I could use some input on a gas mileage issue. I have 1999 Monte Carlo with 150,000 miles. I recently took it into the shop because I noticed a significant decrease in gas mileage (almost cut in half). They said it could be due to a bad clutch on the air conditioning compressor, that was dragging the engine. I replaced compressor, and it doesn't seem to have fixed the gas mileage issue. They rechecked the car and can't find anything else significant besides recommending some maintaince like plugs and O2 sensors. Does anyone have any other suggestions on what could cause such a significant decrease in gas mileage? Thanks

Depending on how old your plugs and wires are, that's where I'd start. If they are still the original parts, they are WAY overdue.

replace the fuel filter, easy and cheap. Will cause bad gas mileage, and alot of other problems down the road.

be sure to take off your gas cap when you change the filter, all you need are 2 wrenches (one side of teh filter is 2 nuts). the one end is just a clamp end. push the plastic pieces together and pull out.

some fuel will come out if you just drove the car, it will seem like alot, but its really not. make sure to have a pan under the filter/line when you take the filter off! and wear eye protection, just in case!

The next thing you should try to notice is if you loose power when trying to accelerate. It could be that your catalytic converter has rusted out and is starting to plug the exhaust ports. It would cause the engine to run "rich" or better known as poor gas mileage, because the engine can not breath. Engines use an A/F ratio (Amount of Air to Fuel mixture). If the engine takes in too much air then it leans out the fuel. If it takes in less air it richens the mixture, hence causes the engine to use too much fuel. I would start with the basics, keep it simple. Check the air filter and make sure it is clean and doesn't need to be replaced. A bad throttle position sensor can also cause bad gas mileage. With the catalytic converter and the TPS you would also notice your transmission shifting at odd times (i.e. the engine revs higher than it should before it shifts). I just replaced the catalytic on my 2002 Monte Carlo SS. I had those symptoms and my gas mileage went from avg. 27 mpg to 13 mpg.