Federal regulators are investigating complaints from more than 150 Ford Explorer owners that potentially dangerous exhaust fumes may be leaking into the SUV’s cabin.

The National Highway Traffic Safety Administration announced recently that it has opened an inquiry into 600,000 model 2011 to 2015 Ford Explorer SUVs after receiving 154 consumer complaints — including one crash — related to the smell of exhaust fumes in the passenger compartment.

According to the NHTSA investigation notice [PDF], investigators will work to determine how the exhaust fumes can enter the passenger compartment and how the fumes have affected passengers — potentially leading to illnesses or headaches.

Complaints indicate that the fumes seep into the compartment when the vehicle is operating at full throttle — such was when climbing hills or merging onto freeway ramps — or when the air conditioning system is in recirculation mode.

“It is happening on acceleration and going up steep grades and there is a sulfur/mechanical burning smell that wafts in,” a driver who alleges the issue contributed to a crash tells NHTSA. “The kids and I, as well as anyone else in the car for long durations have been getting migraines, dizzy, and just flat out sick.”

One day while driving, the man reports that he passed out from the fumes and wrecked the vehicle. He says he was traveling at a slow enough speed that no one was injured.

The man says he had previously brought the SUV into a Ford dealer five times, but nothing had been done to prevent the fumes from entering the vehicle.

According to NHTSA’s notice, Ford has issued two related Technical Service Bulletins over the issue in 2012 and 2014 detailing complaints received by customers and how dealers can fix the problem.

Potential repairs outlined by Ford include adding sealing and undercoating of certain areas of the area floor pan, or adding additional software to the recirculation mode operation of the air conditioning system.

Unlike a recall, these notices only serve to inform dealers of the problem, not vehicle owners. If an owner complains to Ford or a dealer about the issue, dealers are required to respond.

However, some owners reported there was little or no improvement to the issue after a dealer initiated the service bulletin remedy.

“Exhaust enters the vehicle cabin any time the engine is operated at higher RPMS, such as claiming a long hill or traveling at highway speed,” an owner of a 2013 Explorer tells NHTSA. “The vehicle has been taken to the Ford dealer who contacted Ford engineering for a fix. The ‘fix’ didn’t help. I contacted Ford customer service and established a ‘case’ in August 2013. At Ford’s direction the vehicle was taken back to the dealer where it sat for three weeks. The dealer then called and asked us to get the car because they didn’t know how to fix it.”

The owner of a 2011 Explorer tells NHTSA that they brought the vehicle to a local dealerships three times and that the two technical bulletins were addressed. However, the owner remains.

NHTSA says its investigation was opened to evaluate any potential driver related safety concerns caused by the issue.

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