2005 Silverado 1500 5.3L Loss of Performance

[DooWap]

Alright so I have a 2005 Silverado with the 5.3L at around 128K miles that just doesn't have the power that it probably should have. I've seen my gas mileage decrease over the past year by a couple MPG and acceleration has been suffering as well. When towing my boat (maybe a bit over 3000 lbs) you can really feel the lack of power. Poor acceleration and struggling to make it up hills. Between 45-55 mph you can even engine almost stuttering a bit. I put in new AC Delco plugs at 100K and I'm not getting any trouble codes except for an annoying EVAP code that I've been told by the mechanic isn't worth spending money on to fix.

The shop has told me that they can't find anything wrong and without the codes it's hard to diagnose. I've had my buddy tow my boat with his Tahoe with the same engine and similar miles on some long trips already this year and he will get at least 3 mpg better than me and he doesn't have any issues with acceleration or hills. It's starting to really bother me as the truck is in really good shape and should be running a lot better than this even at this mileage. I'd say it's all in my head if I didn't see better performance out of a similar vehicle.

So, anyone have any ideas where to get started? Keep driving it until something fails or I do get a code? I just really don't like sucking down more fuel that I need to or having the truck downshift and the RPM skyrocket to 4500 RPM to get up a modest hill.

[Jeremy airjer W]

Dirty MAF, Lazy Upstream O2 sensors.

Should be fairly easy to see what the fuel trims are doing when your driving. If fuel trims are in the higher end of + or - 10% that might be a clue. Also looking to see what the calculated engine load at wide open throttle might also give some clues. It should be more than 85% if it is not the MAF sensor may be giving the wrong readings.

[DooWap]

Great, thanks for the advice. I'll pull the MAF tonight and spray it down with some MAF cleaner and see if that makes a difference. If not I'll try and get it on a scanner and see what the fuel trims are doing.

[BoxMN]

Just curious, and this is nonmechanic question, but what is "fuel trims"? Thanks!

[Jeremy airjer W]

There are long term and short term fuel trims. They are the adjusted fuel value based on all the inputs the computer gets. Short term are what the fuel system is currently doing and long term is the value over time.

lets say an O2 sensor is biased lean. short term fuel trims will tend to creep up to the +10% or more if it is serious enough. Normal short term trims will vary + or - 5% roughly. Once fuel trims creep up above 10% or Below 10% the computer will typically flag a code for rich or lean fuel trims.

A vacuum leak will make fuel trims go positive. A leaking fuel pressure regulator will make fuel trims go negative.

The two examples that come to mind is an old GMC van with CPI fuel injection. The fuel injectors where not adding enough Fuel. Fuel trims at wide open throtle where at +24% or more. A new CPI unit and fuel trims where back to normal.

The other was on a '95 1/2 Tacoma. The upstream O2 was stuck lean. No codes for the O2 sensor but it had flagged a lean bank 1 code. Fuel trims Where at +12% give or take. I tried every trick to get the O2 to switch lean. A new O2 sensor brought fuel trims back to normal.

Fuel trims are a great piece of info when your trying to put the puzzle together.

Does that help explain it?

[BoxMN]

Yep, I gotcha now. Totally new terms for me, but I grasp the idea thanks!

[DooWap]

Well I cleaned the MAF and it didn't seem to help much. I haven't had a chance to hook it up to a scanner and check fuel trims. I wish I could get my hands on one to take for the weekend when I tow my boat up north, it would be nice to see what's going on with the long drive.