Impeller keeps locking up 2000 21V Rider's Edition

[Pedalmash]

Within the past year, the raw water pump impeller on my 2000 21V has locked up at 4 random times. It only locks up upon starting up the boat while in the water. I have to take the serpentine belt off and manually unfreeze the pump with two of us turning the pulley by hand (it literally takes two of us). Once we unlock the pump, the boat runs fine and there are no cooling issues (there's no cooling issues prior to locking up either). I get home and pull the impeller and it's perfectly fine. I can put the impeller in the housing and turn the pulley by hand just fine. All gauges are normal. Boat runs fine. I've replaced the impeller housing along with multiple impellers to see if this helps, but the impellers look almost new each time I pull them out. I've installed a new serpentine belt. I've checked the trans cooler and it's clear. The boat runs fine on the fake-lake with water flowing nicely out of the exhaust. Water flows out of the blue plugs when checked. This problem is driving me nuts because we can never tell when it's going to happen. Doesn't matter how long we've been on the lake. Anyone have any insight? Any help or even sarcasm would be appreciated at this point. Are the impeller veins flipping somehow when I shut off the motor??? Do I not get my serpentine belt tight enough??? I use a big bar to get that top pulley tight for the belt.

[chpthril]

Are the impeller veins flipping somehow when I shut off the motor???

yes, this is a very good chance this is whats happening. The Merc engine were known for this happening. When the engine comes to a stop, a piston approaching TDC, can be forced back down a little. This results in the engine rotating backwards just enough to flip the impeller. When the boat is restarted, the belt wont flip the impeller, just spins on the pulley.

[freeheel4life]

The other one that happens is the bearing on the pulley shaft starts to go out. Typically you can grab the the pulley and it will have endplay(moves in and out from housing).

[Pedalmash]

Yes, I’ve wondered about the pulley bearings as well, but they’re smooth and the shaft has maybe a gnat’s *** of play, maybe. No in-n-out.

Today I back-flushed all hose lines, except the ones to the manifolds, and no debris was found.

The more time I spend in the bilge, the more I think I’m headed for some long over due boat rehab this fall.


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[NICKYPOO]

This has happened to mine a few times. I still carry a wrench with me in case it does. I can actually turn the wrench with one hand while cranking the boat with the other. Yes, it’s sketchy but it works. I haven’t had it happen in a long time though. I think at least ever since I started revving the motor upon shut down just to keep the fuel system pressurized. Perhaps that helps with the impeller flip issue? Maybe I’ve just been lucky? IDK. It’s certainly worth a try.

[SONIC]

Probably worth replacing the impeller and housing if it's happening often. The housing is plastic and can wear giving you a surface with more friction than necessary, and an old stiff impeller certainly won't help either.

[Pedalmash]

yes, this is a very good chance this is whats happening. The Merc engine were known for this happening. When the engine comes to a stop, a piston approaching TDC, can be forced back down a little. This results in the engine rotating backwards just enough to flip the impeller. When the boat is restarted, the belt wont flip the impeller, just spins on the pulley.

Is there a solution for this? The boat has done this twice since I posted this. I simply free up the impeller by hand and the boat runs fine the rest of the day.


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[Pedalmash]

Have you replaced the housing?

Yep, it’s brand new.


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[freeheel4life]

So you do have the single 5 rib serpentine belt being a 2000 ya?? I ask because some older mercruisers ran dual v belts and had a standalone belt off the crank pulley for the RWP...

[Pedalmash]

So you do have the single 5 rib serpentine belt being a 2000 ya?? I ask because some older mercruisers ran dual v belts and had a standalone belt off the crank pulley for the RWP...

Yes, it’s a ribbed belt and it’s almost new. I’ve wondered about a different belt setup.


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[Pedalmash]

So after talking to some boat mechanics in Havasu today, the third one was familiar with impeller reversing issue and said it’s usually due to the timing being off. He said he’s seen Mercs before that reverse a whole 2 revolutions when shutting off the motor. He said 1 or 2 degrees is all it takes. I’ll post up more after July 8th when he works on it.


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[Pedalmash]

Now that makes sense shortly after I did my impeller it went to a friends shop he has the laptop to look at everything ( was just a general check-up) the boat was new to me. I had replaced the TPS. IAC, MAP and removed TB and cleaned. we might have corrected the timing also. I can't remember. I was just getting ahead of problems that might keep me off the water. The timing thing makes sense. Keeping my fingers crossed for you.

Took the boat to my favorite shop, Fallon Marine. They found the timing off 2-3* btdc. They advanced it 8* btdc. They stopped and started it a dozen times to make sure there was no more locking up. They also found that from the serpentine belt locking up before that there was black belt dust in the flame arrestor and found the throttle body caked with the black debris. Got it all cleaned up. We just got off the lake and she purred today. Was perfect for the wife’s birthday.