I have been installing a water pump and head gasket on my cobalt. When I crunk it up to test it out, it ran fine for a few minutes and started slightly shaking. I removed the valve cover and noticed a little bit of milky oil on the bottom of valve cover. I tried a wet and dry compression test and the compression seems fine. I had to tap the water pump to straighten it out in the sprocket. I may have damaged it but do not want to do all that work for nothing. Is there any advice to verify it is the water pump before hand or anything else it could be.