High idle and emission maint light on HELP!?!?!?!

My 90 xj was running great till a few weeks back it started to idle super high and start really hard then yesterday the emissions maintenance light started to come on sporadically I have been told its probably the mass air sensor but before I buy a part I dont nead nor do I have the money for I nead to know for sure thank you guys for any help you have

1990 XJ High Idle

On my '90, every time I hit the high idle blues, it has been the TPS. On our non-HO 4.0 engines, it's adjustable. Mine has always been shot, though, when it went to high idle. I'm on my third since new. To keep them functioning, avoid watering them (washing the engine)- that's what killed one of mine. If you have an automatic transmission, you may also notice some weird transmission quirks- shifting at the wrong points. The TPS provides input to the tranny too. If you have a LOW idle on a non-HO 4.0, then check EGR function. 4xBob

Emission Maintenance Light

The Emission Maintenance Light is simply a timer designed to let you know that you have reached the scheduled time to replace the Oxygen Sensor. On my '90, I had already replaced the O2 sensor, but the light came on anyway. It is not resettable by an owner- just unplug the timer box, located under the dash near the steering column. Mine was smoky grey plastic- not the yellow cruise control box!! 4xBob

I will see about the oxgen sensor timer box when my girl gets home and maybe do the tps sensor depending on price as soon as I can