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Raw water pump impeller riddle-

3K views 10 replies 8 participants last post by  Ajax_MD 
#1 ·
Riddle me this fellas:

Oberdorfer 202M7 pump on a Universal M30/5424 diesel.
Engine has sat for at least 5 years, properly winterized on the hard.

I remove old impeller. It's not cracked, but is a bit deformed.
I install a new, seemingly identical impeller. The package even said "Oberdorfer" on it, and referenced the pump models it's compatible with.

With the new impeller, the pump simply cannot pull water from the seacock. I put the old impeller back in, and "bam" pulls water and pumps like a champ. I could blow water up to the pump and the new impeller would pull water for a few seconds, and then stop.

Obviously, running with a 5 year old impeller is not acceptable. I tried lubing the new impeller, but it didn't help. The old impeller is not lubricated and still pumps fine. No, the metal hub is not torn out center of the new impeller. The old and new impellers seem identical in every measurement.

Also- What is the difference between the "N" designation and the older, 202M7 model?

What do you think?
 
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#3 ·
Check the 'width' dimension of the new impeller vs. the old impeller. If the width of the new is smaller than the old, then you will not seal tightly against the 'sides' and you will 'break' suction. If you suspect a lesser vane width, do a 'trial' without a gasket and use an automotive grade RTV sealant (form-a-gasket) instead.

Other. When you install the impeller into the Oberdorfer housing, make SURE that the vanes are installed with the correct 'twist', as the vane position in an Oberdorfer pump is NOT 'intuitive'. Rotate the engine by hand, to be sure your impeller vanes are 'deflected' in the correct 'direction' when installing.
 
#4 ·
I went through The same problem years ago, only obd impeller works globe run dry will not, also be sure to get ONLY the thin paper gasket, in spect the cover plate for wear, the new old impeller clearly was a waste of time
 
#9 ·
Since this pump is also used on Atomic-4 engines, I posed the query over on the Moyer Marine forum and that is the answer that I received.

I'm going to pull and measure the width of the impellers with a caliper and attempt to measure the cavity depth of the pump. I can either attempt to sand off a tiny bit of the pump face to make the depth shallower, or I can buy a new pump. When properly primed, the pump moves a lot of water, so I hesitate to just throw cash at a new pump.

The hub is not torn out of the new impeller and this pump does not use a woodruff key. The shaft and the impeller have a flat spot which must align when installing the impeller on the shaft. That serves as the key.
 
#8 ·
If you go through all of this and it still doesn't work, just get the old impeller out and get an identical one by measurement. Also check the intake for debris (I know you wouldn't get suction from any impeller) but that is the most common cause of failure IMHO.
 
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