View Full Version : cooling system question

08-08-2012, 10:34 AM
Have an '84 S&S with a 351 PCM.

I have replaced the impeller every year as part of winter/dewinter without any issues before.

Yesterday I replaced the impeller in my garage (I am getting a VERY late start this year) and hooked up the intake to a garden hose and fired the motor to test. It worked just fine.

Next AM about a minute after launching on a lake that was hours away, the temp soared and bluish smoke came out of the exhaust. I removed the water pump and the impeller was totally chewed.

In previous years I have used dish soap as a lube but forgot this year.

I replaced the impeller and checked for a blockage in the intake line and found none. I fired it and the same thing happened. 2 impellers shot and the nearest one a two hour drive. The pump seems fine, nothing is leaking.

Can the thermostat freeze shut and cause an upstream blockage? BTW the intake hose was very hot all the way back to the hull.

Any thoughts would be appreciated

08-08-2012, 10:40 AM
My guess is you reassembled the water pump backwards. It is possible.

08-08-2012, 10:48 AM
That actually makes sense. How might I have done that and how do I get it right?


08-08-2012, 10:57 AM
If you went as far as disconnecting the hoses from the pump, you may have inverted the pump when you reinstalled it.

There are threads and photos on this forum that show the correct and incorrect way to install the pump.

08-08-2012, 11:15 AM
Thanks for that.
I never did disconnect the hoses from the pump, so I guess that is out.

That thread is pretty much the exact same thing that happened to me but he never seems to get a resolution.

Any thought on the question he asked about the thermostat? Is it possible to freeze shut and create a blockage fo the whole system?


08-08-2012, 06:34 PM
I still make sure the impellor is put in with a bit of lube and the correct rotation. I don't like them having to turn inside out when you first turn the key. Not much room in there! Otherwise yes, I suppose anything is possible. Take your impellor out reassemble and hook your hose straight up and see if you are getting water out the exhausts maybe, at least you will know if you are getting water through the system. Check you strainer on the bottom of your hull. Those pumps don't like restriction on the suction side. Have fun.

08-08-2012, 09:14 PM
Any critters living in your garage. I have heard of mouse nests in there, but I'm not sure how they would be able to climb across the gelcoat to get in. Worth checking though.

08-09-2012, 08:07 AM
Just wondering how you made the connection between the intake and the garden hose? If you actually make a "connection" that is fairly watertight and turn the hose on before cranking, the gardenhose can build up pressure inside the water pump. There is no bypass in these pumps or way for water to go around the impeller. The pressure of 40 to 50 lbs. or even higher on city systems will seal the vanes to the race with such force that when you start cranking each vane will be ripped from the hub as it passes the intake and gets pressureized.This can happen even with the engine at idle with a direct connection if the volume and pressure in greater than what the engine is pumping. With most of the vanes torn from the hub the water then passes thru into the system and out and all appears to be normal untill you put it in the lake and you have no pump.Again just wondering? Also wondering if you found all of the parts from the first impeller?

08-09-2012, 08:53 AM
I would bet on the pump being on backwards as well. It would take a complete lack of cooling for the boat to overheat in 1 minute and shred the impeller. The thermostat will have no effect in this instance- water will always fill the block, as well as bypass the block (RWP--> stat housing--> exhaust), regardless of whether the 'stat is in or not, or whether its open or closed. None of my PCM's have ever had a problem self priming.

The set screw on the body of the pump should be facing in towards the motor. If its facing out, then its on backwards. (This is assuming you have a standard/LH rotation engine.)

08-09-2012, 09:43 AM
2X on the thermostat not being able to cause a blockage in RWP.IMHO I don't think i've ever seen an impeller "shred" from overheating, they do get hot and wear flat spots on the bulbs on the end of the vanes even wear the bulb completely off, but that takes alot longer than a 1min. warmup,and then it's more of a dust.

08-13-2012, 01:46 AM
Hi all

Thanks for the replies.

Found the problem, I failed to get all of the impeller chips from the first failure out of the line. A bunch were stuck in the outflow side of the tranny pump, which is about four feet UPSTREAM of the impeller. when I shook all of those out, and pried a few more out with my finger and a small screwdriver and put in a new impeller, it worked fine. Am not sure how so many found their way so far up stream but they clearly did.

Thanks Byronic, the original deterioration of the first new impeller may be from too strong water pressure. I used to live in the sticks at the end of an old line and had pretty tepid pressure but now live in town with quite a bit more. That could have started the ball rolling as it were. I switched to a passive 5 gallon bucket, as I had to since there was no water tap at the lake and, after priming the line, that seems to work just fine. Will probably avoid tap in future. The connection was made directly to hose at the through fitting and I used the clamp to bite down on a brass garden hose end,