was getting 40 mpg...now suddenly only get 25

[khartley]

Yeah, I was getting 40-45 mpg highway, and then instantly it went down to 25. Anyone that has any ideas let me know. It's for my 95 GS.

Thanks.

 

[Old Mitsu Tech]

Sudden problems usually smack of sensor related issues.

Check the following.
Temp sensor
Air filter
Map sensor
O2 sensor

Air filters are usually overlooked. However they usually cause problems slowly increasing over time.

You can to to autozone to have your vehicle scanned for codes. Even though your light may not be on, there could be a code stored.

I would look very close at the oxygen sensor. If it reports back to the pcm as a lean condition, and it does not have the ability to change. "lazy o2 sensor" The pcm will try to compensate by richening the mixture.

Remember that the pcm is stupid. It always believes what the sensors tell it. Even if the sensors are wrong.

Good Luck

 

[c5chris]

A drastic change like that seems like an O2 sensor. A dirty intake will result on a gradual loss of mpgs.

 

[TURFNNSURF]

That's some incredible fuel economy for a 91 DSM turbo car. Maybe you were runnign very lean beacuse that MPG seems very high. My 91 GSX gets 21 mpg city or highway all the time. When it was brand new that was all it was expected to get. Here is a picture of my original window sticker.

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[c5chris]

It's a GS, usually breaks the 30 mpg mark.

40-45 hwy, I agree it's a little high. It's rated 22 city/32 hwy.

 

[TURFNNSURF]

Sorry my mistake, I didn't scroll over to the right far enough to see it was a 95 GS. I thought it was about the TSi. In that case I agree that the O2 sensor may have gone bad.

 

[Scrubulum]

I agree with everyone else. Bad O2 sensor. They go bad and default to running rich. Last time one went bad on me it flooded my car all the time.

 

[a_scobel]

40-45 mpg?? WTF

 

[Mike Marnik]

yup o2 sensor. Mine went bad and i got crap for gas milage. Its an easy fix especailly for a nt (easier to get out). good luck.

 

[onepimpclipse]

Let's not forget the N/T's have two O2 sensors. Change them both.

 

[xveganxcowboyx]

Let's not forget the N/T's have two O2 sensors. Change them both.

Or whichever one is bad...

 

[Defiant]

Let's not forget the N/T's have two O2 sensors. Change them both.

Only the front will affect mileage. The rear primarily tests the cataclysmic diverter.

 

[gthompson97]

you better consider yourself a lucky SOB for getting 40-45 mpg. i mean, i agree with the low to mid 30's, but 40-45!!! that's insane.

i'm jealous WTF

 

[onepimpclipse]

Only the front will affect mileage. The rear primarily tests the cataclysmic diverter.

I wasn't %100 sure about how much the rear affected the mileage if any. Thanks for setting me straight Defiant.

 

[khartley]

Hey, thanks for the help so far guys. I agree with the o2 sensor...that must be the problem. I wasn't sure which to replace though, but my guess would be the front one since it doesn't seem possible that the rear would affect gas mileage.

Yeah, I didn't believe the mileage I was getting either, but I calculated it over and over and that's what it gets, even with over 150K miles on it. But I removed the power steering/air conditioning belt too, which improved my mileage a lot.

 

[khartley]

Um, ok I changed the front o2 sensor with a new Bosch one....and it did nothing. My car is still sucking down the gas. Even letting it idle for 15 minutes the gas gauge will have dropped a noticeable amount.

Any other ideas? My AWD is getting better mileage than my n/t right now this is NOT RIGHT!!!!

 

[negativezero27]

is black smoking coming out of your exhaust?

 

[khartley]

is black smoking coming out of your exhaust?

nope

 

[xveganxcowboyx]

Test the voltage on the sensor to make sure you did it right.

Also, did you check the air filter?


*edit* Also, how long have you had the car? If you've only had it this summer/fall, remember that cars get worse mileage in the winter. Not usually 15mpg, but it can be a substantial difference.

 

[cb3016]

could be an EGR problem, it takes exhuast emissions and dumps it back into your engine so less air and fuel get in, if its not working you would be using more fuel then normally.
-CB

 

[Scrubulum]

I would look for a gas leak if it wasn't the O2 sensor or the EGR. Timing could also affect MPG. Check ignition and cam timing. Although I think you would have more problems if it is either of them.

 

[khartley]

I don't think it would be a gas leak, cause wouldn't my fuel level drop when the car wasn't running as well? What are the chances that my cat is clogged? I've had this car for over a year, it's got about 152K miles on it now. Air filter is a cheapo but it's relatively clean. Plugs and wires are nothing special but they are in good shape. And this sudden gas-chugging happened suddenly, while I was on a road trip. I checked it today, and I'm getting about 22 mpg highway, and it really sucks it down at an idle.

So I can check a few more of the things you guys suggested, and see what I find.

 

[xveganxcowboyx]

If the gas leak was in the fuel line somewhere it would leak while the car was on and pressure was up.

I can't remember if it's been suggested, but do you have a logger? Can you check to see if you're running rich?

A clogged cat gets pretty obvious as it gets worse. Your power will drop drastically if it's severely clogged.

 

[ninjai_ruken]

When I 1st bought my GS I could fill up before leaving and travel to Rapid City SD and back and still have a half a tank when I got home. Now I never figured up the MPG and I'm not now because I am feeling lethargic right now but Rapid is 240 miles away from were I currently live. So you do the math, I would say it is your O2 same thing happend to me just before I rebuild my motor and made her the 420A-T

 

[Scrubulum]

He said he already changed the O2 sensor. Chances are it isn't that anymore.
Regarding the gas leak, xveganxcowboyx explained what I was thinking. If it was in one of your main lines or return lines (and not the gas tank) then it would only leak when you have the ignition on (and thus the fuel pump on). To test just turn your ignition to the run position and let it sit for a while. If it is anything major you should see a puddle pretty quickly.
Clogged cat is possible but as xveganxcowboyx said, you should notice a loss in power, and it should throw a CEL because the second O2 senor reading would be off.
If you haven't already make sure to check/replace the fuel filter, air filter, plugs and wires.