quickaudi:STFA

All right guys, here's a good one for ya. I usually average around 25-26 mpg since I commute 40+ miles one way to work at 65 mph. In October, I noticed that the mpg was dropping. I vagged the car and came up with an EGT code. I had the EGT sensors (both of them) replaced in December, and I went ahead and did the timing belt et al. while I was in there; this included a new air filter; plugs only have 20k on them. I also put in a cooler thermostat. My gas mileage stayed around 300 miles/tank instead of the usual 400 miles/tank, and my O2 and cat readiness codes never showed ready. I tried clearing codes (only code that came up was the EVAP code, which is expected since it has been cold), but they never showed ready. So I decided to put in a stock thermostat. This didn't help or hinder any. I'm still getting 300 miles/tank. I don't get any codes, O2, maf, nothing except the evap code from time to time.
So, I started logging everything. EGT temps are fine. Cat temps are 400-500 at cruise and 600-700 or so during WOT (I didn't run all the way up to redline.) MAF values give a max of 220 g/s, which is typical of a stock car. Now, here's where it gets weird. O2's run around .84 to .86 at cruise, but when I go WOT, they go to 0. That's right, not 1, which I would think is full rich (unless I have this backwards), but 0. (I've included some of the numbers below.) So, I'm not getting any O2 sensor problems, but that seems (to me) to indicate an O2 sensor problem. Any opinions?
Oh, and to test my theory, since I filled up last night, I haven't gone WOT at all. (For those of you who want to know, yes, I'm going into withdrawal.) My gas mileage has gone from around 300 miles/tank indicated to around 370 miles/tank or so. So, it going WOT causing my O2 sensors to go haywire, or are they haywire themselves? What would cause this: ECU, sensors, bad wiring?
To help diagnosing: stock car and stock ECU, new timing belt et al., stock thermostat, Darintake, 62xxx miles, stock O2 sensors, new EGT sensors, 20k on spark plugs, new air filter, only codes are evap codes, cat and O2 readiness codes are red.

O2S volts pre TWC O2S volts post TWC b2 pre TWC b2 post TWC
TIME
STAMP V V V V
0.84 0.86 0.865 0.855 0.86
1.75 0.86 0.86 0.855 0.86
2.65 0.86 0.87 0.85 0.86
3.56 0.865 0.88 0.855 0.865
4.46 0.87 0.885 0.86 0.875
5.37 0.87 0.885 0.86 0.875
6.28 0.875 0.89 0.865 0.88
7.19 0.835 0.89 0.84 0.9
8.08 0.805 0.835 0.71 0.85
8.99 0.005 0.13 0.005 0.3
9.9 0 0.005 0 0
10.9 0 0.005 0 0
11.8 0 0 0 0
12.71 0 0 0 0
13.62 0 0.005 0 0
14.52 0 0 0 0
15.42 0 0 0 0
16.33 0 0 0 0
17.24 0.03 0.005 0.025 0
18.14 0.835 0.84 0.815 0.83
19.06 0.86 0.88 0.85 0.87
19.95 0.86 0.875 0.855 0.875
20.87 0.86 0.88 0.855 0.875
21.8 0.865 0.885 0.86 0.88
22.77 0.86 0.885 0.86 0.88
23.8 0.895 0.915 0.885 0.91

reefer_bob

You'd think something like that would throw code after code...

davenew

I was with you 100% until scrolling down enough to see your sig.. what'd you say again?

quickaudi:STFA

just to see what would happen. Of course, the O2s will have to be replaced eventually, and the signs (if not the codes) seem to point towards them...

quickaudi:STFA

gets 'em every time.

Mirror|rorriM

Have you checked your tire pressures?

quickaudi:STFA

Besides, I don't see that dropping my mpg from 26 to 20. I'd have to have my brakes locked up or something...

Mirror|rorriM

But you said they're inflated right, so that eliminates them as a possible cause.

quickaudi:STFA

What was the psi difference of the tires?

Mirror|rorriM

The difference this makes will vary by tire and vehicle.