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Discussion Starter · #1 ·
I recently bought a 2000 Impala LS 3.8 with 49,000 miles. My gas milage is averaging 7 mpg. My last 2 tanks of gas lasted 128 miles and 125 miles. I drive a total of 34 miles a day round trip commuting to work. Its all highway except for the 1/4 mile drive from my house to the highway. My job is right off the highway exit. I keep the speed around 60 to 65 mph.

The computer on the car says I am getting an instant 30 to 32 mph on the highway but that is not correct. The computer also says my average mpg is 7.2. These two numbers dont add up so the computer on the car is obviously malfunctioning. I used a can of fuel system cleaner but that didnt seem to do anything.

What can be causing this horrible gas milage? My Cadillac STS is getting at least 300 miles out of a tank of gas and that is with some performance mods and not driving it easy.
 

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I recently bought a 2000 Impala LS 3.8 with 49,000 miles. My gas milage is averaging 7 mpg. My last 2 tanks of gas lasted 128 miles and 125 miles. I drive a total of 34 miles a day round trip commuting to work. Its all highway except for the 1/4 mile drive from my house to the highway. My job is right off the highway exit. I keep the speed around 60 to 65 mph.

The computer on the car says I am getting an instant 30 to 32 mph on the highway but that is not correct. The computer also says my average mpg is 7.2. These two numbers dont add up so the computer on the car is obviously malfunctioning. I used a can of fuel system cleaner but that didnt seem to do anything.

What can be causing this horrible gas milage? My Cadillac STS is getting at least 300 miles out of a tank of gas and that is with some performance mods and not driving it easy.
fail fuel regulator possibility. Does the engine sound abnormal?

car is running rich I suggest not driving it tell you can get it fixed as YOU WILL damage the catalytic converter.
 

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Discussion Starter · #3 ·
The engine sounds and runs fine. It doesnt smell like its running rich at all at the exhaust pipe. I'll look into the fuel regulator. Thanks.
 

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The engine sounds and runs fine. It doesnt smell like its running rich at all at the exhaust pipe. I'll look into the fuel regulator. Thanks.
It's still running rich since efficiency has drop to 7MPG it's consuming three times the normal amount of fuel.


That is still more then a big rig uses with a 40ton load.. :yikes:
 

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Discussion Starter · #5 ·
That is still more then a big rig uses with a 40ton load.. :yikes:
Thats what I drive, loaded with flammable and poison compressed gasses.

Hopefully I get to the bottom of the problem. The check engine light never came on so I guess there would be no codes for a faulty part.
 

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Discussion Starter · #6 ·
I had the car checked out for all the usual problems that could cause bad gas milage. Everything checked out fine. There were no codes in the computer and the EGR valve was operating correctly. I even thought it could be a stuck brake caliper but all the wheels rotate freely. Everyone who looked at it is stumped.

I guess its time to take the car out back and put it out of its misery and put a bullet in this Impala's head. I cant use 3 gallons of gas a day driving this car to work and I cant spend hundreds of dollars buying parts just to see if it was the problem. Its a shame because the car is in excellent condition in and out.
 

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Mr. Handy
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A 00' with 50k miles and gets this horrible of gas mileage? I have no idea what is going on. You had mechanics take a look? How many miles can you get out of a full tank and running the tank empty? What are you relying on to see your fuel level? How much gas did you put in on your 2nd tank?
 

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Discussion Starter · #8 ·
The mechanic put the scanner on the car to see if there were any codes stored or active. Nothing came up. He also tested a few parts like the EGR and some emissions parts but everything was working fine. He said without any codes its a guessing game to figure out which part is causing the problem. My last fill up of gas was 15.2 gallons. I thought the gas gauge could be broken but the gauge showed about 1/8 of a tank left so that is about right for a 15 gallon fill up. I am getting 125 miles from a tank of gas. I am using 4 gallons of gas a day to drive 34 miles. Right now i am at 82 miles and a little less than half of a tank. Everyone i talk to who has mechanical knowledge cant figure it out either. I am at a loss also seeing how i have been building my own engines for my older cars for 20 years now. I guess i got screwed big time buying this car. I should have realized no one sells a mint condition 13 year old car with 49,000 miles unless something is wrong with it.
 

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Did he test the fuel pressure?
 

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Discussion Starter · #10 ·
I am not sure if he checked it but i did bring it up to him that someone suggested it could be a bad fuel pressure regulator. I gotta ask him.
 

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Mr. Handy
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How about your water temp low temp will cause bad mileage

Very true. A stuck open thermostat would cause the engine to NOT get up to operating temperature. Therefor the computer would constantly be trying to make up for cold by feeding more fuel and keeping the idle high. Both of which, would have been noticed by the mechanic, at least they should have been.
 

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Discussion Starter · #13 ·
When I bought the car the thermostat was stuck open. The temp gauge was barely getting past 140 degrees. I changed it and now it runs at normal operating temp. I was still getting no code for the thermostat or temp sensor. As far as the fuel pressure regulator, my mechanic as well as the local Chevy dealer said the oppisite would happen, meaning the car would not start or stall at higher rpm's due to lack of pressure.

I wonder if the computer got stuck in a rich fuel setting from the thermostat being stuck open. I dont know how long it was stuck open for. I will change the coolant sensor and see what happens though.
 

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LIFETIME HILLBILLY
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could it be leaking fuel? Have you smelled any fuel while driving or setting?
 

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Someone is siphoning your gas?

It would seem, to me at least, that getting 7 mpg in this type of car would also be accompanied by the various signs of running rich. Considering there are none noticed, seems we have a mystery. Can you pull a spark plug to check its condition?
 

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Discussion Starter · #18 ·
I have a locking gas cap. There is no leaks or smell of gas. I put cardboard under the whole car and let it run for a while. There was not one stain. I am changing the coolant temp sensor tomorrow and hopefully it will solve the problem.
 

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Discussion Starter · #19 ·
So 3 mechanics, one with 30 years experience and one who is a Chevy dealer mechanic cant figure it out. This is truly a mystery. I changed the coolant temp sensor and that didn't work. The spark plugs are a little dark but that is a sign of running rich. Without codes in the computer I cant keep guessing and spending more money on parts than the car is worth. The odometer shows 49,646 miles but i have a feeling its really 3 times more than that. I am taking it off the road and when i get the title from DMV I am donating it to a children's charity so they can scrap it and get the money. This car has obviously outlived its useful life span. Thank you all for your suggestions.
 

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Mr. Handy
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Tax write off!

Still not the way I would go but, if you can afford it, go for it.

Chevy dealer's are almost the last person you should listen to.
 
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