[Campbellbennet]

Have an 06 buick with the 3800. Mileage was never great 27 when I got it but it has been dropping slowly. Getting 24 now. I did get an O2 sensor error awhile ago but it went away on its own. It has 98k on it I was just wondering if anyone has just replaced the sensors with new and was able to get the mileage back up. If so which brand is the best? I know this has to be running rich, had a 3800 in a 2000 Lesabre that got as good as 36 on a good day and almost always over 30. Same motor, bigger car.

[TXSTARFIRE]

Dont have the answer to your question, but have observed the same between Lucerne and Lesabre. Have a 08 Lucerne with 3800 and very rarely would get 30 MPG on the highway. Multiple 02 Lesabres would regularly get 30 to 32 MPG. I always wondered if the difference was aerodynamics. Lesabre was a great car for visiblility and comfort. After purchasing Lucerne was disappointed on multiple fronts.

[Sirrotica]

https://www.youtube.com/watch?v=_8KvLHrBkf0

Scotty Kilmer video here explaining a few reasons that cars lose fuel mileage as they age. Mass air flow sensor and throttle body getting dirty are common causes, as well as O2 sensors wearing out.

He also talks about brakes dragging due to calipers sticking partially and not releasing.

Tire inflation has a pretty significant effect on fuel mileage, I run most of my cars ar 38-40 PSI to help rolling resistance.

The scanner he shows is also a great tool to track down if the engine management system is working properly. I realize most people aren't going to buy a scanner to bump up their fuel mileage, but using a scanner of a friend would be a possibility in lieu of buying one. The lack of CEL coming on doesn't mean everything is perfect, it just means that the parameters haven't been exceeded enough to turn the light on.

[SRGN]

If the upstream O2s are originals, change them out. Don't use cheap ones. Clean the maf sensor. Change the PCV valve (inside intake under removable cap on top). Reset the fuel trims afterward with a scan tool. After that, use a scan tool, watch fuel trims at idle and cruise. If you are using regular pump fuel with ethanol, mileage suffers. E10 or E15, expect losses in fuel mileage up to 3-5 MPG over ethanol free fuel.

[mgarblik]

The above posts have good ideas and things to check/consider. I just went through a similar situation on a friends 3800 series II Bonneville. His mileage had dropped 20-22% over the last year but the car ran just perfect. Everything, O2 readings, idle quality, fuel trims all good. Problem turned out to be 1 failing injector. As the car would get hot, the one injector, (happened to be #4), would have much lower resistance hot than cold. It is normal for the resistance to go up as the engine gets hot, maybe a 1/2 ohm or so. This one was going lower and therefore injecting allot more fuel. This was an older 3800, OBD 1 with just 1 O2 sensor. No way for the computer to really compensate for 1 bad injector. Replacing the 1 injector brought up the MPG by 4. Took several months to find this. A real tough one. If the other things mentioned don't pan out, check the injector resistance both hot and cold. All 6 should be within .2-.3 Ohms max of each other. I like NTK O2 sensors. I feel Bosch O2 sensors are a poor choice for domestic engines. AC are fine if you can find them.