A 50,000-barrel batch of contaminated fuel from a BP refinery in Northwest Indiana has fouled countless cars in as many as four states since last week.

Roughly 2.1 million gallons of gasoline have been recalled by BP's Whiting refinery after hundreds of reports of hard-starting and stalling vehicles from motorists flooded Northwest Indiana repair shops the past few days.

While the problem is centered in Lake County, Ind., BP executives said the fuel has made its way across state lines into Illinois and Wisconsin, with unconfirmed reports that stations in southwest Michigan may also be affected.

BP issued a statement Tuesday, saying the gasoline was blended at its Whiting storage terminal between Aug. 13 and 17 and contained a "higher than normal level of polymeric residue." The fuel was distributed at BP stations and other retailers in the last week and primarily involves regular-grade gas.

Mechanics say the contaminated gas has a sludge-like consistency, which clogs the fuel system. BP is investigating how the contamination occurred.

Affected motorists are instructed to call BP's customer hotline at 800-333-3991, but customers have reported difficulty getting through to a representative. Gas claims may also be filed at bpconsum@bp.com.

The company also has set up a website to answer questions at bpresponse.com.

Car dealerships and service shops throughout Northwest Indiana and beyond have been inundated with calls about the bad gas, while service bays are jammed with repairs, which primarily consist of draining the fuel tank and cleaning the fuel system. Costs have generally ranged from $300 to $1,200, according to AAA.

"We certainly want to make sure motorists are aware of this," said Avery Ash, a spokesman for AAA. "If you're a motorist and you are experiencing any issues, we would advise you to contact BP and get your car into a facility to have the issue taken care of."

Bill Vlietstra, service manager at Schepel Buick GMC in Merrillville, Ind., said the dealership has received hundreds of calls since Saturday. Those who just topped off their tanks are told to add fuel cleaner and fresh gas. Anyone who filled up with the contaminated gas is advised to bring the car in for fuel flushing.

More than a hundred customers have done that since Monday, according to Vlietstra.

"A few of them, we have towed in," Vlietstra said. "We just line them up outside and get to them as we can."

Vlietstra said the fouled-up fuel drained from cars has a distinctive viscosity.

"We've got glass bottles we collect it in," he said "It's almost like a gummy bubble in the fuel. Gas will always float to the top of water, but this is kind of suspended in the middle."

One of the customers waiting for service at Schepel on Tuesday was Bill Eisman, of Lake of the Four Seasons, Ind. Eisman purchased five gallons at a Speedway station in Hobart on Friday.

"On Saturday morning, I couldn't start it," Eisman said. "It sounded like explosions. I guess it was backfiring. It was a disaster."

Eisman had the car towed to the dealership Sunday, where it was diagnosed with a case of bad gas.

Scott Dean, a Chicago-based BP spokesman, said customers with approved claims will be reimbursed for their repairs.

The Whiting refinery is one of six that serves the Chicago area. It has the capacity to process more than 413,000 barrels of oil per day, according to Dean, who said area supplies wouldn't be affected by the recall.

While individual gas stations occasionally experience problems with water leaching into underground storage tanks, contamination at the refinery level is rare. Dean said the last such incident involving BP in the Chicago area occurred 10 years ago, when heating oil got into a gas storage tank.

Stations receiving the contaminated fuel include BP, Speedway, Thorntons and a number of big box stores, according to Dean. Several gas stations have been shut down in Northwest Indiana while their storage tanks are drained and fresh fuel added, Dean said. The problem has also crossed state lines via fuel trucks into Illinois, Wisconsin and possibly Michigan.

"We definitely now are seeing a handful of customer complaints along the Illinois and Michigan border," Dean said. "It's not unusual for those tankers to cross the border, particularly if it's one of the communities right on the border."

Dean confirmed late Tuesday afternoon that a small amount of contaminated premium and mid-grade fuel was trucked to the Milwaukee area and sold between Monday evening and Tuesday morning before the company halted sales.

Of the approximately 4,500 calls and 800 emails received by BP as of Tuesday afternoon, Dean said about 95 percent of the gas-related complaints were from Lake County, Ind.

A number of the Illinois complaints may have stemmed from motorists who filled up in Indiana.

Napleton River Oaks Kia in Calumet City reported about 40 calls from customers related to the fuel contamination from customers who had crossed into Indiana to buy gas. About 20 customers brought their cars in for service, which ran about $400, including a car rental, a manager said.

While it may take some time to learn how the contamination occurred, the recall itself is no easy procedure, according to Dean.

"Part of the challenge here is to identify these batches," Dean said. "When you recall food, you're able to look at the package and see what the lot number was. It's a little bit more complicated here. We've been pulling bills of lading, reaching out to various distributors that sell BP-produced fuel, and we're working through it."

Dean said it is unclear how much of the contaminated fuel made it into gas tanks.

"We're still trying to account for exactly how much has been combusted," he said. "We are getting loads back and we're doing that as quickly as possible."

Meanwhile, for many motorists, the contaminated gas has long since turned into vapor.

Pat Cappo, 51, of Munster, Ind., filled up her 2005 Chevy Suburban at a Meijer store in Highland on Thursday night, and set off the next morning with her husband to take their son to begin his freshman year at the University of Evansville downstate.

The 250-plus mile trip was uneventful, but by Saturday morning, the car began to demonstrate signs of indigestion.

"It wouldn't start," Cappo said. "It was chugging."