ManduSS

I have a 2000 Monte Carlo SS and lately I have really been noticing how much fuel it's sucking back and not really giving anything in return. It has had some work done to it recently which included:
- Transmission Flush
- Transmission Filter
- Oil Change
- New Oil Filter
- New Fuel Filter
- New Spark Plugs
- Front Bushings
- Steering Tie Rods
- Complete Engine Bay Wash

I don't quite understand what is going on with the car. I am going through fuel way too quickly and I have noticed a severe lack in HP lately and now I believe it's misfiring. I would say it's roughly getting around 12mpg, maybe less than that. It feels more like 160hp rather than 200hp. I've checked for leaks under the car and I haven't noticed any leaking and their hasn't been any leaking anywhere I have parked the car either.

I recently did a test run to see just how quickly it was going through a full tank of fuel. I drove from Newmarket to Leslie & Lakeshore in Toronto which is about a 35 minute drive, 27.62 miles to be exact and round trip, full tank of fuel was gone. The odd time my dad drives the monte and I feel he is being cheap and putting in completely as cheap as you can go fuel in the car when I am putting in Premium. Now as I am aware with the 3.8L MC engines, it is a high compression engine and is not meant to be run on reg. fuel and should be run on prem. fuel.. and I am thinking perhaps the reg. fuel is gumming up the engine.. etc.

I am still learning engines/tuning, etc.

I have been told just to take it into the dealership or a mechanic around town but I would like it know if it's something I could potentially fix myself so I don't have to take it in and get charged an arm and a leg.

Thanks

John86

well u can run reg. gasoline with no problem. it's not goin to gum up the engine. u could have an O2 sensor problem. they don't always set a check engine light. how many miles r on ur vehicle?

Scoper50

One thing I'd like to add about gasoline. The premium label on the pump doesn't mean the gas is better then the regular grade. It simply means it has more octane. The only purpose of octane is to reduce knock and ping in the engine. If your engine doesn't knock or ping on regular unleaded, buying premium fuel is a waste of money. Premium fuel doesn't increase horsepower, it doesn't improve gas milage, and it isn't cleaner then regular grade. Simply put, if your engine runs fine on regular gasoline, you are wasting your money buying premium. It provides no benefits at all. It just emptys your wallet faster.

As far as your poor gas milage problem goes, I'm willing to bet you have an O2 sensor problem. When you first start your car in the morning when the engine is cold, it runs in something called "open loop" mode. During open loop mode the engine will run rich and burn more fuel. Once the engine reaches operating temperature, it goes into "closed loop" mode. Thats where your oxygen sensors come in. The oxygen sensors are located in the exhaust system. Their purpose is to measure the amount of unburned oxygen in the exhaust, which then sends information to the cars computer and adjusts air/fuel ratios accordingly to achieve perfect combustion ratios, hense giving you the best gas milage possible for your engine. Chances are, one of your Oxygen sensors is malfunctioning and the engine is never running in "closed loop" mode. I would also make sure the Air filter is clean.

golfer

Scoper50's condensed version of open loop and closed loopis basically correct however the important component that was left out is the temperature sensor. The control module reads the temperature sensor in cold state allows the open loop and when temp. is in warm state will run closed loop.

If it was remaining in an open loop then the check engine light would be on since it is an emission fault.
If it is a faulty O2 sensor likely the check engine light would come on since this also is an emission fault.

The original poster hasn't said if the check engine light is on ..... I'm curious if it is and if so get the codes.

There are several things that can be the problem:

MAP sensor
low fuel pressure
air filter
fuel filter
PCV valve
MAF sensor???
ignition coils
engine temp sensor
Cat
bad grounds
etc.
etc.
etc.