First, I would like to go over the basics...
Tires. Did you make sure that the tires are properly inflated? Did you recently change tires to a different size and/or brand? This can have a HUGE impact on fuel mileage.
Engine Oil. Did you recently have the oil changed with a heavier weight than normal? 10W30 is what this engine calls for. Did somebody put 10W40 in it? many quick lubes will do just that. Is the vehicle due for an oil change?
Thermostat and coolant condition. If the thermostat opens too soon, the engine will take longer to reach operating temperature which will cause a loss in fuel economy as the engine runs most efficient at operating temperature. About 195 to 215 def F is normal operating temperature. In addition, poor coolant condition will cause thermostats to open too soon and give incorrect coolant temperature readings to the computer. A faulty coolant temperature sensors will do just that.
Did the weather change? Cold temperatures will cause a decrease in fuel economy just as you have described. Back to the engine operating temperature principle.
Did you change fuel stations. Did the fuel station change fuel formulation? Fuel is a big thing today. Ethanol decreased fuel mileage. As little as 10% ethanol in fuel will decrease fuel mileage by as much as 5%.
Finally... the biggest fuel killer of all. CARBON. The evil little word that consumers still today do not believe exists. As the engine is turned off, there is residual fuel in the intake. This fuel evaporates but leaves behind carbon on the fuel injectors, combustion chambers, and intake valves. The Norstar engines are horrible for carboning up. So, a professional upper intake decarbon cleaning and an injector service would do the trick. NO... NOT the bottle in tank stuff. Actually a professional service. By the time you build up enough carbon to effect fuel economy, it's way too late for a bottle of injector cleaner in the fuel tank. The bottle in tank stuff is meant to keep things clean not clean things.
As for the tune up... ACDelco spark plugs right??? Spark plugs on this engine are to be factory OEM plugs. No substitutes. The oxygen sensors age over time and become slow to respond giving you a decrease in fuel mileage. An O2 sensor needs to go from rich to lean (0.1V to 0.9V) at least 7 times in 10 seconds at 2000 RPM. If not, the O2 is too slow and should be replaced.
The only thing mentioned above that is not checked is the O2 Sensor. But then again I do not get any codes. I live in central Florida so not climate change. I did not check the actual coolant temperature sensor, but coolant is in spec. Used gas from same supplier/gas station ... used same fuel in my other Cadillac with no issue. How do I check for carbon build up and coolant temperature sensor functionality?
All other items you mentioned are not a problem.
Slow O2 sensors will show codes only when out of specifications. When they become slow, the computer does not pick up on them for quite a while.
Carbon build up... well... the only way for that to be eliminated is to perform the cleanings. This includes spraying a cleaner into the intake to clean the upper induction and then hooking up a gasoline/cleaner solution into the fuel lines and let the engine run off that.
As for coolant temperature... First, in Florida climate. From a cold engine, you are looking at full operating temperature within 2 minutes at 2000 RPM. If not, then the thermostat is slow. Check the coolant condition for proper 50/50 mixture, PH, and corrosion inhibitors with a coolant test strip. The coolant remperature sensor can be checked using an infared thermometer. Compare the thermometer readings to actual coolant temperature sensor readings shown on the instrument panel or computer data stream. It should be relatively close.
Also consider that the vehicle is aging. As the engine ages, it will progresively lose effeciency and therefore fuel economy.
Well it is about a week since your response. First, I would like to clarify to you that my MPG reduction was not a gradual decrease but a sudden drop from one trip to the next. Therefore I do not buy the "As the engine ages, it will progressively lose efficiency and therefore fuel economy".
My coolant temperature runs between 203 to 211 degree F, but I took your advise and change the Thermostat ... did not make any difference. Checked and clean the EGR valve ... did not make any difference. Did "TOP ENGINE CLEAN " via intake manifold ... did not make a difference. Still no codes beside S061 for ride control.
Any other ideas for the next step?
My next suggestion is the injector service. By hooking up the fuel rail to a cleaning machine and running the engine off a cleaner, this will clean the injectors, valves, combustion chambers, exhaust valves, and the converters. Additionally, remove and clean the MAF sensor. There are 2 small elements in the MAF sensor. Use an electrical cleaner to clean them.
Also... a thought came to mind. Check the brake temperature to make sure they are even from right to left. This will eliminate a dragging caliper.
After any repair, cleaning, battery disconnect, clearing codes, the computer will need about one tank to relearn proper operating parameters. During that time, poor fuel economy will be noticed. So... It may take 2 to 3 tanks before you notice a successful repair. A question for thought... You are calculating fuel mileage based on the trip meter and how much fuel you put in right? You are not depending on the instrument cluster to tell you the fuel mileage?
I will get the injector service done by a qualified mechanic and change the MAF sensor. By the way I had the break checked for any drag when the car is placed in drive. The mechanic at the break shop found it to be working ok.
I do the MPG based on the trip meter at first, then the last few times I did it by the amount of gas added and miles traveled ... I expected to be off on the expected result by about 2 MPG, but it came back close to that showed on the instrument cluster.
Will let you know the outcome.