1998 GMC Envoy Owner's Manual - Page 262

A CAUTION: Operating theengine with the air cleaner off can cause you or others tobe burned. The air cleaner not only cleans the air,it stops flame if the engine backfires. If it isn't there, and theengine backfires, you could be burned. Don't drive with it off, and be careful working on the engine with the air cleaner off. Automatic Transmission Fluid When to Check and Change A good time to check your automatic transmission fluid level is when the engine oil is changed. Change both the fluid and filter every 15,000 miles (25 000 km) if the vehicle is mainly driven under one or more of these conditions: In heavy city traffic where the outside temperature regularly reaches 90°F (32°C) or higher. In hilly or mountainous terrain. When doing frequent trailer towing. NOTICE: If the air cleaner is off, a backfire can cause a damaging engine fire. And, dirt can easily get into your engine, which will damage it. Always have the air cleaner in place when you're driving. Uses such as found in taxi, police or delivery service. If you do not use your vehicle under any of these conditions, change the fluid filter every and 50,000 miles (83 000 km). See "Scheduled Maintenance Services'' in the Index.