1995 GMC Yukon Owner's Manual - Page 296

Before closingthe hood, be sure all filler caps areon properly. Pull down the hood and close itfirmly. Cleaning Your Diesel Engine I NOTICE: If you spray or pour water or any other liquid on your engine when it is warm orhot, or whenit is running, you could cause serious damage to it. you ever clean the engine, clean If it only when it is cold. Engine Oil (Gasoline Engines) It's a good idea to check your engine oilevery time you get fuel. In order to get an accurate reading, the oil must be warm and the vehicle must be on level ground.