Service engine light on. Have replaced air filter and added engine cleaning additive to gas. Any suggestion on how to solve my probllem

Have you monitored the engine sensors when this happens, if not have that done.

The A service and B service is an oil change and a long list of inspections. Unless the service is covered under warranty, or this is a new car and you want some piece of mind, I suggest only getting the oil change. The rest of the "service" consists of someone looking for problems with your car. Here's what's included:

A SERVICE (~ $205.95 + tax) :

- Inspect and record tread depth, and correct tire pressure
- Rotate tires (excludes AMG, Sports Models, SLK, and vehicles with staggered wheels)
- Engine oil change and oil filter replacement
-Includes Mobil 1 synthetic oil
- Lubrication service
- Includes hood hinges, lock cylinders, striker
plates, sun roof tracks and top off all fluids
- Cooling system inspection
- Includes antifreeze protection level, hoses and
clamps
- Brake inspection
- Includes check of pad thickness and condition of
discs, fluids and lines
- Inspect heating and ventilation dust filter, replace if needed. (Replacement additional. Dust filter prices vary by model)
- Function check
- Includes warning lamps, headlights, exterior
lights, seat belts, windshield wiper and washer
- Inspect and lubricate throttle linkage
- Check and clean air filter
- Reset flexible service system counter


B SERVICE (~ $379.95 + tax) :

- Inspect windshield wiper inserts and service windshield washer system (Replacement of wiper inserts additional*).
- Inspect and rotate tires, record tread depth, and correct tire pressure. (Wheel balance additional*).
-excludes AMG, Sports Models, SLK, and vehicles
with staggered wheels
- Engine oil change and oil filter replacement
-Includes Mobil 1 synthetic oil
- Lubrication service
- Includes hood hinges, lock cylinders, striker
plates, sun roof tracks and top off all fluids
- Cooling system inspection
- Includes antifreeze protection level, hoses and
clamps
- Brake inspection
- Includes check of pad thickness and condition of
discs, fluids and lines
- Inspect heating and ventilation dust filter, replace if needed. (Replacement additional*. Dust filter prices vary by model)
- Function check
- Includes warning lamps, headlights, exterior
lights, seat belts, windshield wiper and washer
- Inspect and lubricate throttle linkage
- Check and clean air filter
- Reset flexible service system counter
- Inspect front axle ball joints; check steering play and power steering clutch; and rear differential levels
- Inspect Poly V-Belt for condition
- Inspect starting and charging system and service battery
- Inspect climate control refrigerant

Has the car triggered any OBD codes (ie check engine light goes on) which would help diagnose the problem?
I do not know a way to do "naturally" (no additives or cleaners).
Regardless....since the fuel additive helped once, an approach is too assume that the engine is just real dirty .... so you might try:
1-Add another batch of fuel additive cleaner
2-Pull the spark plugs and clean them ... they are likely to still have some carbon buildup
3-Put in new air filter
4-Clean the MAF (mass air flow) sensor which is in the air intake (between the air filter and engine). Use a MAF spray cleaner, and a Throttle Body spray cleaner (which also, cleans the iac (idle air control) valve.

you might have to clean the tank because the pump might have problems as well the injectors could be clogged or it wasnt only water in gas if you are sure no sugar was added in tank i would check the fuel pump just the same if it is easy to remove

Clean fuel, clean filters, clean pvc, and since its a '99 try an emissions flush, your e-vap canister is most likely cloggeg, or even a faulty egr valve,

New tune up.and air filter,pcv valve,and change the fuel filter,and the oil,replace the oil with a full synthtic oil.Use quality parts from napa auto parts,or some good brand filters and ignition parts.Use a bottle of lucas fuel treatment to clean the fuel system,and injectors,and clean out the plenium(upper intake) with some intake cleaner,spray.

Try cleaning the EGR valve or just replacing it and replace the PCV valve as well.

Cleaning the fuel line was a good idea although it will need time to take effect. Another related problem could be clogged or stuck fuel injectors. Another thing to do is pull the spark plugs. Old or dirty spark plugs also contribute to engine sluggishness. Lastly, bring the car to an autoparts store that features free diagnostic tests. If there are any error codes stored on the car's computer, it would give you an idea where to look for any electrical problems.

Even a small amount of bad gas can cause your problem.
So the first thing you need to do is remove your fuel tank so you can drain all of the old fuel out,then remove the fuel sending unit and fuel pump assembly thats in your fuel tank and clean the filter screen, after that you can reinstall the fuel tank,replace your fuel filter and I would have a fuel injector sevice done.
Unless you are very skilled in automotive repair, leave this one to a pro.

Sounds Like You May Have A Leaking Head gasket Try Some Fix A Gasket Solution From Your Local Auto Part Store. Also Pick Up Some Fuel Additive For Cleaning Your Injectors You'll See A Total Difference. And You Said Fault Lights Are Flashing Or Arent? In The Case That They Are Try What I Suggested Then Reset The Dash Lights By Unhooking The Battery For A Minute Then Plugging It Back In.