I have an 07 Avalanche 5.3 with 102K miles. Just bought it last year so not sure what maintenance has been done. Gas mileage is 11-12mpg. What will help. Should I change plugs. Already installed K&N air filter. Any suggestions?

 

have a look at them and see if they are fuel fouled from running rich. if yes, there is a fuel metering problem.
fyi, you are not far off the epa mpg rating
https://www.fueleconomy.gov/feg/Find...n=sbs&id=23044

 

My son had a '07 Avalanche 5.3L 4WD - even with new spark plugs he never averaged more than 13 mpg. Didn't matter if he was going uphill, downhill, flat terrain or had the wind at his back - 13 mpg. Of course he had all terrain tires that did not help mileage at all.

 

My 08 averages 17 mpg Canadian combined with 285 x17 cooper AT3 [canadian tire motomaster] tires. Has 3;73 gears

 

I'm new here, and I'm not especially knowledgeable about engines in general. I do have a similar issue with gas mileage, so hopefully I'm in the right place to pose my situation in the hope of getting good advice.

I purchased my 2003 Avalanche 5.3L Z71 new in February of 2003. It has been a reliable and mostly trouble-free vehicle until recently. I began to notice oil drops on the garage floor. With 158K miles, I figured it was a small issue. The flow increased to a point where I took the vehicle to my local AAA shop. Long story shortened a bit, on my first visit they pointed out a leak from the valve covers, so I had them replace the gaskets. Drip remained. Took the truck back and this time they pointed to a small trap door looking thing on the underside of the engine. I paid to have that gasket replaced (the oil pan gasket had been replaced 2 years prior). No change. During my next visit, they checked the oil pan again and said it was leak free, but told me the problem was likely a bigger job involving parts under the intake manifold. From the location of the drip, they had ruled out anything involving the main seal. So I gave them the go-ahead.

Over a two day period, they disassembled the intake manifold and replaced the cam positioning sensor and the 2 knock sensors. They showed me the knock sensors which appeared to be corroded from water. When I got the truck back there was no longer a leak. However, first thing every morning I had trouble starting the engine - never before did I have that problem. I also noted a significant decrease in my normal gas milage (never great to begin with). I live in a semi-rural environment where I have, for years, been getting 15 to 16 MPG local, with perhaps 17 to 18 on a steady highway drive, even in summer. After the intake manifold work, aside from the problem starting in the morning, my gas milage never got better than 13.5, even though I drove more conservatively. I brought the truck back again and a few hours later they told me the problem was a procedure that they had neglected to run. Two days later I was back. The truck did start but it cranked longer than it had ever done before. They kept the truck for another 3 days and had a senior mechanic from another location join the search for the problem. After three days I was told they found the problem, a defective knock sensor. After that replacement, the next shot in the dark was the mass airflow sensor.

I'm now well over $2K in repairs which started out as a relatively minor oil leak, with new valve cover gaskets, some trap door gasket, a cam position sensor, 2(or 3!) knock sensors and a mass airflow sensor. Gas mileage still hovers at around 13, and I still have an occasional difficult start. Here is the big question (other than should I find a new service provider), do I need to have the Power Control Module reflashed after all that work, or is some other computer module possibly at fault.

Thanks for your patience reading this, but I'm at the end of my rope.