Once a week my job takes me to Niagara Falls. Assuming I have a valid I94 in my passport, would there be any issues in me driving over the Rainbow Bridge, filling up the car with cheap gas and then crossing back into Canada again? Does anyone know if I would have any problems with the CBSA if I tell them that this was the reason for my visit to the US?

It's done all the time by those living close to the border. Get some groceries while you're there.

As long as you don't fill up with more than $50 CAD of gas it's technically allowed. $50 is your duty free allowance for less than 24 hours out of the country.

US may have issues with that being your reason for travel though, but you're probably fine.

I do it all the time. CBSA in the Nexus office say that if it's in your stomach or your gas tank it doesn't need to be declared.

Though I guess that could be misinterpreted.

I've never had a problem when asked. Shopping seems to be a perfectly acceptable reason.

Been doing this for 15 years and never had a problem with it. Never been asked how much gasoline I have bought.