Paul Walker may not have ever gotten into the car that took him to his death if it hadn't been acting up moments before the crash ... TMZ has learned.

Paul showed up Saturday for a fundraiser at Always Evolving -- Roger Rodas' high end auto shop that specializes in exotic cars. Paul had a stake in the shop.

Sources connected to Roger tell TMZ when Paul arrived ... Roger was watching one of his employees try in vain to back the Carrera GT into the shop, but it kept stalling. Roger decided to take matters into his own hands ... told the worker to get out and took the wheel himself.

As Roger began to back into the garage ... Paul came up and said he hadn't driven in the Porsche yet -- so he jumped in the passenger seat and they took a 20 minute drive.

The fatal accident happened on the way back from that drive -- just 500 yards from the shop.

Paul's best friend from childhood, Nute, grabbed a fire extinguisher and was among the first to arrive on scene. We're told Nute frantically tried to extinguish the flames ... even though it appeared to him both men were already dead.

When the fire dept arrived they tried to pull Nute away, but he was overwrought and punched a firefighter. We're told Sheriff's deputies put Nute -- and several other of Paul and Roger's friends -- in custody, but let them go when they realized the circumstances.

Oh, I didn't see that and no one has mentioned it...would make sense though.