sewer odor from floor drain

I've got a problem with sewer odor coming from a floor drain. I assume this only happens if the trap does not have enough water to close off the air path. And, I assume there are only two reasons why the trap might be dry: evaporation and a leak in the trap. Since water is periodically poured into the trap I'm leaning towards the fact that there is a leak. But before I go to the trouble and cost of tearing up the concrete floor to verify this I'd like to solicit comments from the experienced listnerers out there for suggestions and ideas on what to try to test my assumptions.
Some thoughts I've had:
Is there a video inspection technique that could verify a leak?
Someone suggested pouring mineral spirits into the drain (won't evaporate like water). (Will this cause any problems on a septic system?)
Thanks for any ideas.
John Keith snipped-for-privacy@juno.com
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The normal material is RV antifreeze. Mineral spirits? The guy must have been joking.
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From the google search that Buffalobill pointed me to there was another reference to using mineral spirits so it is not an uncommon suggestion. Your idea of RV antifreeze does sound better! There were other suggestions in the google search such as cooking oil.
Thanks for the tip. John Keith snipped-for-privacy@juno.com
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Cooking oil is a far better idea than mineral oil, as it's far less likely to turn your drain into a rocket engine. Mineral oil is too flamable for that application.
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John Keith wrote:

A third possible cause and probably more likely than a leak would be a vent stack problem allowing a vacuum to form and suck the water out of the trap possibly when the is a large discharge elsewhere.
Pete C.
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Thanks for the suggestion. I agree that a leak does not seem like the most likely source of the problem. I guess I need to examine the vent pipe.
John Keith snipped-for-privacy@juno.com
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see also previous discussions: http://groups.google.com/groups/search?hl=en&q=sewer+odor+from+floor+drain&qt_s=Search
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Great link! With your prompting I poked around google and finally figured out how to search the newsgroups.
Thanks
John Keith snipped-for-privacy@juno.com
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