2008 silverado 2008 Chevy Silverado 1500 5.3 Hard starting after fueling up

[Scott Lamb II]

2008 Chevrolet Silverado 1500 LT 5.3 Vortec. 2WD.     I've got a head scratcher here. So filling up the other day, I went to start the truck, and acted like it didn't have any fuel to start. Either that or fouled plugs. Took a few tries and started hard, idled rough for about a minute and then was running good. It's been cooler outside as its winter, roughly in the 20s and 30s. But the truck was already at operating temperature. It's happened twice now after I've filled up and no other time does it do it.                                                  Few things: Around 3 weeks ago I forgot to replace the gas cap, lost it on the road. Didn't replace it until the first time it started hard. I've recently changed the oil. Along with that I used a lucas oil additive made for variable valve timing. I also used a fuel treatment for carbon deposits. After looking with my telescope camera, it did its job.                                                                                    Now I did get 2 codes. P0171 and P0174. Bank 1 and Bank 2 Too Lean. Definitely running on the lean side. After this last hard start, I had my scanner hooked up reading the live data, Fuel Trim (Lngshft1 and Shrtshft1; Lngshft2 and Shrtshft2) was low. Lngshft1 and ShrtShft Bank 1 showed fuel trim at start up was -10.4% on Shrtshft1 and -25.6% on Lngshft1. Running very lean on bank 1, and bank 2 was also running lean. I did clean the MAF sensor, replaced the air filter. Check for vaccum leak for about an hour and could not find one. Fuel pump is good, runs just fine. Got out the multimeter and everything seems to check out.                                                      I'm thinking its possibly the fuel filter, maybe a bad MAF, purge valve, or O2 sensor. I've been researching for a few days and looking over the truck but cant figure it out. Rather than going through the task of replacing everything and hoping it works, I figured I'd see if anyone else and had similar problem and found the solution. It runs great any other time, MAF showing correct numbers, same with Fuel Trim showing normal numbers. It's only when I fill up. Maybe a fuel pressure issue somewhere.

[swathdiver]

1 hour ago, Scott Lamb II said:

2008 Chevrolet Silverado 1500 LT 5.3 Vortec. 2WD.     I've got a head scratcher here. So filling up the other day, I went to start the truck, and acted like it didn't have any fuel to start. Either that or fouled plugs. Took a few tries and started hard, idled rough for about a minute and then was running good. It's been cooler outside as its winter, roughly in the 20s and 30s. But the truck was already at operating temperature. It's happened twice now after I've filled up and no other time does it do it.                                                  Few things: Around 3 weeks ago I forgot to replace the gas cap, lost it on the road. Didn't replace it until the first time it started hard. I've recently changed the oil. Along with that I used a lucas oil additive made for variable valve timing. I also used a fuel treatment for carbon deposits. After looking with my telescope camera, it did its job.                                                                                    Now I did get 2 codes. P0171 and P0174. Bank 1 and Bank 2 Too Lean. Definitely running on the lean side. After this last hard start, I had my scanner hooked up reading the live data, Fuel Trim (Lngshft1 and Shrtshft1; Lngshft2 and Shrtshft2) was low. Lngshft1 and ShrtShft Bank 1 showed fuel trim at start up was -10.4% on Shrtshft1 and -25.6% on Lngshft1. Running very lean on bank 1, and bank 2 was also running lean. I did clean the MAF sensor, replaced the air filter. Check for vaccum leak for about an hour and could not find one. Fuel pump is good, runs just fine. Got out the multimeter and everything seems to check out.                                                      I'm thinking its possibly the fuel filter, maybe a bad MAF, purge valve, or O2 sensor. I've been researching for a few days and looking over the truck but cant figure it out. Rather than going through the task of replacing everything and hoping it works, I figured I'd see if anyone else and had similar problem and found the solution. It runs great any other time, MAF showing correct numbers, same with Fuel Trim showing normal numbers. It's only when I fill up. Maybe a fuel pressure issue somewhere.

Yours didn't originally come with VVT, that was introduced on the 5.3s in 2010.  Your trims suggest a massive vacuum leak, check the usual places and then look up how to check for leaky intake gaskets while running a graph so you can pinpoint it.  My first thought as I was reading along was bad gas but maybe something popped loose.

[CamGTP]

Lean fuel trims should show positive values, was that just a mis-type?

 

Like +25 means it's adding in fuel to compensate, negative means it's rich.

 

I'd check fuel pressure for the hell of it too.