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Yes, a defective IAC valve can cause an incorrect idle issue. The other reasons or possibilities are: kinked or damaged throttle linkage, improperly adjusted idle screw, defective, loose, or improperly adjusted TPS sensor, vacuum leak, leaking base gasket (carb.), or the timing is off (maybe the distributor has been adjusted or is loose). I hope these suggestions help. Please let me know how your repair goes of if you have other questions, good luck.
The computer controls idle through the idle control valve or motor. Sometimes deposits build up around it or in the air passages it controls. If it is taken off and cleaned, it might help. Use a good quality sensor safe cleaner. Check if the passages are clear. Or if the throttle position sensor is going bad, it could cause those problems. The sensor can be tested with a multi-meter. Have you checked for any trouble codes?
The Idle air control valve is the way the computer controls idle speed. If the computer can not control it, the warning light comes on. Look for gum and varnish in the throttle body or something holding the throttle plate open. Also could be a vac leak or broken hose. Also check for foreign matter in the hole where the IAC goes. Something may be holding the valve open. Even a broken gasket under the throttle body could cause the air leak.
Make sure the idle screw was not turnned causeing tps sensor to read over the spec limit, the next then is to make sure the idle air control valve is working , a bad valve will cause this problem and last is the stepper motor that controls the idle. some of these around this year used steeper idle control motors instead of idle air control valves and in turn the stepper motor is bad it will hold the throtle plate open to much causing the tps reading to be wrong triggering the engine light to come on. the steeper motor will be boltted to side of the throtlebody and theres a small arm to the throtle linkage. i will enclose a stepper motor picture and an idle air valve to show you, but this sounds like the tps is out of range due to this. the last thing is to make sure there are no vacuum leaks, spray crab cleaner around throtlebody base gasket and also the intake mounting gasket and see if the idle changes is so theres a vac leak in that area. good day. okay just did a little research and this truck does have a idle air control valve, the picture is encolsed and is screwed into throtlebody, check that and for vacuum leaks, make sure tps is good sometime it will test good when testing and go bad when it wants, you could have a bad tps and idle air control valve also.
Bad MAF (Mass Air Flow) Sensor - can cause erratic idling and stalling
Bad EGR (Exhaust Gas Recirculator) valve - erratic idling, skipping
Bad TPS (Throttle Position Sensor) controls idle under different conditions ie cold, hot, ac etc
Your problem may be any 1 of the 3 or a combination. Best advice is to get a free diagnostic test done (most chain auto parts stores will do this test free) and come back with the error codes, These codes will help to narrow down where the exact problem lies.
Vacumn leaks will cause a high idle as described..but if its the canister line it will only leak when canister purge solenoid is activated . and will likely not affect idle..When you changed the tps did you set it with a scan tool ?? the tps voltage at closed throttle must be set at .47 volts or a whole bunch of wacky things with the Idle can occur...
po102--maf sensor, could be a bad air cleaner and not letting air into engine, po122--tps pedal position sensor low, replace or adjust tps sensor,or wiring,po507--idle control system high input. for this you can take off idle air control, and clean out carbon from were it mounts on engine. it is just before throttle body at end of air intake hose, it is black. do not push on pintle on end of valve when cleaning, use a old toothbrush to clean carbon off of valve, use carb cleaner but do not soak valve in liquid of any kind. good luck!!