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Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 14405
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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2006 Yamaha F250 4 stroke. Knocking sound coming from what

Customer Question

2006 Yamaha F250 4 stroke. Knocking sound coming from what sounds like the lower unit. Water coming out the tell tale, but overheat light comes on at idle.
JA: Sometimes things that you think will be really complicated end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so the Marine Mechanic can help you best.
Customer: It seems that there is something "loose" in teh lower unit as it makes a knocking sound. It knocks more with more RPM's but does not seem fast enough to be engine related. Somebody mentioned water pump impellor pin may have come out?
JA: Sometimes boat issues seem like they'll be really complicated, but end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Is there anything else the Marine Mechanic should be aware of?
Customer: Nope
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Submitted: 8 months ago.
Category: Boat
Expert:  Jason replied 8 months ago.
Hello my name is ***** ***** am I speaking with? Do you hear this knocking only in neutral, only in gear, or both? When was the last time the cooling system on the outboard was serviced with a new impeller and thermostats? When was the last time the gear lube was changed?
Customer: replied 8 months ago.
My name is***** can maybe hear a very slight knock at neutral, but a definite knock when in gear. It increases with RPM's but not nearly as fast as the rotation of the motor, so I do not think that it is powerhead related
Customer: replied 8 months ago.
I purchased the boat a year ago, and this service work had been done the fall before. I purchased it from a boat dealer that sells Yamaha's
Customer: replied 8 months ago.
I am not looking to repair it myself as much as just to educate myself before calling one of the local mechanics.
Expert:  Jason replied 8 months ago.
Hi Jeff, With any overheat at idle only there are 2 places to look. The first is the impeller, the 2nd is the water pressure control valve. Starting with the impeller, when they get old they will lose their ability to pump at lower rpms. If sand has ever run through the pump, that will prematurely wear it out, and cause it to loose ability to pump at lower rpms as well. On the water pressure control valve.... The way that works is on a properly working system, the impeller generates more water than the engine needs when the engine is at 1/2 throttle or higher. As the rpms climb, so does water pressure. After it gets to a certain point the water pressure control valve opens and dumps the excess water pressure directly down and into the exhaust. When the valves get dirty, what happens is they will stick open. When they stick open there is not enough water pressure in the block to cool it at lower rpms. The knocking is a little more complicated. With that one the only way to tell what is causing the knocking is to get a mechanics stethoscope and listen to the engine block, the middle section, and down on the lower unit, and try to pinpoint the source of the knocking, once the source is located that area can be taken apart to see what is causing it. Does that all make sense to you?
Customer: replied 8 months ago.
I understand the impellor and how it works as I have replaced many of them over the years. The knocking sound is probably my greater concern. I am not familiar with the Yamaha, though. Somebody had mentioned to me a water pump impeller pin?
Customer: replied 8 months ago.
pleaty of sand has gone through the system. We are is southwest Florida and most of our fishing is in shallow water
Expert:  Jason replied 8 months ago.
I'm in SWFL as well. The water pressure control valve being loaded with sand and being stuck open will cause the hot running at idle as well. The water pump impeller pin would not cause a knocking sound. If that was missing/broken the impeller would not spin at all, and the engine would not pump any water. The only thing that stands out that would cause a knock in the lower unit is if the driveshaft bushing was worn out. The there a shaft that runs out of the top of the lower unit and it connects to the crankshaft of the engine. This drive shaft is 20, 25 or 30 inches long depending on the shaft length of your outboard. If the driveshaft bushing is worn out there will be some knocking/slapping nosies coming from the middle section. The noise would be caused by the driveshaft slapping the bushing, because there is to much space/clearance between the two. Did you have any other questions on the issue?
Expert:  Jason replied 8 months ago.
I think that should do it. But I work for tips so I do want to make sure you are happy with my service before you go. If you had a further question on the issue by all means feel free to ask. If not, Just in case you do not understand the way the website works (and some folks do not). You do have to put forth a positive rating in order for it to credit me for helping you. When ratings are not done, the website simply keeps your deposit and they will not credit me. The ratings box is located at the top of the screen. To rate, you must select the star you wish and also confirm it. Please let me know if you run into any problems or errors when trying to do it. If you do have a problem, or if you can not see the ratings box which is at the top of the screen. Please reply back "I rate Jason's service _______" and fill in the blank. Questions do not close out, so if you have to come back later on with follow up questions to the issue you still can even after doing a rating.Thank youJason
Expert:  Jason replied 8 months ago.
Was there anything else Jeff?
Customer: replied 8 months ago.
At idle, water does come out of the tell tale.
Expert:  Jason replied 8 months ago.
I don't quite understand, is that a question or a statement?
Customer: replied 8 months ago.
I guess that it was more of a statement. A detail that I did not remember telling you up front. It seems that the water flow through the tell tale is normal, but the overheat light still comes on.
Expert:  Jason replied 8 months ago.
You did mention that in your initial question, and I did catch it. In your initial question you wrote ". Knocking sound coming from what sounds like the lower unit. Water coming out the tell tale, but overheat light comes on at idle." So I did catch it. The water coming out of the tell tale is only there to tell you that the impeller is pumping somewhat. What you have to keep in mind is when the engine runs 1% of the cooling water exits the engine via the tell tale. The other 99% of the cooling water comes out down and out of the exhaust underwater, where you can not see it. If the impeller was so worn it was only pumping at 1/2 capacity you would still see a stream out of that tell tale. The same thing with the water pressure control valve, if that was stuck open you would still see a strong stream. Did you have any other questions on the issue? If yes, did you want to do it over the phone? It would go a lot faster for the both of us.
Expert:  Jason replied 8 months ago.
It looks like you read my answer without replying back or leaving a rating for my service. Please keep in mind the website does charge you to speak with me. But they will hold your money and not credit me for helping you until you select a rating for me. Thanks again,Jason

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