An employee was troubleshooting the lubrication system for kiln cooler gear. He locked out the system and shut off the compressed air to the lubrication system. Then, he went to unclog the system by pushing the cleanout plunger on the backside of the nozzle while holding the spray bar. His left hand slipped in front of the nozzle as the injector released stored oil pressure, resulting in the injection of air and oil lubricant into his hand at 3,000 psi.