Hi guys,

New to the forum and I didn't see a post regarding this, so forgive me if it's already been covered elsewhere. I have a 2013 Suburban with the 5.3L motor. I'm wondering if, when coasting at a decent speed (say, more than 35 mph), the fuel system continues to supply fuel to the active cylinders. I know the AFM system disables half of the cylinders, so I'm not talking about that. I'm wondering if it completely shuts off fuel consumption when coasting, relying on the transmission to keep the engine turning.

When I check the instant fuel economy while coasting, it often jumps up to 60 or 70 MPG, which is great, but if it shutoff the fuel completely, you would think it would show 99 (aka infinity).

Thanks.

 

My '11 5.3 will go into the '90's when coasting down from 55 mph when in gear. I had a bad MAP sensor causing the emissions self-test not to set and one of its symptoms was instant fuel mileage not going into the 90's on a long downhill coast and staying fairly steady at a high number. If you're only getting up into the 70's on a long downhill coast, I would suspect the MAP sensor. Mine failed with no codes or engine light but my advanced scan tool has 2 MAP sensor tests and mine was erratically failing one test.
Properly running system shuts off fuel until engine RPM gets below aprox. 1500 rpm when coasting in gear.