GMC 4000 Series Service Manual page 479

Sec. 8B
________________ G M C S E R V I C E M A N U A L
Page 468
FUEL TANK A N D EXHAUST
When working on fuel system , fire fighting and
appropriate safety equipment should be provided
in accordance with local fire and safety regulations.
Before making adjustments on Fuel System Units,
be sure Electrical System is functioning properly.
WARNING - CARBON MONOXIDE
Keep cowl vent and heater intakes closed when
operating in congested traffic to prevent deadly
exhaust gases from entering Qab.