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View Full Version : Proof that bad post cat o2's can mess with fuel economy..



frozenpuppy
02-27-2014, 11:04 AM
Just wanted to share this valuable information for whoever might have future problems of this kind. Today while scanning my 2.8 I got all 6 injectors showing open circuits. I cleared the codes and swapped my extra ECM in and was watching my short term fuel trim on my torque app for android, and all of a sudden my radiator fan kicked on and both trims skyrocketed from 1.3ish to 50-55! I don't know if I'm 100% correct here, but I unplugged both of the post cat sensors at the firewall (crappy ones that I bought from rockauto that I've been waiting to send back to them, both have shorted out and have been shorted for about 3 months now. After unplugging them I jumper wired the fan at the harness in front of the P/S tank and clicked it on and saw no high reading this time. Fuel economy has been slightly better ever since. My guess is that the o2's were shorting out, allowing other components to spike the circuit, also effecting the primary upstreams.

What I think it did to the fuel trim was every time the fan kicked on it was severely messing up my trims. The fan rarely kicks on, so I figure 5-10 times a week of turning on should be plenty to rob me of 5-7 mpg overall.

Yes I know im not an electrical wiz. This is simply to help others in the future so dont hate please.

What do you guys think? Time will tell whether this actually fixed anything.

Mad Cow
02-27-2014, 11:43 AM
Rear O2 sensors affect fuel trims, it even says so in the Bentley. It's more to do with compensating for front O2 sensor aging though and it can only affect it certain small percentage. That's a really weird problem you're having though, but then again anything engine related shorting out can wreak all sort of havoc. I think unplugging them disables your long-term fuel trims, so when you replace them your mileage should go up.

catbed
02-27-2014, 11:29 PM
Yep, rear o2 has an effect on fueling. What Mad Cow said is correct.

NeedingAnAudi
02-28-2014, 11:43 AM
Well yeah a bad o2 will mess with fuel economy. It's installed on the car to monitor exhaust to maintain correct emissions levels and maintain fuel consumption.

Mad Cow
02-28-2014, 11:53 AM
Well yeah a bad o2 will mess with fuel economy. It's installed on the car to monitor exhaust to maintain correct emissions levels and maintain fuel consumption.

Lots of people are still under the impression that the rear o2s only monitors cat efficiency and have zero impact on fueling.

frozenpuppy
03-02-2014, 02:34 PM
Lots of people are still under the impression that the rear o2s only monitors cat efficiency and have zero impact on fueling.
Wow! I never knew!

ray4624
03-02-2014, 02:37 PM
thats why they should be coded out, same with the evap.

Mad Cow
03-02-2014, 03:22 PM
thats why they should be coded out, same with the evap.

They serve a good purpose since they measure primary O2 sensor aging and compensate for it, only real reason to code it out is if there's just no room to attach one.

ray4624
03-02-2014, 04:10 PM
What if you arent running a cat?
State of mass says i dont need one so, i dont have one..or evap.
My tune that is going in has them removed to avoid fuel trim issues.

Mad Cow
03-02-2014, 05:34 PM
What if you arent running a cat?
State of mass says i dont need one so, i dont have one..or evap.
My tune that is going in has them removed to avoid fuel trim issues.

I'm running a test pipe with a rear O2, I have the cat aging monitoring coded out but still have the sensor. The way some tuners completely code out the rear O2 disables fuel trims, there's a proper way to do it but not everyone does it so be wary.

ray4624
03-02-2014, 06:10 PM
i assume United Motorsport did it correctly..or so i hope.

RENOxDECEPTION
03-02-2014, 10:06 PM
Interesting thread, maybe I should monitor my STFT with my current O2 config.

Avant Nate
03-02-2014, 10:52 PM
I thought that the conventional wisdom was that the rear O2 sensor only monitored CAT efficiency, so what happens to fuel trims when you use a spacer? How can it properly measure the ratios after passing through a cat on a stock? I'm having a weird fueling issue, and I wonder if this has any effect.