City economy very low

dougman

Original poster
Member
Mar 15, 2012
20
Hi everyone,
My 02 with 182K miles on it has had a gradual, but overall dramatic decrease in city fuel economy over the past year or so. The type of driving we have done hasn't changed in any dramatic way, and my driving for economy has definitely stepped up after seeing the decrease, but still seeing quite a large effect. I used to be able to see 16-17 MPG without too much difficulty. This was real, calculated MPG. I'm now down to about 10.5 or 11. Interestingly enough, my highway fuel economy is the same as it's always been - between 20 and 22 depending upon speed and terrain. I've kept up maintenance as far as fluid changes, tire air pressure, etc. I don't know what would cause such a loss of city economy without affecting highway. I did change my O2 sensors about 2 years ago - using Denso universals. Apparently the TB isn't too happy with them as I frequently get OS heater over current CELs, especially when it's cold out, or when the vehicle is started and left to idle for any length. I will change them soon to OEM Delcos, but I'm not sure that will do too much.

Other than economy, I don't notice any changes at all, except for rough idle due to bad engine mounts. This engine is a replacement (my original had the cracked sleeve issue), and has 86K on it.

Any thoughts would be appreciated. I can't afford to replace it, nor do I want to, but $60.00 fill ups every 160-180 miles is getting really annoying.
 

Wooluf1952

Member
Nov 20, 2011
2,663
Milwaukee, Wisconsin
You're getting close to new plug time. (AC Delco Iridium 41-103) Also cleaning the t-body might help. Air filter?
 

CaptainXL

Member
Dec 4, 2011
2,445
you could unplug the 02 sensor and see if it gets better mileage. Top off and run for about 1/3 tank fill it up and calculate mpgs.
 

dougman

Original poster
Member
Mar 15, 2012
20
Wooluf1952 said:
You're getting close to new plug time. (AC Delco Iridium 41-103) Also cleaning the t-body might help. Air filter?

Oh, I failed to mention that I did replace the plugs (Delco Iridium 41-103) 5K miles ago, along with my thermostat and air filter. The thermostat was opening early, so I thought that might have been it. It made no difference.
 

CaptainXL

Member
Dec 4, 2011
2,445
Then the cat is more than likely pooched if it was running like that for some time.
You said it's been gradually getting worse. More evidence of a clogged cat via bad tstat.
 

dougman

Original poster
Member
Mar 15, 2012
20
I'm afraid that could be it. I know it's not awful, because I still have really good top end power. I'll have to see if there's a muffler shop that can do a back pressure test. Last time I looked at it, I could only get a small peek at the core and what I saw looked okay.
 
Dec 19, 2011
31
Sounds like what I've got going on.. except same thing with highway. I had to stop 3 times from atlanta to destin for gas. Was getting 13.4mpg highway.

Fairly new plugs
Clean air filter
Sea foam treated the fuel system
new oil
fairly certain that the Cat has been replaced once already.
I'm going to the dealership tomorrow to get the service history print outs.

I'm getting about 180 mptank.
I tow a landscape trailer for work and have noticed it seeming sluggish more recently.
 

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