If you have a sour milk smell in your car, it can be a serious distraction while you're driving and for your passengers. This embarrassing situation requires some investigative work to correct. The key is to identify the source and then take proper measures to clean and deodorise the area. Even after you do remove the problem, since your car is an enclosed area, the smell may have permeated the interior, so a thorough cleaning is necessary.
Open all of your car doors and investigate the interior of your car for the source of the odour. Check under your seats, in your ashtrays, in your glove compartment and in the middle compartment between the seats. It could be an old carton of milk or coffee container under a seat. It could also be a milk spill or coffee spill you never properly cleaned.
Concentrate on the source first. If it is a carton of milk or something similar, throw it in the trash. If it's an old spill of milk on the carpet or upholstery, use a carpet extractor to get the stain out. Mix two cups of vinegar to one gallon of water and pour it into the extractor. Vinegar is a natural cleaning and deodorising agent. Use the extractor on the stain--it will inject moisture into the surface, then pull out the stain and most of the odour with it.
Use the carpet extractor to clean the rest of the interior carpet and fabric seats of your car. Use a more diluted solution of about a half cup of vinegar to every one gallon of water in the extractor for the remainder of the car if you want. Be sure to clean all carpet surfaces, including under the seats. Allow the car to air out with the doors still open.
Sprinkle the surfaces of your carpet and seats with baking soda. Allow the baking soda to sit and absorb any leftover odours. Vacuum up the excess powder. Spray the interior of the car with an odour-neutralising spray such as Febreze.
Don't worry about your car smelling like vinegar. The vinegar smell only lasts for a few hours and goes away after a while.