of its adjustment range.

 

[msfeldt]

Latest update on belt

Went to Complete Yacht in Fort Lauderdale today to address the belt issue with the owner and their Yanmar mechanic of many years. There are two belt sizes available for the 3YM20 depending on serial # of the engine, 19" and 20". Unfortunately I've already got the 19" and they don't make anything smaller. They called Yanmar which didn't have an answer as well. The Yanmar mechanic suggested trying to find an 18.5" belt at local auto parts stores, etc. The closest I could find was 18" but I think it may be too small to get on. My other thought which I hate to do is take my dremel and lengthen the length of the slot on the water pump bracket so I can slide it over a little further to put more tension on the belt. I just don't remember how much "extra metal" there is on the one side of the slot...probably not much.

Unfortunately I can't get to the boat now til Sunday, we will see how it goes and I'm going to don the SCUBA gear and clean the entire bottom and prop.

As frustrating as this is (and I am pulling my hair out), one of the great joys/frustrations of owning a boat is having a seemingly insurmountable problem and figuring it out. That said, if this gets too expensive I'm taking the Lin and Larry Pardey approach and going engineless.

 

[msfeldt]

Latest update

Spent Sunday and Monday on the boat. Cleaned all the barnacles off the prop and bottom. Found a small piece of fishing line wrapped around the prop. The belt mystery is over. My receipt shows a 19" belt but when I got on board it's a 20" belt that was installed. Luckily my spare was a 19". Swapped out the two belts, no more problems with overheating, and she comes up RPM's no problem. Likely the belt slipping caused the overheating and the dirty prop and fishing line caused excess resistance preventing coming up to speed. Thanks again for everyones help.