so around 350 miles per tank. thats actually better than i get i average 200-250 miles per tank

 

If you are getting around 350 miles per tank that don't sound too bad. I get just a bit more than that daily driving my 2000 5.3 ECSB Silvy, only mod is a K&N CAI.

If you had a issue with cats, O2's, or something related you should get an engine code letting you know there is a problem.

How many miles on your current spark plugs and wires? When was your last oil and oil filter change? Have you tried the Sea Foam treatments?

 

its an 03 with 73000 Kms, so its like 40,000 miles or so. It may be time for a tune up as well. I bought it from a dealer last november. Not too sure if they did their usual gm tuneup or not. Might try plugs and wires for now. And maybe look into the computer diagnostic if not. elaborate on the sea foam, I have not heard too much about it. thanks

 

i bet its the MAF needs cleaning from an overoiled filter

you arent getting "bad" mileage maybe not as good as you used to but still not bad for a truck

 

Yeah, your mileage in not bad. I bet the dealership didn't do any kind kind of tune up before you bought the truck.

For the Sea Foam check this out: https://ls1tech.com/forums/showthread.php?t=534376

Hope this helps.