I have a 99 Ford Expedition. I bought it a few weeks ago with 106,000 miles on it. To make a long story short, I put some bad gas in it, it ran rough for a while, then cleared up, but the check engine light is still on. Skip to the bottom if you dont want to read the details.

So when I bought the truck, it had a broken fuel tank strap so I didnt want to put more than a few gallons at a time in it until i could get it fixed, for fear of the fuel tank falling off from all that weight. I stopped at a gas station and put about 5 gallons of 89 unleaded in it. I think with the higher gas prices people dont use much other than "regular" so the upper grades just sit. Anyway immediately after putting this gas in it began to run horrible. I drove home, fixed the fuel tank strap, drove to another gas station and put in one can of fuel injector cleaner and one can of drygas and filled it up with regular. After running it through it seemed to run much better, but the SES light was still on.
I disconnected the battery to reset the light, and then it began to run horrible again. After a few days it ran fine again. Then I drove it 700 miles roundtrip to pennsylvania. the whole time it ran great, but the SES light was still on.
Next I replaced the Fuel filter and reset the SES light with a code reader. Cloudy stuff came out of the fuel filter. It is running even better now and the SES light stayed off in the beginning. Now the light came back on. THe codes were P0171 and P0174, both indicate a lean mixture.
Earlier before I changed the fuel filter I had the codes read and these codes came up along with one that had to do with the fuel trim being out of limits, as if it could not adjust it enough to compensate. I think the fuel filter being plugged up had to do with this.

So here's it is again summed up. Got bad gas and ran it through. Runs good now, but the SES light is on. I changed the Fuel Filter, but even after doing so I got 2 codes. P0171 and P0174. Any suggestions. Will the system just adjust itself. I just changed the fuel filter today and only drove 10 miles or so max. I reset the codes with a code reader and did not disconnect the battery.

Thanks in advance

 

Follow the hose from the PCV valve back to the firewall and where it goes into the back of the intake. Betcha the rubber elbow at the fitting has a hole in it. Lean codes both banks are almost always a major vacuum leak.

 

I checked the pcv valve and hoses as best as i could without replacing stuff. I also checked the brake booster. Nothing appears to be leaking.

One other thing i noticed is that the heater never seems to get very hot. I think the engine warms up to 180 or so. Dont know if that is enough, maybe the thermostat is stuck open or something. would that cause these codes in any way?

It has also been suggested to me that the egr may be plugged up from the bad gas. I bought a bottle of "guaranteed to pass" as suggested by the garage where i had my inspection done (failed cause the light was on for being lean).

Any suggestions?