92 grand prix mileage...tranny possibly?

[92grandprixx]

I have been having issues with fuel mileage with my car we'll I replaced the o2 sensor... am now getting 23.72 now thats town or highway doesnt matter....I am wondering if its tranny related? The car doesnt always seem to go into full lock up...whats your rpms going down the highway at 65? Mine are right around 2-2100...Does that sound about right?...also it seems to slip down into drive alful easy if u just let off the gas a little It just doesnt seem right the car has 149000 miles.... any help here is greatly appreciated!!

[richtazz]

Your RPM at 65 is normal, so it's not a torque converter slipping. When you say around town or highway, do you mean you took it on a long drive and got the same mileage on a steady cruise for a 1/2 tank or more as you do in normal combined driving?

[StevePT]

I think you're a little too paranoid. It doesn't sound like there's any issues with your car. 23mpg with hwy/city driving isn't bad this time of the year. Your car takes a lot longer to warm up, you're running oxygenated fuel and the environmental conditions (water, wind, etc.) all add to lower fuel economy.

The thing to do is go for a road trip, lock the cruise at 60mph and then fill up once you've used at least 1/2 a tank. If in straight cruising you're getting up around 28-31mpg, then the car is running well.

[92grandprixx]

Steve you could be right!!.Afterall it is winter,lol well kinda ...very weird winter here in wi.I know it bothers me tho as i drive to watch the gauge drop almost effortlessly...,But Maybe thats normal too.I know the guage likes to move around alot too as the fuel gets lower is that normal? or is that a sending problem?..I havent taken it a on a long drive yet,but will sometime here i suppose to just see if i am just being overly concerned!

[StevePT]

Don't worry about how gas needle moves. Mine 97GP will stay up around the 3/4 mark for about 100 miles, drop to the half mark by about 150 miles, drop to the 1/4 mark at around 250 miles and then my light goes on right at around 300 miles. This time of the year I've been getting the low fuel light at around 280 miles.

If the thing is going up and down though, then you might have a sending unit problem but there's no reason to worry if it just moves down at different rates throughout the range.

The only thing you should care about is how many miles you drive and how many gallons it takes to fill it up. Also, for the most accurate readings always stick the nozzle in the same distance and when it clicks off, don't round off your price. Use a little notepad and record the miles on the trip meter and how many gallons it took to fill it up.

[92grandprixx]

Steve...Thanks .. Right now I am at a little over a 1/4 tank and only have 216 miles !....Also I noticed that as it gets closer to 1/2 tank and below the gauge moves around alot driving from take off etc?...you think that could be the sender?Either way right now if it goes to 260 and it takes the 14 to fill it again thats only 18.5 miles a gallon! which isnt good enough as far as I am concerned...I havent replaced that cat.. maybe I would be money ahead to hook it to a scanner?

[maxwedge]

Mileage is severely reduced if is cold out, especially in short trip driving.

[92grandprixx]

I agree,but I drive 15 miles to work its all highway..I just think something is not right is all..... thanks for your reply

[StevePT]

Definitely should be doing better than 18mpg even this time of the year if you have a 15 mile drive to work each way.

It would be worth it to invest in a scanner to check realtime data and see how your sensors are reading, make sure the thing is properly switching to closed loop mode, etc.

If you have a laptop there are many inexpensive scan tool systems that interface with laptops. Or if you don't have a laptop I'd look into something like an Actron CP9145. They go for about $150-200 on EBay.

[92grandprixx]

Update...
filled up car last night took 14.04 gallons...and had 292 miles...works out to 20.79....is that ok?I wish i had a big trip coming up to see how it would do all highway!

[StevePT]

That's about right on especially for this time of the year. You're going to be getting about 3mpg less just because of the fuel, warm up time, road conditions, etc.

I got about the same mpg number the other night when I filled up and my drive consists of 25 miles each way to work, mostly highway with a bit of stop and go on the highway and I have to push some hills on the way back home.

[92grandprixx]

Thanks Steve...So what year do you have? Another question for ya...my tranny when its cold will not go into lockup till the car heats up some....Should I try a filter change ?Seems like that would help it cause once its warmed up it seems to shift fine?and go into that final lockup phase