[heathj1]

Hi everyone Haveing a problem with my wife's daily driver 1999 lumina car 3.1 V6 230K miles. Car use to get 26-30 mpg now its getting mid teens. The service engine soon light isnt on. It still runs good plenty of power. I just checked and changed air filter, last spring I changed all the plugs and wires. What do u think?

Thanks Heath

[LATECH]

Start by doing a live data scan and look to see what the fuel trims are doing. If they are subtracting fuel( more than 5% on the long term trims) then there is a problem.Otherwise a nanny cam may help catch the guy with the siphon hose.
If the trim is subtracting fuel then it is showing that it is trying to compensate for a rich running condition.Unmetered fuel is getting in somewhere.Like a leaking fuel pressure regulator, or to high fuel pressure etc.
If the trims are adding fuel to the xcess of 5% check the MAF reading to see if it is not wacked and dumping to much fuel. A normal MAF reading should be around 4-5 ish at Idle and increases while accelerating.
Have you check it over well for a fuel leak?

[LATECH]

Also dont forget a lot of stop and go and cold weather will kill gas mileage. A cold engine needs a lot of fuel to warm up to an efficient place stoicheometrically speaking,and a lot of stop and go gets 0 miles per gallon at a stop light. The corn ethanol "oxygenated fuels" are also less efficient at the exspence of mileage in order to keep the air cleaner as winter cause richer more pollutive mixes for a longer duration than if the daytime tmp was 70 farenheight

[heathj1]

Is there anything I can check or change before having it hooked up to a scanner? There is a self serve junkyard close to house if u think there is something I should to swap if part is expensive. I have taken the warming up in the morning out of the equation by taking the car to gas station filling up and then going on 150 mile road trip.

Thanks

[CJ_1080]

Pull the oil fill cap. Is there a milky/snotty residue on it? If there is, your lower intake manifold gaskets are leaking! This is a common problem with them. Air may be getting in past the leaking gaskets. Then the computer will compensate by adding more fuel. Good Lucy, its not that hard to change or terribly expensive (maybe 80 bucks).

[80PontiacWagon]

clogged fuel injectors maybe? I'd try pouring a bottle of cleaner in the gas tank. You can pick some up at your local walmart.

[DanielC]

Take LAtech's advise. You kind of need to hook the car up to a scanner. Your car is an OBDII car, and there is a standard hookup port on the car, usually under the steering column.
Many auto parts stores have scanners, now, and they will hook up to your car for free. There is also a possibility you may need to go to a GM dealer to hook your car up to the scanner.

Check for the obvious. You can still hook up an old school vacuum gauge to the engine and find out a lot of information.

I was able to get a program off the internet, and an adapter that I bought mail order, that hooked up to my 1996 Ford Aerostar, that allowed me to use an old Laptop computer to read the engine trouble codes. It required some research, and wiring the cable from the serial port on the computer to the car adapter. I probably spent about $120.00 on it, plus the laptop computer, that I already had.
I have occasionally noticed that is a late night infocommercial for a "cardoctor" code reader. that might be an option, Just watch how much they charge for "shipping and handling".
You do not need a $2000.00 scanner. you can find them for a lot less.

If you are really only getting about 15 MPG, instead of 26 to 30. you will in a short time spend enough money in gas to pay for a scanner, or the charges a shop will ask to hook up to theirs.

[heathj1]

Can there be trouble codes with out the service engine soon light being on?

Thanks again

[DanielC]

I believe there can be. It is called a pending trouble code. For example, on some cars, if you start the engine with the gas cap not tight, it can cause a pending code with the evaporative emission system. If the problem is not fixed, (the gas cap is tightened), the check engine light comes on, steady.
This may cause the folowing codes, or other codes.
PO442 code, Evaporative control system, leak detected.
PO446 code Evaporative control system, vent control problem.

By the way, a check engine light on steady, means you need to get it checked soon.
If the light is flashing, it means you need to have it checked as soon as it is possible. something is wrong that may damage your engine, or its control systems.

[lmsport]

Exhaust restriction? plugged catalytic converter?

Bad oxygen sensor?

To use so much extra fuel with no operation issues or CEL is unusual.

[heathj1]

Do I need to replace o2 sensor?

[T-bucket23]

In most instances a rapid drop in mileage is due to a faulty pressure regulator, I have seen this quite often. It is however difficult to diagnose without the proper tools. Sometime if you pull the vacuum line off the regulator it will be full of gas, this is a sure sign it has failed.
As Latech stated the fuel trims will most likely tell the story.

[DanielC]

Modern engines are too complicated to just guess faulty parts, and replace them. It is a waste of your money and time to do this, unless you get really lucky. You could go play the lottery as an investment strategy, but that is not recommended either.

Hook the car up to an OBDII code reader, and it will tell you if the O2 sensor is bad.

[LATECH]

No Code / Symptom only diagnostics need to be performed by someone who knows EFI and can interpret Data streams.
Basic test such as leak down of fuel pressure (injectors) also need to be performed.A live data stream will reveal whether the problem is mechanical or electronic...if you know wht you are doing.
At this point I would have to say you need to check around for a reputable garage that performs dianostics and take it there.
You dont go to McDonalds for a steak... Dont go to Jiffy Lube for Diagnostics.
Pay someone to diagnose it. Youll save money in the longrun, trust me.

[scrot]

Originally Posted by T-bucket23

In most instances a rapid drop in mileage is due to a faulty pressure regulator, I have seen this quite often. It is however difficult to diagnose without the proper tools. Sometime if you pull the vacuum line off the regulator it will be full of gas, this is a sure sign it has failed.
As Latech stated the fuel trims will most likely tell the story.

Check/replace the coolant temp sensor too.