Brad Keselowski has been fined for criticising NASCAR's move to electronic fuel injection (EFI) in a recent newspaper interview, according to several sources contacted by The Associated Press.

According to the sources, Keselowski was fined $25,000. The fine has not been publicly announced, and neither Keselowski nor NASCAR would either confirm or deny the report.

Instead, NASCAR spokesman Kerry Tharp told the AP that: "We did talk to Brad following his recent comments that were highly critical of the series moving to electronic fuel injection next season. We made it clear to him that these kind of comments are detrimental to the sport, and we handled it accordingly with him."

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Keselowski made his comments last week during an appearance at the NASCAR Hall of Fame. "It has less throttle response, and it's harder to get to start (because) it takes a computer to start the damn thing," he is reported as telling the USA Today newspaper. "It's a pain in the ass. I don't see where fans get anything from it," he said, adding that the system being introduced was already 35-years-old and far removed from modern day fuel injection systems.

"I'm not a big fan of it at all. Carburetor technology is 50 years old but is very simple. The benefit of a carburetor is that it's very, very easy to police. That's why NASCAR stuck with that," he had added. "They've been pressured into switching it through the green initiatives. In reality it's no more efficient than what we have, and it costs a lot more."

While unpopular with fans who want transparency in the sport and for drivers to be able to speak their mind, secret fines are not uncommon in NASCAR - although by definition, it's hard to know just how many have been handed out by the sanctioning body.

Ryan Newman was reportedly fined $50,000 earlier this year for allegedly throwing a punch at Juan Pablo Montoya in the NASCAR hauler at Darlington in June, during an unsuccessful private meeting between the two drivers and officials to resolve an ongoing feud.

Newman is said to have previously been handed a fine in 2010 for comments criticising the style of racing on superspeedways, in which he had said: "Racing for a championship shouldn't be a lottery ... [Talladega] is not about someone's car handling or motor being better. The cars might as well all be kit cars for these two races."

And Denny Hamlin revealed that he got a slap on the wrists for critical comments that he made in posts on Twitter, in which he implied that NASCAR may have intentionally influenced the outcome of races by throwing a debris caution at key moments.

"I understand this is show business," Hamlin had said in his original 2010 comments. "There is always debris that they could throw a legitimate caution for, but I think that sometimes they just kinda let it go when maybe things are getting mixed up, and other times, when things are spread out, let's tighten it back up. You don't have to be so smart to realise that these things are just by chance."

As with the ongoing debate about 'Boys Have At It', it seems that drivers are still struggling to identify a consistent line that they are not supposed to cross.