E90 consuming higher gas than normal

[julls]

I have a 330i E90 2006 with 190k miles. I normally use Shell V-power (and topped off my gas with SeaFoam on last refill at Shell). I have now switched to using Chevron Techron; however, after my first refill with Chevron, my fuel guage suddenly appears to be dropping faster than normal. I topped off at a Chevron station few days ago, then drove a few miles and noticed that my gauge dropped down close to a quarter tank. I found that odd, so I went back to the same gas station, topped off again (it took a little over a gallon at the station to completely fill the tank). I know for sure that a little below a quarter tank requires a few gallons - so it appears this is more a problem with the reading than actual usage. Also, from the station to my house is less than a mile - but the needle has already dropped a bit even before getting home. I could literally see the needle shift down while driving home. Could this be a sign of a failing O2 sensor? Something else to keep an eye on? Thanks for your input.

[Pilgrim]

Use the car computer to read gas mileage, and check it by filling and calculating mileage manually. Fuel gauge movement is no way to judge consumption. Hard data will help you.

[julls]

@ Pilgrim - correct, I have manually calculated and I know for sure that the Guage is considerably off. What I want to know is how to fix this sudden issue of my Guage dropping real fast. Never had this issue before.

[Pilgrim]

Tried pulling codes? They can be present without a check engine light. Since you're in CA, I assume weather is moderate, so it's not an issue of cold weather reducing gas mileage.

[marvinstockman]

You may be suffering from the 'law of unintended consequences', which in your case may be that the cleaner did something to the intank wiping resistor on your fuel pump, that measures fuel level.

[julls]

I took the car to the shop and I am being told the issue is due to a faulty fuel pump. The entire fuel pump system was only recently replaced (after to which I topped off my gas and applied SeaFoam/cleaner). Indy is saying they checked and the part appears to be defective. Not sure if it got ruined by the SeaFoam cleaner - thankfully the system is under warranty. They just replaced the entire system again today so I'll have to wait and see after I pick up the car (though I still have some of the SeaFoam in whatever is left of my gas).

[julls]

Got my car today with the fuel pump replaced - still seeing same issue. Wary of bringing my car to the dealership

[julls]

I have taken my car to a new Indy and he diagnosed the issue - the tool is showing that there is a problem with the left fuel sending unit. So it seems plausible that the Indy who replaced my fuel pump probably screwed up (twice). Will have to take it back.

[julls]

Seems to be an issue with the fuel sending unit. Will have to take it back to the shop that did the initial replacement.