Mpg Drop / Inconsistent Highway Rpm

mcinnis · February 8, 2010

Vehicle: 2006 Silverado C1500 ext cab 2WD,5.3T, A/T, limited slip 3.23, all oem exc. K&N Air Filter, 124,085 miles

Operating Environment: North Florida, 90% Highway 70 mph, 10% stop and go, conservative driving all, occasionally used for towing

When this truck was new it would routinely return over 19mpg average on 87 octane, unloaded. I have routinely serviced the vehicle using WIX filters(oil and trans ), use 5w30 Amsoil, use Amsoil ATF, and have changed the plugs once( replaced with OE equiv. ACDelco ). In the last 3 months I have had a significant drop in fuel mileage down to 14-15mpg average with no change in driving route or style. There are no DTC stored or displayed. The last issue with the truck was a P0449, which was repaired with replacement. This seemed to have no effect on the mileage before or after.

I have noticed that the truck seems to run at a different rpm depending on the time of day. In the morning, it will run at or around 2000 rpm at 60 mph in O/D. The rpm drops as the engine operating temperature comes up to around 1500 rpm at 60 mph. However, on several occasions, with the engine ( assuming trans as well) at operating temperature the rpm at 60 mph has been around 1300 rpm. This lower rpm number has only been seen after going WOT from a rolling start( driveway to highway ) Cruise control maintains all of these figures if engaged. All rpm numbers come from the display and all figures taken on highway with selector in O/D ( D).

These two things may or may not be related, but any insight as to the cause of this would be much appreciated.

Green_Monster · February 8, 2010

This is not my expert area, but it sounds like you TC is not locking up all the time. When at hwy speeds and you push on the gas (not so much to kick downs a gear) do the rpms change on your tach? If they do I have to go with my first response.

mcinnis · February 9, 2010

"When at hwy speeds and you push on the gas (not so much to kick downs a gear) do the rpms change on your tach"

If the gas pedal is pushed slightly the rpm barely creeps up, if pushed more ( but less than kickdown ) the rpm spikes several hundred and then begins to rise steadily.

Does this answer your question?

mcinnis · February 11, 2010

Where can I find a diag. flow for determining if TC lockup is the issue?

gmtech4 · February 12, 2010

there would be a code for TCC ,also the TCC does not disengage like the old 90's trucks ,they are now and has been for awhile,they are PWM which means they are applied (ramped) on and off slowly.

you need a professional to check with a real scanner called the tech 2 at the dealer