'99 Pontiac Grand Am - Horrible Fuel Economy ~13mpg city

Your still huntin huh? Good for you.
Did you have the oxygen sensor wave patterns checked liked Chris said?

Computer Controlled Engine Management System:
This is what contols your air fuel mixture among other things. It consists of the Computer, inputs such as sensors and outputs like solenoids and valves. The inputs evalute or report tho the computer, the computer makes adjustmetns based their data and tells the outputs what to do.

Catalytic Converter: "plugged" or restricted? You will get a code if this is the case and the cel will be lit. A plugged converter will cause the motor to run quite rough.

†Mass Air Flow Sensor. Heard somewhere this is only in the V6s, is that true? Some 4 cyls have them, not sure about yours.

Fuel pressure regulator - controls fuel pressure when vacuum is reduced to allow flow through injectors. Runability or codes would be expected to be set if there was problem.

†coolant temp sensor. How does this one relate to poor fuel economy?
This input sensor tells the computer how cold or warm the engine is. The engine wants to get hot as quick as possible to reduce emissions. This sensor reports it based on the coolant temperature. The colder it reads, the more fuel the computer gives the motor.

†Spark plugs
Pull a plug and look at it. Measure the gap, either replace it if they look bad or clean and regap them. Replace the rubber boots at the same time regardless of which way you go.

If you go back to your original post, Chris mentioned this, as I was incorrect. A lazy O2 as he said may be the source of your fuel milage. An expenve scanner can read the wave pattern, It may be cheaper for you to just replace it however.

CCEMS

CCEMS?

How many miles since:

plugs
transmission filter
coolant flush (& also last date. Of coolant flush that is.)

Dpn't worry about the fuel filter, do it another 30,000 since the last one.

"I really don't know much about cars and I hope it is cool that I am posting all this"

It's very cool 8) As as much as you like and as often as you like. :D

Your car does have a MAF btw.

Generally if there is a problem with the computer sytems, a performance problem or cel will come on. There is a data stream that can be monitored with the more expensive scanners. One who is up on this can evaulate what is going on to narrow down or find causes. Such is the case with you oxygen sensor wave form. This form should be "jumpy" versus straightline. The coolant flush on yours should be done every five years with dexcool. Dexcool is the coolant that gm uses and is a long life coolant. Should any green coolant (older tyep) be added, then you can have it flushed and refilled with the green, but now it is a 2 year coolant. Anytime a lower rated coolant is combined with a longer life, the coolant bcomes reduced o the lower common denominator.

If you decide to do the tranny filter:
https://www.2carpros.com/forum/93-ford-taurus-vp53989.html#53989

Call a parts store and ask for a mass air flow sensor and see what they say. Can't believe everything you read :wink: , including here. The MAF is an input to the computer and evalutes how much air is coming into the throtle bore. This has nothing to do with fuel economy, it is more for preventing acid build up that eats away at the internals such as the heater core, removes or at least reduces hardened particles that can cut the seal on the waterpump, and replenishes the cooling and freeze/boil protection qualities needed for the motor.

Again, I would focus on the upstream o2 sensor, sense the rear was replaced.

Let me get this otta the way first. :Oops: Your engine does not have a MAf sensor, perhaps 4 cylinder GMs do not. I know others such as ford and Dogde do however. My labor give.4 to change the MAF that you car doesn't have :roll:

As far as the dexcool. It does form a sludge around the radiator cap area this should get cleaned out. I have no proof otherwise. Early years of dexcool 95-98 there seemed to be a higher incidence of the coolant getting muddy-ish. I couldn't tell you why, but I don't see it much anymore. It has nothing to do with finding leaks. If you can't find florecent green, yer sure as hell not going to see purple.

This you referred has some filtered information. The chevy 3.1, 3.4 and 3.8 motors have flaws that are inherent in them. Poor designed intake manifold that can't handle the heat, poorly designed gasket that require an upgraded design. The coolant doesn't eat up gaskets. I have read more articles on coolant than I care to reflect on. I use dex-cool on a car that came with it. There are guys that drain it out and put the green in as soon as possible. I'm okay with that too. I had to research the coolant again today on a chevy Aveo that has blue coolant. The dealer didn't have an immediate answer for me, suggested consulting the owner's manual, cause it has to be in there. It wasn't, just reflect to use the "proper coolant". The only info I found was it has to meet astm 1825m standards. :Evil:

Please post the transmission post, that doesn't sound like something I would "never ever" say. The exception may be if it was a lincoln that had 100,000 miles or more than had never had a filter put in. Can't think of any other instance. :

W :D e'll be here