2001 Monte Carlo SS VIN K.

Getting really bad gas mileage. When I bought this it had some issues and I knew it was going to be a "project" car going into it. So far I have repaired a vacuum leak, new front and rear O2 sensors (A/C Delco) and a new Catalytic converter (Evan/Fischer).

I can't actually define the gas mileage 'cause to be quite honest I can't afford to fill this damn thing up LOL. I drive around town during the week with about a 6 mile trip to college back and forth. I seem to be using about ½ a tank of gas after only a few days. Which in my estimation is waaaaay too much. I can literally see the gas gauge move from one day to the next with just a simple trip to the college and back (12 miles round trip). My wife's Trailblazer doesn't get this bad of gas mileage LOL.

So there is no CEL on. What else can lead to horrible gas mileage like this without throwing the CEL? I have checked the tire pressure, cleaned the MAF, switched to 5W/30 motor oil in an effort to squeeze more gas mileage out but nothing seems to work. The overhead display always tells me the same thing 27.6 MPG and the "instant" has not worked since I bought the car so I don't think that 27.6 MPG is accurate.

Any suggestions on what to check or change? Fuel filter? Fuel pressure regulator maybe?

 

You have no true basis for knowing if it's good or bad mileage by not filling the tank and calculating it.
Reason: The gas gauge is tuned per the PCM to only move the at certain levels. If you fill the tank 100%, the needle on mine does not start moving until I drove almost 50 miles (give or take), then once it begins to move, it goes a LOT faster. Per what I've seen in the PCM, it can sit on E for about as long as it sits on F (which accounts for the low fuel light coming on and you still have about 30-50 miles depending on your driving style.
So trying to say your MPG is bad based on how much you put in and the needle movement when you have no idea how the needle moves from full across to empty OR calculating your MPG, you won't get anywhere.

Fill it up, then calculate it.

 

FYI - Helpful tips with high mileage cars and fuel efficiency:
- Change plugs (AC Delco or NGK, NO BOSCH) and wires
- Change fuel filter
- Change Air filter
- Change O2 sensors (you've already done that).

 

Jason is right. In order to properly gauge your gas mileage you need to fill your tank and set your Trip gauge and your MPG, Average Speed, to Zero. At about a quarter tank or less, fill the tank up again, take how many miles you have driven and divide that by the gallons of fuel it took to fill it up. Do that 3 times, add those totals and divide your answer by 3. This will be your Combined Average Gas Mileage. Now you know you if you are getting good or bad mileage. Remember, since most of your driving is Local your mileage is going to be lower. But at least then you will know.
Once you have that number you can use it to predicta problem or to see if a mod you did gave you better mileage, aka made you engine run more efficiently!

 

Makes sense gentlemen. Actually maybe I am getting "OK" mileage as mine does the same thing as The_Maniac's in that when I do fill it up it has set on full for a loooong time and then starts dropping drastically around ¾ to ½ a tank or so.

And as far as resetting the "MPG" and "average speed" to 0 how do you do that? I see a reset button on the Home Link module but it doesn't seem to do much. If I hold it down I think I got it to reset a couple times but then it like counts up to 50+ MPG and then down to like 10 or so and seems to be all over the place. Is that the way its supposed to work?

So I'll fill it up and try that but how in the hell do I reset the "MPG" and "average speed" indicators on the Home Link which I'm assuming you are referring to?

Thanks guys.

 

You MPG and Speed are Averages. And they are not really accurate till you get to about half a tank. The one to really pay attention to is your Range. It automatically when you fill up. But it uses the MPG, Fuel Used and Odometer to calculate it. That's why you should reset Everything. And Holding the RESET Button will reset Each of them Individually!
I get from 22 to 24 MPG average. Not bad for an 11 year only car with 175,000 miles on it and an Old Man driving who Doesn't always drive The Speed Limit!

 

OK so I reset the MPG which was reading 27.6, I reset the fuel used which was on 104 and the Range which was on like 314 I think? But the one that reads "inst MPG" reads -E- for some reason? Is that some kind of error or something? Is that one important and if it is how do I get it to read? Its read -E- since I bought this used.

Also how do I use the Range reading? What exactly is that for?

 

The Instant MPG doesn't work in my car either. I wouldn't worry about it. As long as you could reset your others. I use me average MPG all the time. To the Point that I trust it. It is actually one of the reasons I bought the car. I had it in my BMW and don't want to live without it. It is a good toll for knowing your gas mileage and how efficient your engine is running.

 

So can I rely then on this MPG reading of 27.6 as being accurate? Or after ½ a tank I see you are saying?

I'm coming from a '96 Olds Cutlass that had a 3.1 in it and I'm just used to the way that gas gauge used to read. I drove it for so long that I know about what my gas consumption should be for common travels I make on a daily basis. Maybe I'm over analyzing things here on the Monte who knows?

 

One rule of thumb that I use on a car that would get good gas mileage is to check the RPM at your drive speed. If you are out on the interstate and doing 75 miles per hour and your RPM's are below 2K in overdrive chances are the vehicle will get better mileage. If your at 2400 or more not so good. Usually the performance cars the SS and those will be geared different.