Pup99

So recently my impalas fuel millage dropped off. Down in the 19-20 mpg range. I started looking at things for what changed. I noticed the rpms seemed high, I thought I was told by a friend with one if I kept the tach around 2100 on the interstate I would get good gas millage, now at 70mph it reads 3000. I took the car in for it and they decided it needed a new catalytic converter. This didn't change the 3000rpm but new if you try to speed up to pass someone it down shifts and it's up around 5000rpm. The car didn't have any trouble codes either.
So what I am wondering is what other people are seeing in the tach around 70, or any other ideas that I am missing.

cleveland63b

fuel mileage can be effected by a lot of things. When was the last time you did spark plugs? fuel filter? checked tires and tire pressures? Using the same gas station?

Pup99

Plugs and wires have less than 30000 miles on them, never changed the fuel filter, but the car only has 68800 miles on it. The motor runs great. All my gas comes from the same stations that are owned by dining I work for. I am more worried about the transmission at this point,I don't remember the tac being so high on the interstate before.

tech2

you needed a fuel filter 30 000 miles ago. they should be changed every 30 000 miles.



what engine do you have?



I will check the rpm at 70 with my 2004. Mine has a different final drive so mine may be higher. 3000 seems high.


is the trans shifting into all 4 speeds. is the tcc locking up?

tech2

test drove at 70mph:
in 4th gear rpms were 2200......instant fuel economy reading was 25mpg
in 3rd gear rpms were 3000......instant fuel economy reading was 17mpg


so accelerate from a stop and count the gear shifts.....the car is probably not shifting into 4th

Pup99

I don't see it dropping to fourth gear, but I wasn't sure if it was just me. The car had the 3.8l series two.In all honesty I don't know how to tell if the tcc is locking up. The fuel filter may have been changed by the dealer that sold me the car, they had serviced everything else, but I am not sure. But with what your saying for your rpms I am pretty sure I was right and the Chevy dealer screwed me, I know they didn't leave town limits because they didn't put enough miles on it to go out of town. Thanks for your help tech2,I really appreciate it

tech2

read/write for me the description of the problem on the work order



not sure how they get to cat replacement unless the work order description was horrible or the tech just diagnosed stored codes.

Pup99

Driveability concern- customer States vehicle won't go into 4th gear

Engine not running correctly- found 8psi back pressure- found converter up- replaced converter and test drove vehicle-ok now

(Tech:67) A




I wrote on the envelope that I put my keys in, not going on overdrive, and told the service manager all about it when I made the appointment. I am debating on how exactly tio handle the situation tomorrow.

U.Nanimous

I'm with tech2, your probably missing a shift. I don't actually get on the expressway much so I can't say RPM's at 70MPH but I believe it is about 1800RPM at 40MPH and around 43MPH there is a shift that brings it down around 1100PRM.

tech2

if there was 8psi of backpressure, the engine probably would not run at 3000rpm.
Go directly to the service manager and tell them nothing has changed since the convertor replacement. An oem cat is expensive. Ask him how does the shop handle a miss-diagnosis.


be polite but firm. just ask questions. if you get rude, they will not help you.


Someone will have to cover the cost of the cat repair....should not be you. What ever fix they do next....lets hope they do not absorb the cat price into this repair. The repair may end up being a hard part failure trans rebuild.