Keke Palmer and LaKeith Stanfield tried putting the pedal to the metal on some good deeds Tuesday ... but their free gas event ran into a little road rage when cops got called to the scene!

The stars pulled up to a Shell gas station in L.A. to promote their upcoming movie "I Love Boosters" Tuesday ... offering free gas to the first 70 drivers who rolled through -- although one latecomer ran outta fuel after missing the cutoff and called the police!

Thankfully, officers were able to pump the brakes on the complaint and other side dramas pretty quickly ... while Keke and LaKeith kept the vibes cruising smoothly.

And honestly, the event still looked premium grade. One lucky person even drove off with a full $100 tank ... while Keke posed for adorable pics with fans and babies.

Meanwhile, LaKeith kept things heartfelt -- talking about the importance of community support during tough economic times and calling attention to rising prices squeezing everyday people.

So despite the minor blow-up at the pump ... the event still delivered unlimited smiles per gallon!