What is the part number of the GLM impeller? Hopefully 89623.
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www.volvopentastore.com
3862567
21213660
Replaced by
21951348
89623 GLM Impeller Kit Volvo Penta 21951348,18-3276-1
www.boatingsolutions.com
If it is 89623 then you're possibly dealing with a defective, wrong type/size, not spinning type impeller issue or you have air in the line between the drive to the impeller housing. You may have to manually help prime the water up to the housing better/more to make this easier and not hurt the new impeller if it's the correct one, etc..
Was the old impeller in bad shape? If not, maybe as another test, put it back in and see if it helps.
Carefully put your backside of your hand on the housing to see if it's getting pretty hot quickly once you start the engine, the housing should stay relatively cool by feel from the water.