No water flow AFTER replacing impeller

[pdecker]

I have a late model raw water cooled A4. I replaced the impeller for the first time in three years. The old one was in good condition, with no broken vanes. Coolant flow and temperature has been fine for the whole three years.

I'm doing the ICW to Florida in a few weeks, so I replaced the impeller. A couple weeks ago, I pulled the boat out, painted the bottom, and made sure the intake strainer was clear. Now, I get almost zero water flow and the temperature quickly goes up to 190 F once I start the engine.

I removed the water pump cover plate and removed to make sure it was installed correctly. Everything seems to be installed correctly. I started the engine again, got a dribble of water, and then nothing, following by high temperature. What could I have done? What could it be? I'm thinking blockage somewhere, but how and where?

[ndutton]

Do you still have the old impeller on hand? If yes, compare the thickness to the new impeller. Unless the impeller fits against the faceplate and the back of the pump cavity its performance can be reduced to nearly zero.

[JOHN COOKSON]

1. Remove the new impeller and compare the thickness to the old impeller.
2. Reinstall the old impeller.
3. Install the hose on the intake and pour some water down it then turn the engine a bit to prime the pump.
4. Raise your right hand and read out loud: "Don't fix it if it isn't broken".
5. Have a good day and a great cruise.

TRUE GRIT

[BunnyPlanet169]

Recent experience

I've been through this recently.... It's pretty simple to at least localize the problem. For each step, reconnect the last good step until the blockage shows itself...

Thru hull valve to pump? Take the hose off the pump and see if there is clear flow. This should be an alarming amount of water even in a 3/4" hose. Air pockets before the pump can cause more problems than you would imagine.

Pump output? Put a longer hose on the pump output to a bucket and run the engine for 10 seconds - about a gallon if you're lucky, and a half gallon at idle wouldn't surprise me.

Bypass valve? Do you have one?

Are you using a thermostat? Take it out for testing.

Take the hose from the thermostat output into a bucket and run the engine for 10 seconds. Same flow as from pump? Bypass vs no bypass?

Take the hose off the exhaust manifold output, and run the engine for 10 seconds. Same flow as from pump?

[pdecker]

All good ideas

I'll try them all until the thing works, and I'll let you know what worked here. Please keep the ideas coming, too! The new impeller LOOKS like it is the right size, but I'll remove it and compare it to the old one (which I had to partially destroy to get it off the shaft).

[JOHN COOKSON]

Maybe You'll Luck Out

Try priming the new impeller before doing anything further.

TRUE GRIT

[BunnyPlanet169]

+1 for what John said.

I was flummoxed for about 5 minutes till I figured out the pump won't self prime. It relies on water pressure from a below-the-waterline inlet. I had put a longer hose into a bucket for acid flushing and guess what? Nothing happens when the hose has air in it.

[edwardc]

+1 for what John said.

I was flummoxed for about 5 minutes till I figured out the pump won't self prime. It relies on water pressure from a below-the-waterline inlet. I had put a longer hose into a bucket for acid flushing and guess what? Nothing happens when the hose has air in it.

Others have noted this too, and I'm really puzzled.

When I did my rebuild, I had the engine on the bench and a 5-gal bucket on the floor. It had no problem priming a dry system through that 3+ feet of empty hose.

It seems there are cases where it will prime, and cases where it wont.

But I have no idea what makes the difference.

[BunnyPlanet169]

My favorite physics joke

It's like the thermos.

It keeps hot things hot, and cold things cold.

How does it know?

[Marian Claire]

OK. I will be the jerk and ask. Did you open the RW intake after servicing the pump? Other than that if you need anything as you pass thru the Oriental/Neuse River area let me know. Be happy to help if I'm at the boat.
Dan S/V Marian Claire

[JOHN COOKSON]

+1 On Being Puzzled

Others have noted this too, and I'm really puzzled.It seems there are cases where it will prime, and cases where it wont.
But I have no idea what makes the difference.

I'm RWC.
For my ~ first 20 years of A4 ownership I never had to prime the water pump after servicing it.
Then maybe ten years or so ago the pump wouldn't prime by itself after I took it apart.
Now I prime the water pump any time I take it apart to avoid the "drama".
Also the heat generated by running the impeller dry doesn't do it any good.

TRUE GRIT

[ndutton]

John, Bunny

There's a chance your inability to dry prime as the pump ages may be due to cavity wear. It results in the same internal leakage across the edges of the impeller vanes as the earlier picture shows. Same symptom, different cause.

About 2 years ago I measured the cavity depth on mine and compared it to Oberdorfer's published specification and sure enough, I had measurable and significant wear. I figured I'd give repair a try before replacement (where's the risk?) which thus far has been successful. Mine primes dry from a bucket, no problem.

Once again, another no flow report:

http://www.moyermarine.com/cgi-bin/shopper.cgi?preadd=action&key=KTAS_01_563

[BunnyPlanet169]

Neil - you are certainly correct that internal tolerances must make a difference in the ability to prime or not, and any wear from nominal must make self-prime more difficult. Especially blow-by on the faces. FWIW, mine is a 2 year old MMI. I've had some 'dry' experiences this year, however, and I'll be looking at it more closely on the hard this Fall. The flow sensor is a definite for me.

[ndutton]

The flow sensor is a definite for me.

It's cool that the sensor is fully compatible with the existing alarm system instead of having to pay for redundancy. Way cool.

[thumbdoctor]

I've noticed on a lot of pumps that don't self prime the cam shoe has irregular wear (taper towards the cover plate). This reduces the displacement of the pump too.