2001 GMC Yukon SLT 5.3L getting P0171, P0174, P0300

[arkhamfallout]

Hello, first time posting. My brother in-laws Yukon started getting the P0171 and P0174 codes the other day so we cleaned the MAF sensor and the throttle body as well as replaced the engine air filter but the codes came back (he cleared them). Then he got the P0300 code soon after. The car runs just fine and he feels no loss of power from before he got the codes. The only thing he did mention is that when he remote starts it on a cold start, it sounds like it struggles for a second but then it's smooth. We made sure all the intake connections were tight and secure as well. He just bought the car a few months ago and isn't sure what was done recently, I believe the fuel pump was replaced a few months ago. We are looking to replace the spark plugs this weekend to see if that helps with the misfire code but for the P0171 code would something like this help figure out where there might be a vacuum leak? https://www.harborfreight.com/fuel-pump-and-vacuum-tester-62637.html

I have a basic scan tool and was able to pull some of the live data so I will try to attach it here or include it in the post if it helps. At one point there is a spike to 2000 RPM and that was just him accelerating to see if that also helps with a different RPM data set.

Any help would be appreciated!

 

[Fless]

Welcome to the forum!

Which engine is this? (What's the 8th digit of the VIN?) Trying to determine if this is a Flex Fuel engine.

The short term fuel trims look good, but the long terms are in the teens, adding fuel. They're not totally out of whack, but they should be better.

A few suggestions:

When you buy your spark plugs, be sure to get them from a reputable source. Lately we've been seeing some counterfeit plugs (primarily from online vendors) that cause misfires and don't last more than a few thousand miles. Think about changing the wires, too, if they've been around a while.

You want the fuel pressure gauge that is for fuel injection systems -- you'll be seeing 60+ psi at the passenger fuel rail during your tests. You can likely "rent" one from your auto parts store's tool loaner program. Follow the book test with the key on, engine off process. Also read the fuel rail pressure when it's running; it should be in the low-mid 40 psi range. Say, 43-45 psi.

Will your scanner look at individual cylinder misfires? If so, take a look at those and see if one is causing the over-fueling.

Any exhaust leaks? between the heads and the cats?