I have started to experience this issue the car burns gas like crazy. I get less then 100 miles on the tank. No check engine lite is on. I have replaced plugs and wires about a year ago. Car runs fine. The DIC shows 10 miles per gallon. I have a v8 car that is better on gas now. Any suggestions what can cause such a poor gas mileage.

Change the upstream O2 sensor. It controls the amount of fuel that the computer sends to the engine and a significant decrease in fuel mileage is one of the signs that the sensor is going bad.

I changed the one before the converter about 7 months ago is this the one you are talking about?

Yes, but it is possible that it is bad even though it was replaced. Next, are you sure that
you don't have a fuel leak or is there a possibility that someone is stealing your gas?

Did you get bad mileage before you changed the sensor?

Also check your air filter(dirty) or check vac line to f/p regulator for fuel....check for codes, just to see if you can communicate with the PCM....

did you gap the plugs properly? Is the car sluggish?

i did not gap teh plugs i thought they come pre gapped the car runs fine no sluggishness

definately remove the plugs and check the gaps on all of them. Some of them come pre-gapped but I never trust them. always check the gap. what kind of plugs did you use? change the pcv valve. also, clean the throttle body and use seafoam in the gas tank as well as in the intake.

for comparision sake I get 19.7 mpg in my Comp G and I definately use the gas pedal

i am definetely not getting anywhere close to that. I will check this weekend. The car runs great lots of power so i dont know.

Update had the guy scan the car with gm tech II tool he says no codes come up.

did you check the plug gaps yet???? also take off your maf and get the maf specific cleaner (green label) and clean the wire/sensors off very well, do not touch the wires!!!!!!!!! I have seen this have a significant improvement.

i have not done this yet its too cold to work on cars in NY now

Check your catalytic converter. It's a common problem on these cars

The car has brand new converter from GM and no engine light is on

Update i had the car scanned again and i got a code for missfire cyl # 4. could something like that cause the car to run rich? I had the missfire issue befor it was on cyl 3 the light never came

What happens when you have a misfire? Incomplete combustion.....when that happens, all the OXYGEN in the cylinder is not burned, so it goes into the exhaust, and it detected by the O2 sensor.......the O2 sensor tells the PCM, you have a lean condition.....so what does the PCM do? It commands the injectors to run richer....

My tech guy saud he does not know what it is i have replaced all plugs and vires about a year ago also new converter and ne 02 sensor. Its an intermitent misfire. No engine light is on.

coil pack?

This past Saturday i went to get the car checked out again and the technician showed me with his tech II tool slight misfire on cylinder # 4 I would say its about 20% he told me that he can reprogram my ECM with the latest update and if that does not fix its a bad injector. I asked him about the coil he said he is confident its not the coil but a bad injector. He offered me 2 options to run an fuel injector cleaner its a system with pressure or replace the bad one. I did have him reprogram the ECM but its too early to tell if that fixed the issue. What do you guys think of this ?

It depends on what fuel injector cleaning system they use. If it the one where the car's fuel rail runs directly on the machine then yes it is worth a try. I know Midas calls their method a fuel injection cleaning and all they put in is valvoline fuel injector cleaner and clean the throttle body and intake, which you can do yourself. throw some lucas real fuel system cleaner in and run 91-93 octane with a half tank of fuel.

You may want to check the plugs for any damage, ie. cracks in the porcelin and gaps like I said before. check the wire itself for resistance. if all checks out then it is a possibility that the injector is bad. also to eliminate a coil you can switch it into another position to see if the misfire changes.

The guy pulled the plug and he says by the way it looks its an injector. The one he offerd me was to run the machine through the fuel rail.

Had the injectors cleaned this weekend with system through the fuel rail and still have missfire code PC 300 at this pont i dont know what steps to tka e or who to go to to have the issue fixed. Please advise

Dealer would be the best place to go! Otherwise you will just keep paying someone to guess what is wrong. At this point it is either a bad plug, plug wire, coil pack, injector or even a lower intake gasket leak. lots of variables.

*To eliminate the coil swap the coil pack in question to another position and see if the misfire follows.
*It is possible that you have a bad wire although it has been replaced. Did the tech check the gap in the plugs?
* if you are feeling up to it you can even move the injector to see if the misfire moves.

what are the symptoms of lower intake leak ? the missfire is random now so its impossible to isolate but i think i will have to take it
to the dealer its driving me nuts.