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could be a number of things, from a vacuum leak to the idler needing cleaning. to a bad connection on the auto box sender unit. can also be a overcharging alternator this would change the ecu readings and enhance them, but also damage the ecu and the battery. 14.03 volts should be the max any higher then its a problem.
hunting or surging ilde. name the 2 RPMS, low and high , is it hot or cold engine this issue.? 400rpm or 1000 or 2000 all are diff.causes.
no year, stated. so cant help.
but ill answer in the generic,
1: the ECU (got a carb,? )
the ecu controls idle speed at all times.
if not , there is a problem (you)
A: air leaks in the induction will make motor race at some point the ECU/ISC loses control. and hunts. trying hard to correct it.
b: egr stuck open idle too low, iSC cant correct it,, bad EGR.
c: engine EFI running way too leak starving for fuel so it hunts for 2 reasons < ISC hunting, and lean does that.
lean is gross power loss and opening throttle bogs the engine.
so is double bad. ISC makes it worse,
all this breaks down to 2 reasons.
1: ISC bad. in some way (gummed up. or restricted inside)
2: bad fueling or air leaks. vacuum leaks.
try answers , the hot cold question and the 2 speeds, of the hunt.
lowest rpm and highest of the hunted range of RPM.
take out idle air motor stuff a plastic bag in the hole deep and pack it in there hard hold it in there,2 people, start motor dont take your hand off of key engine may run away with rpm and need to be turned off if you didnt seal that hole well if it idles really low and doesnt race then bye air idle motor ifit still races the same then check for vaccumn leaks if you cant under stand then dont try this
sounds like you have a vacum leak in one of the vacum lines or you need to change the idle air control valve. start the engine and raise the hood and lisen for a vacum leak and check all vacum lines for dryrot. if no vacum leak can be found then replace the idle air control valve, this controls your idling
It sounds very much like the idle air valve is faulty as I have just had same sort of problem I fitted a new idle air valve and it has cured mine. Unfortunately these idle air valves are not cheap. They are about £70 from ford but you can get them on ebay for about £30 from partsdome
need to do a diagnostic ,but you could have an air leak on the manifold,but if you did then it would race on idle,try looking at the air flow control valve.If you cant find the fault then take the car to a dealer to fault find it as you might evan have a problem with the ECU and it needs re-programming.Its all very well someone saying this or that but these parts are not cheap and if its not diagnosed properly then its cost you more than going to dealer.
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