reduced fuel mileage

[H8 PVMT]

I have a 2000 Yukon with the 5.3 and recently I've seen a large drop in fuel mileage and I'm not sure why. recently I've changed the plugs/wires, cleaned the air filter (K&N) cleaned the throttle body and replaced the gasket, I've cleaned the Mass Air Flow Sensor, I've replaced the O2 sensors (all 4 of them). I've replaced the fuel pump and filter as well. The only thing that I have't done ever is replace the cats and muffler (275k on them).

Any ideas? When I say it's dropped I used to get about 290 miles to the tank - 20 gallons, now i'm around 240 I know it's not great on mileage but what the heck.

Thanks in advance for input.

 

[iamdub]

Has this been on one tank or a few? My point is maybe you got a bad tank of gas.

Did this happen right after cleaning the air filter or did you do this in an attempt to resolve it?

Can you tell any differences in the way it runs versus when it was getting the better mileage?

 

[AppocAlexx]

it is getting to be summer, but people always forget about the environment when it comes to fuel mileage. That stuff can change it up quite a bit.


Could also check tires, low pressure bogs down the truck causing bad fuel mileage, even one can make a difference. Also could check into suspension, bad parts going out will cause drag as well. But really the list can go on and on.

 

[iamdub]

^^^ All good points. I immediately saw at least 1MPG more by upping my tire pressure 3psi. Judging by my tire's wear pattern, this is perfectly fine and maybe even beneficial in that sense. It may have helped more than that 1 MPG, but it's hard to be accurate when my gf drives it more than I do and she fills up at different stations. I use the same station (ethanol-free) and even try to use the same pump (so it clicks off at the same point) to eliminate as many variables as possible.

 

[H8 PVMT]

Has this been on one tank or a few? My point is maybe you got a bad tank of gas.

Did this happen right after cleaning the air filter or did you do this in an attempt to resolve it?

Can you tell any differences in the way it runs versus when it was getting the better mileage?

It's been a couple tanks and it didn't correlate with the cleaning of the filter. I will say that it seems to be bogged down a bit, under power. But, to be fair, I've been driving a new to me Acura so it's hard to tell.

 

[Danny3737]

Are you sure you haven't been accelerating a little faster from a stop because the Acura accelerates faster? That could easily be the difference.
Things as simple as hitting a pothole knocking the front end out of alignment or a dragging caliper will do the same thing

 

[H8 PVMT]

Are you sure you haven't been accelerating a little faster from a stop because the Acura accelerates faster? That could easily be the difference.
Things as simple as hitting a pothole knocking the front end out of alignment or a dragging caliper will do the same thing

Yea I'm pretty sure, my wife drives it 99% of the time. I did put my summer wheels/tires on it and was going to take it in for an alignment.

 

[iamdub]

Sounds like you've pretty well covered the usual suspects. Your cat converter(s) may very well be clogged. No SES light? No hot brake smell after driving? Trans shifting normally?

 

[H8 PVMT]

SES comes on P0171 & 172 which is why I changed the O2 sensors. No smell from brakes and trans seems to be shifting normally. The cats have 275k on them... maybe they are clogged?

 

[iamdub]

Those codes are both for Bank 1, one is rich and the other lean. Still getting the same codes after replacing the O2 sensor or no more code(s)?

If the original sensors weren't bad, then they were accurately reporting that the exhaust is actually rich or lean. But, maybe a sensor was bad and reporting that the exhaust was lean. So the PCM unnecessarily richened the fuel mixture which sooted up the 275K-mile cat converter and the O2 sensor. The cat was then clogged and the sensor is partially choked with soot so now it's reading the exhaust as lean... Just a guess on a plausible story.

The conflicting codes both being for Bank 1 intrigues me.