well i did everything in the stickies and from other posts about this and i know its old to some but my mileage just started to suffer even more. at first after i changed the oil, fuel and air filters, cleaned tb, checked tire pressure i was getting about 330 miles to a tank. i wasnt complaining at all then but all of a sudden within the past month my car is only getting about 250 miles to a tank. i always use 93 octane and still drive the same as before. i really noticed a boost in mpg when i changed the faulty O2 sensor but now its worse than ever. i do have a cel which is telling me a bad knock sensor but i didnt think that would hurt the mileage that bad. between that and what feels like my tranny slipping (automatic)which just started does anyone think that there could be something esle or could these two problems be killing my mileage that much. sorry for the long post but with high gas prices i need help fast.

 

A knock sensor will NOT turn on the CEL so something else is wrong... You probably have to replace both of the front O2s and KS

 

When was the last time the spark plugs and ATF were changed?

 

You just answered your own post. Check Spark plugs also they can have a serious affect. NGK BKR5E11 about $2 a piece.

 

my knock sensor dropped me from 19 mpg in the coldest of winters to under 14 mpg. you problaby have another bad O2 sensor as well in the front somewhere. if both the KS and the O2 is bad, then itll explain your awful mileage.

 

You obviously didnt do EVERYTHING. You couldnt have done that much research or good reading in the stickies if you couldnt tell how much a faulty knock sensor would affect you're gas mileage. Change it out, and watch the fuel needle stay afloat.

 

are you sure??? b/c a day after i bought my car my engine light came on and i took it back to the dealership and they say it was a knock sensor and replaced it for me

 

thanks for the replies, i guess i should check the plugs and atf. my cel used to be on and when i pulled the codes it pointed to a faulty front right O2 sensor and that was it. replaced that, cel went away for a bit and came back on, now i get a code for the knock sensor (0304) and another code that is undefined (0706). my mileage was good for awhile after the changes but not anymore. as for the knock sensor there are so many different opinions on this board i didnt know what to think, i hear different from everyone so i just figured it wasnt that big a deal. anyways will do the suggestions and see how it works. thanks again

 

change the KS. with a faulty KS & front o2, I got 330 to the tank roughly. I got a new KS and now I'm getting 370+ in this ungodly heat and with the a/c on.

 

KS code is a ghost code - it does NOT turn on the CEL... There was probably another thing wrong that set off the CEL

 

so then how can i figure out what is really wrong or causing the cel to come on. like i said i get the 0304 and a 0706 code which appears to be undefined. i hate to spend the money on a new ks if that isnt even the real problem.

 

I had a cel turn on for just the knock sensor also.

 

what is your Long Term Fuel Trim at? get a scantool and find out if it is fuel related or air related problem. also whats your fuel pressure at.

 

0706 - Fuel injection system, lean or rich (right bank)

Being your getting poor gas mileage, I'd say you have a leaky injector causing you to run rich. The rich condition is probably causing some engine ping, detected by the KS which is pulling back timing and setting off the KS code.

Pull your plugs and take a good look at them. The cylinder with the leaky injector will have a dirty spark plug. Change that fuel injector, clear your error codes and see if your gas mileage improves. Check for the KS code about a week later, if it's still there replace the KS.

 

Can a bad KS cause you to get other codes as well? I have a CEL on with three codes, 1-undefined, 2-KS, 3-EVAP, I have the KS on order. Gas mileage is ~27mpg car runs fine but a little sluggish in the 2K range, I'm hoping KS cures that but I'm concerned about the other codes. I can't find a lot of write up on EVAP, don't even know where to start.

 

KS code is set off by other things... Fix the other things and reset the CEL - check the ECU for the KS code a week later (it's a ghost code - it won't turn on the CEL) to find out if the KS is bad or not...

 

okay this is a few days old but i just went out and rechecked my ecu codes. like i stated earlier i was just getting a 0305 and 0706, now i reset my ecu and my mileage went back up to about 340 miles to a tank. i decided to go ahead and recheck the codes again just for grins and now iv'e got a list of codes 0503, 0706, 0307, 0304. i just checked the stickies and looked up the codes but do yall think i should go ahead and just replace the 02 sensor for the 0307, and i am not too sure what the 0503 one was about. i just dont want to go replacing parts that dont really need to be fixed but figured the 02 sensor should be done anyways. about the other codes, anyone ever dealt with these and how much or how hard are they to fix. thanks for any help.

 

From the ECU sticky, the 0503:

0503
Diagnostic Trouble Code 0503 indicates a problem with the front Oxygen Sensor on the right cylinder bank. The right bank is also called the rear bank. It is cylinders 1,3,5. A normal signal sweeps back and forth between 0.2 volts and 0.8 volts. This malfunction was detected when the Engine Control Module received a signal from the sensor which was a constant value of approximately 0.3 volts.

Possible causes include ...
- Harness or connectors (the sensor circuit is open or shorted).
- Defective Oxygen Sensor

Replace both defective O2 sensors (0503 and 0307), reset the ECU... then drive around and check for codes again in a few days. If you still get the 0706 and 0304 - post back.