I sometimes get a sewer smell coming from garage where there is a big drain. What happens happens when it smells, it doesn't happen all the time. What wrong?

Trap is probably drying out. Pour about a pint of cooking oil down the drain.

Can you post a pic of the drain in the garage? If it is what Im thinking it is, you may need to just pump the pit out and clean it up real good, then fill the pit back up with water. Does this big drain have a metal grate over it?