Poor Fuel Mileage, what to check?

[gpr]

I have a 2002 Silverado with 5.3 lm7 engine. In the last 6 months I have noticed the fuel mileage has gone down. When i first got the pickup i would get 15-16mpg almost all the time. Now I'm getting 12, but don't know why. It had 90,000 miles when i got it and now has 157,000 miles. It is pretty much completely stock, and I have tuned it and turned off most of the torque reduction etc...

I did a quick scan today and LTFT, maf and map seem inline. What else should i check? I pulled the intake and cleaned the maf wires. I did notice when cruising down the interstate that i got a slight bit of KR. Not sure why, and I was wondering if it was real or false knock. Attached is a screen shot of it as I had cruise set. I'm running the stock spark tables

I had serviced it up a few years ago and put new iridium plugs, MSD wires, knock sensors, fuel filter, and o2 sensors. I was thinking i could replace all of those again to service it up, but don't think that would help any as I'm not seeing signs of it missing etc...

I have noticed once in a while while idling it seems to idle maybe 200rpms higher then it should, but doesn't do this all the time, and I was thinking vaccum leak. However, there is really only two vaccum lines, one for the brake booster and one for the fuel pressure regulator and they look fine.

I was wondering what i should log to see if there is a sensor or something that would cause this? Or how would I know if maybe the intake has a leak?

[gpr]

Here is another screen shot of knock later in the log as I was cruising at 75mph. Is this real knock? what would cause this driving down the road with no load?

[joecar]

Post log file and tune.

Make sure air cleaner filter is clean/new.

Make sure catalysts are not clogged.

[gpr]

Air cleaner is fresh, and the cats.... lets just say there is no possible way they are clogged. :laugh: Other than that the exhaust is completely stock.

I pulled a trailer this morning and made another log file. It was colder this morning and i didn't see any KR this time, which is surprising with a load. I did notice the LTFT are higher. Refresh my memory, if the ltft are positive then the engine is running rich and if ltft are negative it is running lean, is that correct?

[joecar]

Yes, engine would be running lean, and the LTFT is correcting the condition by adding fuel.

Positive LTFT indicates any of:
- air leak (i.e. not accounted for in MAF and/or VE tables).
- MAF and/or VE tables are under-reporting.
- MAP sensor is not working correctly.
- IAT and/or ECT sensors are under-reporting.
- air filter clogged.
- cats clogged (or exhaust otherwise clogged).
- other possibilities.

I'll take a closer look at your files later today.

[gpr]

I would assume that if something was leaking or had an issue my LTFT would be substantially off. I did notice today that they were higher then yesterday at 6.3 and 4.7%, where yesterday log was only showing 4.7 and 2.3%.

I checked the map and key on, engine off it shows 12.3 which is normal for this elevation. Also compared its values while engine running to old logs and it was similar to years ago.

IAT seem to be correct with current weather temp, and the ECT seemed normal.

Maf looked normal as well but i don't know what a stock 5.3 should flow exactly, was comparing to old log values.

Next thing I was gonna check was pull all the plugs to see what they look like, then check the compression of each cylinder. I was also wondering if maybe the injectors could be plugged up? My dad had a 6.0 years ago that the injectors were getting plugged and not misting fuel so it actually caused a slight misfire. I don't have that but maybe there is built up on the tips?

[joecar]

The amount of LTFT depends on the size of the air leak.

Also, if you run fuel with different stoich you will see LTFT.

Also note that a misfire will cause a lean condition (unburnt oxygen) which drives LTFT positive.