98 Ford Expedition starting problem

[1293Mini]

I have a 98 Expedition 5.4L 4x4 138k miles A few months ago it started to not want to idle when started, I was told it was a dirty MAS and that it may have to be replaced. It had the original battery and had slow starting. I finally replaced the battery and had no more problems. Within the last two weeks, all of the sudden it started up again, and now sometimes the air bag warning light flashes severel times and then stays on till I stop and when the engine has warmed up many times it will start and idle and the air bag light goes out as normal. I do change oil and air filter regularly. Any suggestions.

[Highwayman]

You said replacing the battery solved the problem for awhile, but it's back again. Have you checked the battery voltage when cold and when warm? That it doesn't want to idle almost seems like theres not enough juice in the system to fire the sparkplugs. Hard to tell with computer controlled vehicles, though. Check for trouble codes in the computer as well, may give you a clue. Check Idle Air control circuit, and o2 sensor. With that many miles on the motor, you're going to be replacing sensors more often, as this is about the time many of them quit. Just some thoughts...

[1293Mini]

Thanks Highwayman I have a code reader that hooks to laptop and when I run all the test everything checks out O2 sensors ok and idle ok. The last couple of days it has been starting just fine no air bag light, idles up and then down. as it should.
With as man miles as it has it would be good PM to cahnge the O2 sensor anyway. thanks again

[JonKraatz]

You might want to also check that the idle spped is correct. At this many miles it is often a idle air control motor or the gasket underneath it going bas. That seems to be a common problem with Fords that use the modular engine.
If the idle speed is correct, check the alternator output at idle. If the alternator is not putting out sufficient voltage at idle or has a bad field, it will disrupt the signal to the PCM and cause the engine to run rough. This occasionally happens on Fords and the computer does not store a trouble code for it when it does. Can drive you crazy looking for the cause.