Reports of bad gasoline being sold in the Bismarck area are being investigated by the Mandan Tesoro Refinery.

The reports from motorists who said they had engine problems prompted at least one Bismarck retailer to voluntarily stop selling 87 octane unleaded gas on Thursday.

Mark Leischner, owner of UNISTOP stores in Bismarck, Mandan and Washburn, said his stores received dozens of calls from customers who were reporting engine problems after filling their tanks.

Similar problems have been reported at other gas retailers in the area.

Tesoro issued a statement at 6 p.m. Thursday that said: "The Tesoro Mandan Refinery has become aware of customer complaints involving all grades of gasoline, except for premium gasoline without ethanol, supplied to the Bismarck/Mandan area.

"Tesoro is currently investigating and will provide more details as they become available."

Jill M. Saletta, vice president of corporate communications for Tesoro, said customers who have experienced engine performance issues related to the gasoline in the Bismarck-Mandan area on or after Tuesday evening should contact Tesoro at 877-876-5383 for further instructions.

It's not known how many gas stations or customers have been affected.

Trevor Hendrickson of Bismarck said he filled his car Tuesday night and began experiencing problems the following day.

He said the car, usually driven by his wife, started hard the next morning, to the point it had no power.

He towed it to a mechanic, who drained the gas tank, and he told Hendrickson he thought there was diesel fuel mixed in with the gasoline.

Hendrickson said his repair bill was $223, and he was told by the gas station's manager to submit a claim to the store's insurance company.

Hendrickson said he filled his tank with 87 octane regular unleaded gas.

On Thursday afternoon, Leischner said he narrowed the problem down to a gas shipment he received from the Tesoro Refinery in Mandan two days earlier.

He said ethanol-blended gas was not affected at his store.

Two of his stores - one in Mandan and one in Washburn - received loads from the refinery Monday and have not reported problems, he said.

Leischner said most people with problems said their tanks were almost empty when they filled up. He said the refinery took a sample of his gas from his store on Century Avenue for testing, but he had not heard back from Tesoro as of late Thursday afternoon.

Leischner said he was frustrated about the situation.

"Most of the people who have called have been very nice about it," he said.

Vehicles meant to run on 100 percent gasoline or E-10 - which contains 10 percent ethanol - won't run well or at all on E-85, other than late model hybrid or flex fuel vehicles, said Tim Geloff, owner of Auto Therapy in Bismarck.

Some vehicles may run on diesel, but sluggishly and badly - drivers may see a lot of gray or white smoke in their exhaust, he said.

Damage to vehicles would probably be minimal since it's unlikely that drivers will drive too long with the wrong gas mixture because the car will run badly or not at all, Geloff said.

How much of a problem the wrong fuel will cause also depends on how diluted the mixture was when it was put into the vehicle, Geloff said.

Those who believe they bought bad gas should not drive their vehicles, but rather take them in and have them inspected, he said. Bad gas should be drained by station owners or mechanics, he said, because gasoline is considered hazardous waste and can be pricey for owners to dispose of.

Drivers who have a problem should take their vehicles back to the place they purchased the fuel, he said.

Leischner said he was telling his customers as much as he knew about the situation, which wasn't much.

"I stopped selling the

87 octane on my own ... now I have 20,000 gallons of gas I can't sell," he said.

(Reach reporter Brian Gehring at 250-8254 or Tribune reporter Karen Herzog contributed to this story.)