Low ideling engine,have a check engine light on,what could it be?

Often the IAC (idle air control) has built up carbon, but if the check engine light is on, read the codes. EGR, O2 sensors, bad torque converter or solenoid, low fuel pressure, or many more possible causes.

Sounds like there is a vacuum leak. I's there a check engine light on? If so is there any lean codes?

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You might check the pulleys that the serpentine belt runs on. Make sure all are free wheeling and not loose or rough or make noise. You would need to remove the belt to check them.

If the check engine light is not on, I doubt there is much you can do.
It sounds like one of the engine temp sensors or one of the other sensors. The engine temp sensor and the intake air temp sensor tell the computer what the outside air temp is and the coolant temp which allows the computer to regulate the amount of fuel going thru the injectors. Generally if one of the sensors has failed the check engine light should be on.
It could be low fuel pressure too.
You may have to have a shop check the sensors with a scanner and the fuel pressure with a gauge.

Their is a couple things to check before replacing the oil sending unit. 1. Verify the wire connection to it is secure. 2. Verify engine is at proper idle and not to low.

if it over heated and now has a miss there may have been some internal damage to a cylinder check the plug and wire and if they are good do a compression test

Your describing a vacume leak or a mis adjusted throttle body. Does it ever idel down? What engine is in it Desiel? If it's gas the AIS motor might be sticking or is bad. Does it run and is the check engine light on? are all the fluilds correct level? coolant level low can make the coolant senser lie about the temperture of the motor to the ecm and it'll command a high idel. Once the idel motor goes as high as it can it might be shicking and you can remove it and wash it off with carb spray and while it's out with the wire connected turn the key on and off with it in your hand and spray it with cleaner till it moves smoothly. With it retracted install it and try again, any vacume leak will make it idel fast. The trans mission will shift hard if the vacume line to it is off and it's on the back of the intake manifold and it goes to a modulater on the side of the trans. it can be off there too.

the coolant temperature sensor can cause the condition you're experiencing. it should be located where the radiator hose enters the top of the engine. is there a check engine light on?

sounds like the throttle position sensor, I think that is the one that makes it idel rough

Just trowing some suggestions out there, but, I would check the idle air control valve (solenoid) or the throttle position sensor. If they check out then my last guess would be the map sensor or the vaccuum to the map sensor. Hope I was able to help. Best wishes.