I have a furnace that's about 30 yrs old. The pilot stays lit and it kicks on for about 30 seconds and then blows out along with the pilot light. We have vacuumed out the flue and the whole furnace with no results

I have a furnace thats about 30 yrs old. The pilot stays lit and it kicks on for about 30 seconds and then blows out along with the pilot light. We have vacuumed out the flue and the whole furnace with no results
Does it go out after the blower kicks on or right after the burners kick on?

You might need to get a real good carbon monoxide, tester/monitor,, sounds like a cracked heat/combustion chamber is possible,, very dangerous

does it go out after the blower kicks on or right after the burners kick on?
It goes out after the burners kick on