Boomer1974!

I have an 05 GMC Sierra 1500 with 5.3 it was smoking on start up first thing in the morning. Had a compression test done all cylinders are between 3-5 lbs loss. Shop said replace valve seals so they have done it twice and it smoked worse!!!! After research we had them replace the valve covers and it fixed the smoking issue. Now the truck has less power runs so bad it won’t get over 50 mph has code for mass air out of limit multiple cylinder missfire and when I took the intake tube off there is oil in the throttle body and it blew the gaskets out of new valance covers!!!!! Is it building up to much crankcase pressure?

Swathdiver I drove to a town 14 miles away last night and I was getting 15.4 mpg with switching to the 89 octane. Taking it to get windows tinted on Wednesday so we will see what kind of mpg I get then. The shop is about an hour away and then I will be driving to my daughters game and then home so I will be putting some miles on it then. Will keep you updated.

ProComp 6 in I bought the truck last year already lifted it is a Tuscany conversion. Not sure on the calibration of speedometer. I am going to put a McGaughys 7-9 on it in about 6 months I am getting around 12.9-13.5 on the highway running 87 just switched to 89/91 so will have to get back with you after I run it for a bit.

Swathdiver it is a 2012 Chevy Silverado 1500 crew cab with 6.2 L with 3.73 gears with a 6 in lift 325/60R20 BFG MT

Thank you for your reply but those have already been checked and secured and tensioner is good.

I have a 2012 Chevy Silverado 1500 with the 6.2L. My voltage is 14.45v running around town but when I drive it for long distances or I let it idle for 10 minutes or more it drops to 12.67v and stays there. Yesterday I drove it for an hour and it dropped to 12.67v got to my appointment and when I came out 45 min later it said start vehicle battery low so I ran it for 20 min and shut it off and about 2 hours later it was still at 12.67v. I have put a 160 amp alternator on it from gm this is my 4 or 5 alternator. Any ideas?