My 2001 Chevy Blazer LT had an oil leak (I was told it was the rear seal?) and a large crack in the radiator (I didn't know until it nearly overheated). I replaced the radiator. Things went well for about three weeks. Then my water pump "went out" (broke, crapped out, FAILED). I replaced the water pump and thermostat. Before cranking it back up I checked the oil level and the tip of the dipstick wasn't even wet! (Strange...even with the leak it's never been bone dry.) So I added a gallon of oil and refilled the radiator. I accidentally (fortunately) left the cap off the radiator and cranked the engine. After about five or six seconds, oil and coolant started shooting out of the radiator. I killed it and got out to look. OH NO!! Now I have 10w-30 in my radiator!! What happened? Why was there oil in the radiator? AND What should I do to fix it? PLEASE HELP!

 

Sounds like the radiator had a faulty oil cooling passage.