6hp, Very strange water pump problem

[RPJS]

Model 6BA79E

I have fitted a new impeller, the pump body and base plate were both in very good condition.

When started the motor pumps little or no water and overheats quickly. If I remove the plugs and turn the motor over with a hand drill it pumps loads of water, replace the plugs and try again it stops pumping. The water level in the tank is well above the pump, there are no blockages in the water pipe or passages.

My first thought was a head gasket leak allowing cylinder compression to pass into the water jacket, I pulled the head fitted a new gasket skimed the head 0.005" reassembled and still had the same problem. The compression is OK 105/99.
I have run compressd air into both cylinders at 70psi and found no noticable leaks.
I think I need to be looking elsewhere for the problem but I'm not certain where to start, anyone got any ideas?

Thanks

 

[OBJ]

Re: 6hp, Very strange water pump problem

Have you replaced the t-stat RP?

 

[RPJS]

Re: 6hp, Very strange water pump problem

I've tried running it without the t-stat in and it made no differance but I have not fitted a new one, I did test the old stat in hot water and it opened OK, also tried running with the stat cover off, again plenty of water with the plugs out and almost non with the plugs in.

Could pressure in the exhaust system be leaking into the water jacket??

 

[OBJ]

Re: 6hp, Very strange water pump problem

I would ask how "hot" is the block getting? After idling for a while, can you put your fingers on the flat area on top of the block for several seconds?

The exhaust cover has two gaskets. Possible the outer gasket has blown and allowing exhaust gas into the water circuit......the exhaust cover (outter) should be fairly cool to the touch with the engine running.

Did you notice any heavy deposits when you took the head off in the water passages?

 

[RPJS]

Re: 6hp, Very strange water pump problem

The block is getting very hot, far to hot to touch.

I have just removed the powerhead and pulled the exhaust cover, it's hard to tell if the gasket had blown but I have 2 new ones, the thin plate that seperates the exhaust from the water is quite badly buckled so I will make a new one. However the seal area looks OK.
The exhaust cover was very hot, water splashed on to it hissed.

All of the water passages are very clean the motor has only ever been used in fresh water.
I will make a new plate tomorrow and get it all back together and see what happens.

Thanks for the help OBJ, I'll let you know how I get on

 

[OBJ]

Re: 6hp, Very strange water pump problem

While you have the block off, check the rubber grommet at the top of the water tube where it meets the block. Make sure it hasn't mushed over restricting water flow to the block.

 

[RPJS]

Re: 6hp, Very strange water pump problem

There is definatly no blockage in the water pipe.

I hooked the hand drill up to the drive shaft and spun it up, a spout of water hit me right in the face, if you've ever seen a cartoon of some fool looking down the end of a hose pipe you will get some idea of what I must have looked like.
Even more stupid with a 240volt drill in my hand.

I won't do that again in a hurry.

 

[RPJS]

Re: 6hp, Very strange water pump problem (solved)

Re: 6hp, Very strange water pump problem (solved)

Found the problem.

The ally casting under the pump has a hole corroded through into the exhaust chamber, with the plugs out no air was passing down the exhaust, put the plugs in and the air being forced through the exhaust was being drawn into the pump causing it to lose pressure.

 

[OBJ]

Re: 6hp, Very strange water pump problem

Glad you found the problem RP....thanks for the come back on the problem's fix.