A number of vehicles broke down shortly after after filling up at the station in Branksome, Poole.
The supermarket said it was carrying out "further research" after tests on the fuel came back as inconclusive.
About 20 motorists were initially thought to have had problems but the firm said the number of new cases being reported was increasing.
An excavator was seen at the forecourt but Tesco would not give further details of work taking place.
A Tesco spokesman said: "We have been working urgently to respond to customers' complaints and get people back onto the road as quickly as possible.
"Our customer services team have now contacted every known complainant and are proceeding on an individual, case-by-case basis to resolve their problems."
Hertfordshire County Council's trading standards is also dealing with the supermarket - which has its head office in Cheshunt - as part of the investigations.
Council spokesman Simon Hoggett said: "The council's actions will be governed by what the tests show.
"Is this a case of fuel being contaminated, or is it a case of the wrong fuel being put in the wrong tanks? We just don't know."
Anyone who has not yet reported a problem to the supermarket should call the customer service team on 0800 505555.
The forecourt of a Tesco fuel station is being dug up after drivers reported engine problems after buying diesel.