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-   -   weird mpg problems (http://legacygt.com/forums/showthread.php?t=194204)

Billy Dagwood 10-05-2012 08:12 AM

weird mpg problems
 
I have an 05 LGT sedan 5MT with ~95k, normally tuned to the cobb stage 2 off-the-shelf map.

I recently took it on a road trip from Boston to Cape May, NJ -- that's about 400 miles each way. On the way, the car repeatedly threw the same several codes, as follows (although the car felt perfectly fine to drive):

P1518 - Starter Switch Circuit Low Input
P0852 - Neutral Switch Circuit High
P1153 - Front 02 Sensor Circuit Range/Performance (High) (Bank 1 Sensor 1)
P1152 - Front 02 Sensor Circuit Range/Performance (Low) (Bank 1 Sensor 1)
P0133 - Front 02 Sensor Circuit Slow Response (Bank 1 Sensor 1)

Anyway, since I was away from home, the car felt normal, and I had time pressure and commitments in NJ (I was going down there to shoot a wedding), I decided to go easy on it but still clear the codes with my AP.

When I got back to Boston I noticed that my mpg around town (city driving) was much worse than usual, ~14mpg.

I took the car in and got a new 02 sensor, as I thought that after 95k miles, that was probably the culprit. Since then, the car hasn't thrown any codes at all, and my HIGHWAY gas mileage seems weirdly high, but my CITY gas mileage is still lousy. I flashed down to the stock map (does the ECU need some time to "learn" the new 02 sensor...?). My mileage on the highway since then is still great, and my city mpg is a little better, but I'm still not convinced.

Thoughts?

Billy Dagwood 10-05-2012 02:07 PM

nobody?

Billy Dagwood 10-05-2012 09:29 PM

*tap tap* is this thing on?

BarManBean 10-05-2012 09:53 PM

How long has this been going on? 1 tank? 2 tanks?

How are you measuring highway mileage vs city mileage? Just guessing? Reseting your trip meter repeatedly?

It sounds like the O2 sensor went bad, which will make the car run rich and negatively impact MPG. Sounds like you fixed it and your overall gas mileage is likely back to normal.

Billy Dagwood 10-05-2012 10:45 PM

Yeah, I reset the trip computer repeatedly so I could monitor the two separately. I also pulled my air filter tonight and washed it out (AVO washable), so that will hopefully help a little too. I'll also check tire pressures tomorrow when it's light out. I'm still on the same tank from when the sensor was changed. Probably need gas tomorrow.

I still don't think it's back to normal, though. Does it take time to adapt to the new O2 sensor? I'll admit I'm not entirely sure how the ECU does its figuring regarding all this...

BarManBean 10-05-2012 10:47 PM

Quote:

Originally Posted by Billy Dagwood (Post 4103119)
Yeah, I reset the trip computer repeatedly so I could monitor the two separately. I also pulled my air filter tonight and washed it out (AVO washable), so that will hopefully help a little too. I'll also check tire pressures tomorrow when it's light out.

I still don't think it's back to normal, though. Does it take time to adapt to the new O2 sensor? I'll admit I'm not entirely sure how the ECU does its figuring regarding all this...


Well, resetting it constantly won't exactly give you accurate impressions of you MPG unless you are letting a decent number of miles pass each time. My city driving will cause the "average" MPG reading to fluctuate between 12mpg and 20mpg. It usually averages out to 18.6 b/c I have a shitty commute.

I guess it could take time to respond to the O2 sensor. If you really want to do a hard reset, unplug the negative battery terminal for 15+ minutes and see if that helps.

Billy Dagwood 10-06-2012 06:38 AM

I mean, I let it go for a good 20-30 mi between resets, but still, I see your point. Around town I tend to average out around the same as you, or at least I did until now. But there's a big difference between 14 and 18.

When I fill up again, I'm going to reset and leave it for the whole tank (like normal), and I'm also going to do the math out at the end of the tank to see if it's actually accurate.

iNVAR 10-07-2012 11:32 AM

you should not be running a stock map on stage 2 harware.

Billy Dagwood 10-08-2012 11:30 PM

Quote:

Originally Posted by iNVAR (Post 4104410)
you should not be running a stock map on stage 2 harware.

I think I see what you're getting at. Yesterday I got these codes:

P1518
P0852
P1153
P1152
and a new one, P0546 (EGT Sensor -- too hot)

and these ones today:

P1518
P0852
P1153
P1152
and a new one, P0402 (Catalyst System Efficiency Below Threshold (Bank 1))

I did not get P0546 again this time around.

Tomorrow I'll clear the codes one last time and flash back to stage 2. If I get more codes, I'm taking it to the shop.


Interestingly, throughout this whole time, it's been running absolutely fine, to the point that if the CEL and cruise weren't there, I'd never know there was anything wrong.


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