A while back my check engine light came on. I had it scanned and the guy told me I need knew oxygen sensors. So I replaced to ones I could reach and took it in for the other (2003 Ford windstar, BTW). A few months later the blasted light came on again! This time the guy said to clean the mass airflow sensor and change the air filter. So I did. Months later, the stupid light came on again! This time I ended up replacing all the spark plugs and wires, the pcv valve, and the fuel filter (just for kicks). It was my first time doing all that and it was quite an ordeal. I had my arm buried so deep behind the engine trying to get the plugs out that passers by may have thought I got eaten by my car! After that everything worked great. Now the light has come on again and I’m burning fuel like mad! It seems like I’m filling up all the time. A little help, please.
Does the van seem to run fairly decent or is it bucking and jerking, etc.?
Belching any black smoke?
Are you comparing the miles traveled to the gallons needed to fill it up?
Without knowing that and what if any codes are present it
s difficult to make much of a guess.
In the event the van is running somewhat ragged and possiby belching smoke you might check the fuel pressure regulator to see if the diaphragm is ruptured. Remove the vacuum hose and see if there’s gasoline in it. There should not be.
That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here.The van is not emitting any black smoke and has not bucked since the light came on. I remember it did kind of rumble for a few seconds and thats when the light went on. I’m not sure about exact gas mileage, only that it’s much worse than before the CEL came on. I did have the codes run at autozone each time the light came on and it said something about bank one and bank two. I seem to have misplaced the slip so I don’t remember exactly what it said.
misfire bank one and two? or bank one/two lean fuel mix?
Lean fuel mix bank one and two. I did some more research and I’m thinking it may be a vacuum leak. Plausible?
My (Guess) Definite possibility on the vacuum leak
A vacuum leak below the throttle plate will often cause a rough idle which can vary in intensity based on the size of the leak. Bad leaks may affect off-idle performance.
A leak in the intake tract above the throttle plate may not affect the idle but could cause bucking at engine speeds above an idle as this can affect the MAF sensor quite a bit.
Vacuum leaks will often affect driveability on a Ford more so than others and will generally cause a lean code, or sometimes two lean codes. Stalling and surging are very common. Is this van doing any of this? The unmetered air flow will get picked up by the oxygen sensors and cause the system to dump more gas into the engine, causing poor fuel economy.
When you changed the plugs, what did the old ones look like? Did they have any soot or dampness on the ground electrode? Did you gap the new ones (many DIY’ers forget to)?
A look at fuel trim values would tell you if some type of compensation was taking place. It is odd that I am the one told that the days of carbs and points and guessing with diagnostics are over with when it is others that will not embrace the tools of the trade that will solve this problem. OP you need to get a scanner and take a look at the parameters that control your engines fuel management, the days of “winging” repairs like this, well lets just say they can be long.
I would check those oxygen sensors that you installed to make sure they’re properly connected and that the wires are where they belong. Have seen way too many diy installs with melted O2 wires to ignore this. Not to mention, it doesn’t cost anything to check. I would also go with Oldschool on this. Need a scanner to watch fuel trims and other things that give a clue. It shouldn’t be that hard for a decent tech to find and repair this problem in my opinion.
Could be a vacuum leak. The '99 Windstar was notorious for this (same basic engine as yours). It had a defective front valve cover design that allowed the engine to suck significant amounts of oil up through the EGR hose and into the air intake, resulting in horrible carbon deposits and a vacuum leak between the two parts of the intake manifold. 
The first thing I’d probably do is pop the top half of the intake manifold and see if it’s oily. If so, get thee to a parts store and get new port seals and isolator bolts, clean everything as best you can, and flush out the EGR hose. Then check the front valve cover to make sure you don’t have an old one somehow.
http://autorepair.about.com/library/illustrations/bl860a-lib.htm
[i] I did have the codes run at autozone each time the light came on and it said something about bank one and bank two. I seem to have misplaced the slip so I don't remember exactly what it said.[/i]
No, it did not say something about bank one. It said something like [P1234] A good mechanic knows that while the code might relate to bank one, it does not, by itself say to replace this or that. It only means it is getting an unexpected signal from that area. What is missing is the real mechanic who can consider the code and other factors and have hands on to the car, to make the most probable suggestion for fixing the problem.