It could be in need of a tune up. Poor driving habits also effect the mpg, just saying.
The truck needs a tune up.
Could be in need of a tune up.
Time for a tune up.
I am assuming this did not occur before the tune-up and isn't a cam wear or bent pushrod problem. Check your firing order. The firing order is stamped on the intake manifold.
There is nothing to tune up on a GMC Envoy except to change the plugs at 100,000 miles. Follow the manual for regular oil and drive line lubrication fluid changes. www.trailvoy.com Envoy enthusiasts forum!
At what degree do I set my timing for a 1985 GMC s15 pick-up truck?