1991 Ford 150 - 8 cylinder , that hunts at idle , voltage meter also hunt going to low then normal. Check engine light on and also battery light comes on. Any help..............

You have rvc/regulated voltage control, that circuitry is in your pcm. I'm not saying the pcm is faulty, at the moment, I don't know? You also have a battery temp sensor in the battery tray housing, that might cause problems?
Check for applicable charging system trouble codes, if you haven't already.

It is likely that the engine management system has lost its base line calibration. If the battery is weak and during the crank to start the voltage dipped low enough the Engine Control Module lost its calibration. If the pedal is depressed during the crank the TPS (Throttle position sensor) will lose the idle position for the throttle. The hunting and surging and resultant stalling is the ECM seeking out the limits and re-establishing the throttle idle plate location.

If battery is OK than running the car allowing the calibration of the vehicle to reestablish itself will correct the problem. My experience is that this happens if the gas pedal is depressed during crank with a very very low battery voltage. Always the best way is to not touch the pedal during any cranking starting procedure. Or after a battery replacement.

To accelerate the calibration process attempt to run the car ingear tring not to allow the car to stall. After afew munites the car will have reestablished its normal running calibration and you will be allset.

no engine type stated, there are 6 engines used.
but bad idle is caused by 2 things.

• misfire.
• or lean AFR, lean fuel mix.. causes HUNTING

the ECU can also cause the hunt if it can not hold hot idle to spec.
most cars is 800rpm.

Your regulator in your alternator is going bad,go to Autozone and have them test it,it's over charging,could mess your battery up.

First ensure that the throttle interior is really clean, a surface layer of grime can upset the ECU's ability to meter and govern the air entering the engine. If the throttle body interior is found to be really oily and dirty check on the idle air control valve and make sure that it too is cleaned up. Next put the ignition switch in position 2 (on, but no start) and depress the accelerator fully and release. If there is an issue with the throttle position sensor this procedure can help the ECU relocate the throttle plate position. Start the engine and if it is still hunting locate the air mass meter (AMM) on the air ducting immediately next to the air filter and disconnect the electrical connection. Does this remove the hunting aspect?... the rev speed might change but for the moment concentrate on the evenness of the tick-over. If the idle is even when the AMM is disconnected but hunts when connected then look at replacing your AMM. Try spraying the AMM with no residue electrical cleaning spray to see if this improves matters.

is the alternator charging??if you have a battery charge light ..put ignition in start position..battery light should come on..if it dosnt the volatage regulator is shorted..if it does come on start engine..it should go off once engine has started.. buy a cheap dc voltage meter..radio shack 6.00 ..start engine,, get your voltage meter put red lead on battery positive black on neg.while engine is idleing it should read 13 to 14 volts..if you have a charge gauge on your dash check to see what your reading is..put headlights on at idle if it goes below 12 volts..its not charging..unless you have a short..then battery will lose charge overnight or after a few hours depending on short..that should cover most of it..happing hunting

check for a vaccuum leak this is where i would start if it rolled into my shop. possibly a PCV valve sticking or a hole in the hose, this hose should run from the valve cover, or crank case and then to the intake, or carb.

These trucks have what is called a smart charge system. It normal for the alternator light to elluminate when the truck requires no charging or the idle is low. The most likely reason you have low idle is throtttle body coking which requires cleaning or a bad idle air control valve. Also clogged injectors will cause a low rough idle but if that was the case misfires would show up and can be seen while they happen using a diagnostic data scanner.
And the first thing we always look at is the air cleaner. If you have a K/N filter the oil can contaminate the MAF senor. If the sensor cannot see and feel the air then it won't be able to add fuel causing a low idle, The MAF can be sprayed with brake cleaner to clean it. Usually you will get a p0171 and or a p0174 if the maf was causing a lean condition.

try cleaning the throttle body first then if that dont work you will need to replce the iac motor wich is located on the bottom of the throttle body your car is hunting for the correct idle position and either the sensor inside the iac motor is dirty or it is bad

i have a 1991 f-150 and had the same problem try changing the fuel filter and put gas treatment in the tank alone with fuel injector cleaner if that doesnt work you mite check the fuel pump and see if any rust or dirt is in the tank cloging things up