sup ppl i have a big mpg issue.

i filled up the other day, and 180 highway miles later i have less than half tank left. i have 114k, replaced the plugs, fuel filter, air filter, cleaned the maf sensor,
all within 10k miles ago...im cali emissions so i guess i would have to try and replace both oxygen sensors pre cat. but without a cel, would i burn that much gas from a bad or 2 bad oxygen sensors? i know i used to get better mpg

 

i have a bad o2 sensor and im getting around 250. thats on regular.

 

A lot of posters say a bad Knock Sensor will cause terrible MPG.

Have you checked yours visually?

 

? Check knock sensor 'visually' ? Just check the codes ... that's probably what's wrong with your mileage. The cheap and easy fix is the 'resistor' bypass. I've had mine in for about 3 years, and get about 30mpg hwy. Harder is to replace the sensor. Your O2s probably are OK unless you get codes for them too.

 

Well, take it out and see if it has a giant crack down the middle.

Mine was in rough shape and didn't throw a code.

Car felt great and got Much better mileage after slapping a new one in there.

 

The knock sensor doesn't set off a CEL, just a 'ghost code' when you read them.

 

i dont have any codes. although a few months ago it mysteriously turned on, for the o2 and ks, and my boy used scantool to delete the codes, and it hasnt came back on ever since. what is this resistor bypass your talking about? i've heard about it but what size resistor, and what wire(s) does it get tapped in line with?

 

I would be most interested in the "resistor bypass" as well...??? My hands are bit too big to get at the knock sensor without removing the intake.

thanks.

 

resistor bypass? just replace the ks, it can be had for 30 bucks or so.

 

resistor bypass not for the ks, but the o2's

 

ttt?

 

I need to disconnect that 02 sensor on the rear side of the catalytic converter. Does anyone know how to do this? I've already got most of the old muffler system removed. I still need to remove the old cat, but it is "attached" to this sensor.

I'm trying to do this today (October 16th), so any quick advice would be much appreciated.


DISCLAIMER: I've already searched the "Noobies read this first stuff" and did a general search for catalytic converters on the forums and haven't had any luck.

Thanks!

Jeremy
(94 Maxima GXE w/240,000 miles)

 

Bad front O2 sensors would trigger a CEL and you would be running like crap. Bad rear sensors would trigger a CEL, but don't affect performance or emissions. Bad ks WILL NOT trigger CEL and will be evident by decreased economy and poor performance. Change you KS, or replace with 470,000 Ohm (470K) resistor.

There is no resistor bypass for the O2. The O2 signal has to vary and change, while showing a lower signal than the primaries. There are circuitry units called simulators, or you can use the spark plug anti-fowlers, but they still require a good sensor.

The resistor you heard about was for the KS.

 

damn i didnt know a ks could cause that bad fuel economy then from what your saying...i barely got 200 miles from full to empty

 

i did the ks 470k ohm resistor bypass today. i disconnected it at the subharness and soldered them in line with each other and taped them up for a factory look, and holy **** i immediately felt a power gain. i hope i can regain my decent mpg again...thanks guys!

 

I've also noticed my mpg go down to about 16.2 mpg from 19 mpg in the city and no codes.
Will check KS. =) thanks for the info.

FYI it is fairly easy to replace the KS, if you have small hands it's a 20 minute job and the KS is like $10 off of ebay. There is a write up with pictures and everything somewhere on the forum.

 

i seen it and its not something i have the patience to do lol. i honestly looked and was like...**** that ill just add a resistor lol

 

If I understand correctly... the KS is a piezoelectric device that generates a voltage when the engine vibrates. The ECU uses this voltage (feedback) to adjust the timing... yes?

If that's so... and I'm not questioning the results from those who have made this mod... but am curious... how does a fixed resistor generate the necessary voltage (feedback) for the ECU? or is the KS/ECU relationship different than what I have read?

Thanks...

(my hands are too big to change the KS without tearing into the intakes, so the resistor mod is quite appealing but would like to know how it works before I give it a try)

 

With a resistor the ECU will see a good resistance value and believe there is a god KS installed. Voltage is only generated in the presence of detonation, therefore the ECM will think you have a good KS, but wont detect a knock. this is why you need to run premium fuel.

 

Not that either of these posts were related but just wanted to highlight what asand1 said for anyone considering the KS resistor mod. Be sure to run 91 octane or above.