Fuel Milage

**jdkopveiler** · 09-16-2009

I bought a 98 grand prix GT recently, which only gets about 21-22 miles per gallon on a tank (85% highway driving-70MPH). I had a 97 GT about a year ago that got around 26-27 MPG doing the exact same driving. Any ideas on what would be different? It seems to run as good or better than my old one outside of the milage.

Thanks!

**Jakegday** · 09-16-2009

could be MANY different things..... ill start by asking if you've ever replaced the front o2 sensor? and is your MAF screen still in place?

**dualmonger** · 09-16-2009

how often are those items to be replaced? and.... one size fit all? any "better" o2 sensors on the market?

**singularity** · 09-16-2009

bosch has always been my preferred brand, irriguardless the car that i have been installing it into.

**Jakegday** · 09-16-2009

ive read nothing but bad things about bosch oxygen sensors. that caused me to buy a denso. i changed it at around 90,000 miles and it made a significant difference in the poor mpg problem i was having

**jdkopveiler** · 09-16-2009

I have tried the obvious things (plugs and wires, air/fuel filters, tire pressure, etc.). I have not replaced the 02 sensor. Where is it on these (converter, exhaust pipe, manifold)? I also have not looked at the MAF sensor yet...thanks for the suggestions.

**matt5112** · 09-16-2009

Its right on the top of the down pipe. The primary O2 sensor that is.

**jdkopveiler** · 11-16-2009

Tried the 02 sensor and it didn't change anything. What kind of RPMs should this thing be running down the highway. It is currently around 3000 at 70mph which seems high to me. It shifts through all of the gears, but is it possible that the convertor isn't locking up?

**Zef_66** · 11-16-2009

Originally Posted by jdkopveiler

Tried the 02 sensor and it didn't change anything. What kind of RPMs should this thing be running down the highway. It is currently around 3000 at 70mph which seems high to me. It shifts through all of the gears, but is it possible that the convertor isn't locking up?

Yes, very possible. At 70mph, I am right around 2000 rpms.

**intervention** · 11-17-2009

i only get 20 highway and 17.5 city, and i drive it nice. so, ya.

**jdkopveiler** · 11-17-2009

I checked on the way home and I cannot feel/see the converter locking up. It shifts through 1-2-3-drive, but then stays at about 3100 rpms at 70 and never drops. what controls the lock up on these? does it get a command from the computer, or is it from a switch like the older lock up converters? I did clear a "P0742-torque converter clutch circuit stuck on" code with my OBDII tester back when I got the car. what exactly does that mean? Thanks again for all of the help!!

**kfish** · 11-18-2009

Originally Posted by Zef_66

Yes, very possible. At 70mph, I am right around 2000 rpms.

yeah at 70 it should be at about 2000.

**skidoojames151** · 11-18-2009

not on a gt with 3.29 gears but ya probably on a gtp...i get about 2500 rpm at 70...2000 at 60.

**kfish** · 11-18-2009

i have a gt with 3.29 gears. its about 2k-2100 for me. weird

**Feather Head** · 11-18-2009

My Gt is at like 2100 at 70

**chantspel** · 11-19-2009

Originally Posted by Feather Head

My Gt is at like 2100 at 70

Ditto on my 98 GT. I usually get around 26-27 hwy so I'd say something is up.

**BwolfGT** · 11-19-2009

I only get about 17mpg. It sucks.
So replacing the oxygen sensor helps?
I also have no MAF screen, is that lowering it?

**shawngtp** · 11-19-2009

You need to take your car to a transhop and tell them what is up. Might be something easy. Your car in not going into overdrive there are many things it could be. Anybody else have any ideas?