hi everybody...i'm new to this forum and would be appreciated if anyone would have any insight on this. Recently I had my CEL light on and took it to an auto repair shop and they found out that it was because of the knock sensor. And finally it was replaced. But then the gas mileage dropped dramatically from 19 mpg (city) before it was replaced to around 12 mpg (city) after it was replaced. I don't have a clue of what might go wrong. Do you think they gave me a bad/second hand knock sensor to made a fool out of me or is it related to some possible adjustment of the ECU with the new knock sensor (if it's new)? Or any other possible explanation...hope that I can learn something from you guys here, thanks!

 

Dont know a reason why the mileage would drop like that. Do you have a check engine light on?

 

The CEL light is not on anymore. The mileage just dropped from heaven to hell. I have been using 93 octane all the time....I just can't think of other reason to explain this. Any further insight? Would a bad quality knock sensor results in something like this? This is crazy...

 

Mmm, take it back to the shop??

 

I'll take it in to that shop to ask them, although it's been more than two weeks since they replaced the sensor. Thanks for you guy's input.

 

I just dont know how or why that sensor would have any thing to do with mileage.

 

That sensor could affect timing. If it were always retarding timing you would lose power, and that would cause you to get on the gas more?

 

Yea its possible, BUT you should also notice a difference in the way it drove. If it was throwing it off that much to be using that much gas you should be smelling that its running really rich and should notice the power difference and responce, plus if it is really running that rich to be using that much fuel the cat wont last long and will be throwing more codes soon.

 

So today I took my car back to that auto shop and asked them, they said they also don't know why. They kept saying that they replaced the sensor with a new one (I was suspecting that they put a used sensor...) and reset the computer after the replacement and the CEL was no longer on like it's supposed to; if something is wrong with the sensor or its wiring, the CEL light would have stayed on. This is what they said. Looks like they implied that they have done their job and just have no idea why the gas mileage would drop like that.
By the way fsttyms1, I do notice loss of power on top of loss of gas mileage. It's like when I step on the gas, it just won't go as fast as it used to. I'm totally lost on this instance man...half gas mileage = double $$$ spent on gas = damn...

 

So is the new sensor really messing up my car now like you guys mentioned? A new sensor won't do something like this right? God hopefully not...

 

Honestly, I think you should just pay 90 bucks and have the dealership diagnose it. They can stream live data to see if the knock sensor is indeed causing a timing retard.

My guess is that the shop messed something up. Maybe they don't even realize it, but a simple new sensor installation shouldn't cause a problem like this, unless the sensor is defective or incompatible.

 

I'll consider your opinion, thanks. I think if the dealer can verify it's because of the new sensor then I can show the proof to that auto shop and have them fix it again, hopefully..

 

I agree with this. The dealer has real time diagnostic equipement and can see whats causing it

 

Just want to post a follow-up. About two days before I was to take the car to the dealership to have a check, I think the gas mileage is suddenly go back to normal by how I feel about the driving (power was restored and the gas won't "gone so quick"). So I will give it one more period of time to observe my car; hopefully the problem was solved (actually I didn't do anything, it seemed like it just became normal, so the new knock sensor needs soem time to warm up or something (scratching head..)) Yet I want to thank you guys for opinions; this forum is so nice and informative!