Bad fuel mileage

[patrick w. hunt]

Hello - I just joined and do not know the correct way to enter a question. Hopefully someone will read this and kindly tell me as well as helping me. I recently bought a 2006 Sportcombi that would barely run. A mechanic installed a new throttle body and spark plugs and it is a rocket - it is very fast. I am 76 years old and drive like a 90 year old man, very slowly. Here is my problem. It is getting 8 mpg! The tailpipes are clean - inserting a white cloth in them brings out very little black soot. Any suggestions? I can deal with 8 mph. Shouldn't it get 20-23 or so mpg?

[nordwulf]

You can start a new thread in the Workshop forum for each new issue you need help with.

8 mpg definitely is very bad and 20-25 mpg is common.

Do you have a 9-3 or 9-5 SportCombi?

[patrick w. hunt]

9-3 with a new throttle body and spark plugs.

[9-3 Aero]

Is this 8 mph fuel mileage the one read on the SID, or computed by actual filling quantities and your own recorded driven miles?

[tunnanxwd]

Nice to know the year and the type engine, the 9-3 can have up to 3 engines ... older are "t" (small t)l, capital T and finally the 6 cylinder 2.8 depending on year can be 255 or 280 horsepower.

When was the last time the car had plugs, what tire pressures are you running?

[patrick w. hunt]

Actually computet mpg over four tanks of in town driving. I drive very carefully - I 76 and drive like an old man. The car has new spark plugs of the correct kind and a new throttle body. Inserting a white cloth into the tail pipe there is no evidence of rich running. The car has 30 psi per tire. I touch the wheels after stopping to see if they are hot, which would indicate a dragging brake. The car runs very well. I even thought someone was stealing gas, but no one is touching the gas cap. It is the Ecotech 2.0t. It uses no oil.

[patrick w. hunt]

Well the poor fuel mileage is partially explained. The trip mileage odometer is wrong. It reads 1.5 miles when it should read 3.1 miles. That makes my gas mileage 16.5 mpg vs. 8 mpg which is much better but not really good.

I don't understand why or how the odometer could be so far off. On a separate subject, does anyone know where the alarm module is? My alarm goes off at strange times, always preceded by the automatic door lockers locking and unlocking feversishly.

[9-3 Aero]

Odometer: Pulses are generated in the (rear) wheels sensor, converted to rpm by the TCS/ESP module, transmitted on the P-bus to the Engine Control Module (ECM) which converts rpm to vehicle speed. The CIM picks up this information on the P-bus and transfer it to the I-bus, where the Main Instrument Unit (MIU) uses it for speedometer - and odometer displays, after internally processing the information.

Thus could the problem be in the MUI processing of the speed signal for odometer display, given the speedometer reading is what you believe it should be - is it? If not, could programming of the speedometer be in question, however not very likely, noting the deviation odo reading / actual distance. The programming is performed to compensate for wheels fitted with different circumference. Programming is done with the Tech 2 GM TECH 2 Diagnostic tool - A quick word before buying on Ebay | eBay

What is the current size / dimension of the wheels?

To possibly get some ideas regarding the alarm going off unpredictably, a clarification is appreciated: Are the doors really locking AND unlocking, apparently by themselves - without being commanded by the remote control? There are no fuses to pull which could kill the alarm, for obvious reasons.

[patrick w. hunt]

The wheels and tires are all the correct size. The speedometer is correct. The alarm goes off shortly after the door lockers go into a frenzy, locking and unlocking as I drive. This car is nine years old and no one will steal it. I wonder where the alarm module is? After finding it, I wonder how it would be disabled. This one is the kind that uses the horn.

[9-3 Aero]

The logic of the anti-theft alarm is in the BCM, located in the dashboard, below the steering wheel column. However does this module as well control lightning and other "tasks", operation of which will be disrupted if module is disconnected / removed.

From my perspective is the culprit in first place the feverish, uncontrolled operation of the door locks.