We have tried plastic covering, air purifier, baking soda, scent drops, and bleach to get rid of foul odors in our dirt crawlspace. Our home was built five years ago and the crawlspace is approx. 40'x80' and 3'in height. The smell has been there from the beginning and is not due to water leakage. Any suggestions? The odor comes through forced air heating vents and cold air returns. We are desperate!

Woops. Our crawlspace is 32'x40'not 40'x80'.

If there is no moisture leaks, and your smell is coming from the air vents, then your smell is coming from the condensate in your A/C(in crawlspace?).

Unless you have a sewage problem that is causing sewage to back up into your A/C:
You can fix with a)mildew inhibitor, b)fix the drainage problem in your A/C, or c)move the condensing coils to a higher location(This is what I would do) which would allow proper drainage of the coils.

Chartre