Burning up impellers

[Bradmh]

I have a 2020 Suzuki 9.9. I’ve gone through 3 water pumps since I got it last summer. Can’t figure out why. It’s getting expensive. Cavitation plate sits about even with the bottom of my jon boat. So it’s plenty deep. Never ran it out of water. The water I run in isn’t sandy or even muddy. The motor has never even touched sand or mud. I’m out of ideas. Everything is installed correctly. Suzuki paid for a shop to do the first one and blamed me. They were far from helpful. Any ideas?

[Moonlighter]

This is certainly not normal, and is also not something I have seen reported on this forum in the last 7-8 years.

So it does seems to be unique to your situation. So whether that is an installation issue or how it’s being used is the question.

So, some questions for you:


How do you flush the engine after use?

Have you EVER had a temp alarm when you are going along?

Is there always a solid stream from the telltale?

Where is the cav plate (actually its proper name is the anti ventilation plate) in relation to the water surface when the boat is planing at cruise speed?

Do you get any prop ventilation at any speed or when turning corners?

[Bradmh]

You may be on to something here. No temp alarm has come on. I do not flush the motor because they say not to use ear muffs on these 9.9’s. There is always a tell tale stream until there’s not and the impeller is melted. I’m not sure exactly where the ventilation plate is. I’ll pay attention to this. I’m not sure what to look for with prop ventilation but I’ll research it. I think you are on to something with these last two though. I’ve had the power randomly cut out a few times. I stop, turn it off, check for any blockages and then start it again and it’s fine with telltale stream coming out. This happened yesterday. Lost power, let off throttle, no stream, shut it off, I could smell the impeller was toast so I didn’t run it anymore and went home.

[Moonlighter]

Prop ventilation in turns sounds like the prop is slipping and losing grip on the water, the engine revs increase and you notice a loss of thrust. When you straighten up, the grip of the prop is regained and engine revs drop back to normal.

The AV plate is that large plate directly above the prop. You called it the cavitation plate. When setting the engine height on the transom, using the level of the AV plate with the keel is, at best, just a starting point, and a rough guide at best - because every boat hull is different and a flat bottom will be vastly different than a V hull.

The AV plate should be just skimming the water surface when the boat is travelling along comfortably at planing speeds. It should not be submerged in the solid flow of water, but neither should it be sitting well above the water level. So the best way to determine whether your engine is at the correct height is to get the boat onto the plane and look over the stern at the leg to see where it is sitting.

As you may be aware, the water pump itself is located in the leg ABOVE the height of the AV plate. So if you flush in a barrel of water, its best to have the water level sitting 3-4” above the AV plate.

As you may have gathered, I am wondering if your engine may be set too high, and may therefore be compromising water flow to the pump.

When you say that power has randomly cut out, that is a big hint that there may have been an overheating issue. You should be getting some kind of indication though, via an alarm sounding.

I am 90% sure that the 9.9 can be hooked up to the Suzuki diagnostic software (SDS) that Suzuki dealers are trained to use. This system records all events that happen that cause fault codes or that activate the engine’s fail safe system.

Do you know if the dealer connected your engine up to check for codes? If not it might be worthwhile getting that done to see if the codes stored in the ecu give direction to what has been happening with those power cutting out issues. If it does show those kinds of issues then it would also be worth finding out why you havent had alarms going off.

[Bradmh]

They didn’t hook it up. They weren’t very helpful. Neither was Suzuki. They said I had to of run it out of water.

[Moonlighter]

The only way that an impeller gets melted is when it is run without water circulating, right? I’ve never heard of a impeller being melted any other way,,perhaps if someone else is aware of how that could happen they might chime in.

It doesn’t take long, even a few seconds run dry can destroy them if the engine is at Full revs. I have seen people at the boat ramp do a “test start” of the engine with the boat still on the trailer in the carpark before launching, that is absolutely crazy and a sure way to destroy an impeller pretty fast.

So you have to figure out how that is happening, and prevent it from happening again.

[Bradmh]

The only way that an impeller gets melted is when it is run without water circulating, right? I’ve never heard of a impeller being melted any other way,,perhaps if someone else is aware of how that could happen they might chime in.

It doesn’t take long, even a few seconds run dry can destroy them if the engine is at Full revs. I have seen people at the boat ramp do a “test start” of the engine with the boat still on the trailer in the carpark before launching, that is absolutely crazy and a sure way to destroy an impeller pretty fast.

So you have to figure out how that is happening, and prevent it from happening again.

Yeah that’s why I’m stumped. I would never run it out of water. It happens with the motor in water.

[Handsomerob1980]

What kind of boat is your 9.9 on? size, hull design, draft if you know it? might help determine why you may be losing water flow at higher RPM.

[Moonlighter]

What kind of boat is your 9.9 on? size, hull design, draft if you know it? might help determine why you may be losing water flow at higher RPM.

Agreed - and the best way for us to help would be to post some photos showing the motor mounted on the transom so we can see how its positioned relative to the keel etc!

[Solarman]

One other possibility, the water inlet tube could be pulling in exhaust? If this happens, the exhaust (especially at higher rpms) pressures could be interfering with water going into the pump.

I've had this problem with larger motors, but am not very familiar with the 9.9 motor.

Good luck, post back on what you find out.

[2014DF25ARS]

Do you sit in the bow of the boat with the motor idling, like for fishing? If so, you could be lifting the motor higher out of the water than you realize. And if you start the motor while sitting in the front, or if you start the motor with the boat on your trailer when launching the boat, you could be running it dry. When I first installed my DF25A I ran it in a barrel with the water level just above the water intake grates. I let it run for about 30 seconds that way, hoping it would start to pee. A year later, my motor started overheating at idle, and I had to change the impeller. No problems since, but I am careful to never let it run dry. As Moonlighter said, the water level must be 2-3 inches above the lower horizontal plate on the foot of the motor (where you unbolt the foot from the leg) when the motor is started, in order to "prime the pump". I.e. the impeller must be underwater when you start the motor. It is not sufficient for the intake grates to be under water.