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Catalina/Capri 25/250 Sailor's Forums

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Outboard cooling water pressure drop

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tapinkham
1st Mate

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USA
36 Posts

Initially Posted - 07/07/2002 : 20:07:23

Hi,

I have noticed a distinct drop in the water pressure from my '98 Yamaha. I am in saltwater but this is only the third summer I have used this outboard.
This has been a fairly sudden development.
Any ideas?

Thanks,
Tim

Edited by - on

Leon Sisson
Master Marine Consultant

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USA
1893 Posts

Response Posted - 07/07/2002 : 22:33:31

Tim,
<BLOCKQUOTE id=quote><font size=1 face="Verdana, Arial, Helvetica" id=quote>quote:<hr height=1 noshade id=quote>... distinct drop in the water pressure from my '98 Yamaha. I am in saltwater ...<hr height=1 noshade id=quote></BLOCKQUOTE id=quote></font id=quote><font face="Verdana, Arial, Helvetica" size=2 id=quote>Funny you should mention that. I have a 1992 Yamaha 9.9HP 4-stroke long shaft I use in saltwater, and have recently experienced a similar problem.

The past few times I used the motor, I noticed that upon start up, the reassuring telltale stream of cooling water didn't appear until after I'd revved the motor once or twice (which I don't like doing to a cold motor). After that, the cooling system seemed to work normally until the next start up. This problem seemed to be getting worse, So I investigated.

First, I checked for mud dauber wasps in the tell tale tube, but didn't find any this time. Next, I opened the thermostat housing and started the motor. I noticed an interesting thing I still can't explain. With the engine idling, instead of water pouring out at the thermostat, exhaust gases came out (more below).

After some head scratching and shoulder shrugging, I pulled the motor, planning to replace the water pump. With the motor on a workstand, I poured water down the thermostat opening on top of the engine. The dry cooling system eventually filled, and then began to slowly drain down. The water seemed to be leaking out only at the cooling water intake screens, indicating it a bad pump impellor.

I removed the lower unit from the leg (a sickening tale of burning torches, poisonous chemicals, repeated extreme violence, and very course language). I tested the pump by submerging the lower unit up to a bit above the cavitation plate in a garbage can of water and spinning the drive shaft (in the correct direction, clockwise from above) with a 1/2" drill motor. The 2-speed drill has a max speed of about 1,000RPM in low gear, and 2,500RPM in high. I did most of my testing in low gear, as the problem seemed to be at low engine speeds.

The only two ways I could get the pump to prime (below 1,000RPM) were to either submerge the lower unit all the way up to the base of the pump (top of foot casting), or to splash a teaspoon of water down into the pump discharge port. In high gear (2,500RPM) the pump would self-prime if the cavitation plate was submerged at least 2" or so. Once primed, the pump would put out water down to maybe 200 or 300RPM. If allowed to turn slower or stop, the pump seemed to drain back and lose prime, and wouldn't pump water until splashed or spun up to 2,000RPM or so.

I dissassembled the water pump, but didn't see any reason for this odd behavior. One impellor blade was slightly cracked where it joined the hub, but didn't look bad enough to affect performance yet (although that certainly justified replacing it). Next I tried a spare healthy water pump assembly from my parts doner motor. It worked a tiny bit better, but didn't cure the problem. Neither of these used pumps looked particularly bad inside. I've seem much uglier pumps work for years on american brands.

Shrugging again, I bought a new water pump kit from the Yamaha dealer ($30). While at the dealer, I showed their mechanic my lower unit, described the no water at low speed problem, and asked if he'd ever run into that. He shrugged too, and said no. He suggested that maybe I wasn't running the motor deep enough. I am, the cavitation plate is well submerged when the problem occurs (both on the boat and in the trashcan). We both visually inspected the water pickup passage below the pump (because of exhaust gases noted above) for casting porosities, corrosion pinholes, cracks, gaps, etc. None were observed. There was no water in the gear case oil.

After very carefully installing the new water pump kit, and retesting in the trashcan, the losing prime problem is still there, but only at a very low RPM. So low, that I don't think the motor could run that slow.

Until further enlightenment occurs, I'm going to say that maybe this low water flow thing is a characteristic of this model, and that they are real picky about changing the water pump frequently. I'm not real happy about that. Thirty dollars isn't too bad, but beating that lower unit apart every year or two isn't my idea of good for the motor, or a fun time.

I'm going to put mine back on the boat, but not right away. First I'm going to replace the thermostat and control cables, and give it an oil change and tune-up while I've got it off the boat.

By the way, the factory shop manual pretty much sucks. It has vague and insulting instructions like, "if the part is damaged replace it", without defining "damaged". There are an annoying number of Japanese-to-English translation screw-ups, nameless parts, etc. It doesn't actually tell how to get anything apart that isn't eager to come apart. About all it's good for is torque values and the exploded drawings, like in a parts book.

Let me know what you figure out. Any one else have any insights?

-- Leon Sisson

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OJ
Master Marine Consultant

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USA
4382 Posts

Response Posted - 07/08/2002 : 10:07:38

I just picked up my 1995 Yamaha 9.9 4-stroke from the shop for the same problem - impeller was in perfect condition. They said there was blockage between the water pump and the power head. They suspected a nest of some kind and suggested flushing the motor every spring and fall. Flush kit was ($14.99.)

Steve Madsen
#2428
OJ (Ode to Joy)

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