Going the wrong way
 

I have a problem. 8 of my last 10 tanks have been under 50 mpg, and those 10 tanks average 2.6 mpg worse than average. They've brought my average mpg down from 49.6 to 49.3 mpg.

I changed the air filter and checked spark plug gaps. New tires may contribute to the low mileage, and I was fighting headwinds on several of the recent tanks, but the trend is worrisome. I'm gong to replace the front O2 sensor to see if it helps.

Suggestions?

Date..........mi...gal......mpg
2013-01-29 268.7 6.05 44.41322314
2013-01-30 364.5 7.13 51.12201964
2013-04-13 460.6 9.42 48.89596603
2013-05-02 422.2 8.771 48.13590241
2013-05-02 321.2 7.464 43.03322615
2013-05-03 240.6 5.156 46.66408068
2013-05-06 328.5 7.3 45
2013-05-06 119.7 2.837 42.19245682
2013-05-07 356.3 6.913 51.54057573
2013-05-07 341.3 7.431 45.92921545
0
............... 3223.6 68.472 47.07909803
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New tires DO initially have higher RR, but they should've "broken-in" by now.

How many miles on her Sentra?

regards
Mech

New tires and air filter went on at the end of April. I checked spark plug gaps yesterday. They were all within specs. The car has about 88000 miles on the odometer.

Here's a possible smoking gun. The emissions drive cycle readiness flag hasn't been set up since last year. The O2 sensor readiness flag is set, but it went off briefly at the end of January during a trip to Colorado.

Rather than replacing a possibly good O2 sensor, put one of these bad boys together.

DIY Air / Fuel Ratio Meter

Get it in closed loop (i.e. > 1800 RPM, steady throttle) and the O2 bar should cycle on / off quickly.
I've been using that and a vacuum gauge to get better FE out of the Lancer. Finding the wires and tapping onto them carefully is the hard part.

check egr also

My Ultra-Gauge has an Air/Fuel ratio display, IIRC. If not, it has direct O2 sensor Bank 1 1 and O2 sensor Bank 1 2 voltage displays. I'll check them the next time I drive the car.

The voltage is 'at the time of the read'- ODBII is 10400bps.
I'm not sure you'll see the voltage cycle like it should over ODBII.

It continuously goes up to 0.8, and down to 0.2, repeatedly whilst you maintain cruise (i.e. 1800RPM or more, steady throttle, closed loop operation).
At idle, my sensor stays at 0.8 and slows back to 0.2, and cycles slowly (due to volume - idle RPM is low volume).

That'll tell you the O2 sensor is working OK!
The other thing to think of is you'll get a code again if the O2 sensor is useless.
Vacuum style leaks produce O2 sensor codes (air after the AF sensor).

02 sensors get "lazy" before they fail to the point where you get a CEL. It may be worthwhile to replace one at 80K, but that is a matter or personal choice and my 17 year old truck with 128k, with what appears to be the original sensor, still seems to be working properly. I may replace it this year mostly as a preventative maintenance and to see if I can get a little better mileage, but no CEL. An EGR passageway cleaning is another good idea but again you are facing the possibility of it being an expense that is hard to justify from strictly a cost effective perspective. OBD2 kicks a code when EGR flow drops to the point where the code would be tripped. Basically I never tried to "sell' that type of preventative maintenance to most of my customers until the CEL showed the problem had reached a point where something needed to be done.

On the other hand while riding a Cab in Baltimore when Pop was having some surgery done at Johns Hopkins, I noticed that every single Cab had a CEL and ABS warning lights on and the drivers just seemed to ignore them.

regards
Mech

Nobody else thinks the emissions system might leak enough to cause my mpg loss?

I know with my cab company the drivers pull the fuses most of the time so as not to run up the mileage. When the odometer fuse gets pulled it trips the cel light.

To SentraSE-R did you change the brand of tires or could they be another gen of the same tire with more weight different compound? If your O2 was bad in a Sentra your MPG would be a lot worse I would think.

The car is a 2006 Scion xB. I did a major seasonal tire swap from P195-65/R15s (OEM are P185-60/R15) to P205-50/R17. I've run the same size 17" tires before, but the old ones were Goodyear NCT-5 (V)s, run-flats that weighed 26 lbs each. The new ones are Cooper Zeon RS3-A all seasons weighing ~22 lbs each.

Wow those run flats are some heifers, I bet it helped getting rid of those.

Have you compensated for the difference in tire height? The different height will throw the speedo off a good amount. Taller wheels will make the miles read less, shorter wheel will show more miles.

Yes, I calibrated my UG against my GPS. When the odometer shows 100 miles traveled, I've actually traveled 109.9 miles - a significant difference.

The OEM tires/wheels weighed 36 lbs each: 17 for the steelies, and 19 for the tires. The run-flats weighed 48 lbs: 22 for the wheels, and 26 for the tires. I haven't put the new wheels/tires on the scale, but they should weigh in @ 44 lbs for the combo.

The wheel and tire package seems good from what I know, but if that is the only thing you have changed recently I would still be leery of some unknown effect going on. Maybe from tire patterns or rubber compounds, or maybe side wall stiffness.

Otherwise if you haven't already check your brakes for dragging, and wheels bearings (with heavy wheel/tire packages wheel bearings don't seem to last as long), tire rods, ball joints for any play. They are all pretty easy to check for if you have a jack. Did the old tires have any strange wear that would be an indicator also.

Brake drag, intake manifold leak, clogged EGR - those are the points I would suggest to check.

Alignment was way off, so I had that done when I replaced the tires.

I'll check the EGR tomorrow. Thanks for the suggestions.

Since every vehicle I own now has brake drag, I opt for that.

My cobalt of all the weird things drags less if I high pressure power wash and wax the brake area thoroughly, no idea why this works, I am wondering if I am getting crud and rust off.

My insight I managed to reduce brake drag by installing brake friction reduction springs from napa, they don't fit quite right but have improved though not eliminated my irritating brake drag problems. (at least I don't need to drive to the floor to go 55mph in 3rd gear any time it rains and gets cold)

Cheers
Ryan

For brake drag generally all you have to do is just pick up some brake caliper grease, and wire brush and lube the two slide pins. You can usually go for a drive and when you stop put your hand on the calipers and feel if one is hotter than the next. The hot one is the culprit. It is still a good idea to lube the slide pins frequently anyways as general maintenance, and it's pretty easy to do for the novice DIYer

Unless they are rusted stuck or the no one has changed the brake fluid in ages, and rusted the caliper pistons. That takes years of neglect so don't worry about that too much with the xb

My O2 sensor Bank 1 1 readings went from .078 to .66 and did cycle repeatedly between those values.

Drove 80 miles today. Brake drums, rotors, and calipers weren't hot at all - maybe 90 degrees - so I don't think I have brake drag.

I watched the O2 sensor voltage cycle between .078 and .880 with cruise control on @50 mph. I guess I'm going to have to find the EGR next,