Increase then decrease idle when engine is cold especially

What Colorado do you have ? 2 wheel or 4 wheel drive ? Sport ,sport LS 3.5L , Z71, Z71 LS. etc. what size engine ? an does it have electronic throttle control ? I have factory service info an it is not showing that code .
DTC P0506

Circuit Description

The throttle actuator control (TAC) motor is controlled by the powertrain control module (PCM). The DC motor located in the throttle body drives the throttle plate. In order to decrease idle speed, the PCM commands the throttle closed reducing air flow into the engine and the idle speed decreases. In order to increase idle speed, the PCM commands the throttle plate open allowing more air to pass the throttle plate. If the actual idle RPM does not match the desired idle RPM within a calibrated time, this DTC sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:
DTC P0506 Idle Speed Low
DTC P0507

Circuit Description

The throttle actuator control (TAC) motor is controlled by the powertrain control module (PCM). The DC motor located in the throttle body drives the throttle plate. In order to decrease idle speed, the PCM commands the throttle closed reducing air flow into the engine and the idle speed decreases. In order to increase idle speed, the PCM commands the throttle plate open allowing more air to pass the throttle plate. If the actual idle RPM does not match the desired idle RPM within a calibrated time, this DTC sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:
DTC P0507 Idle Speed High
Cleaning the throttle body mite take care of it !
Cleaning Cleaning An Electronic Throttle Body Safely

Hi Stephen, The problem with the idle speed being to high could be with the cold start up idle motor, located on the throttle body, more especially if the idle speed is erratic with surges of increasing and decreasing speeds. If the idle speed is high and without surging the problem could be a broken or loose fitting vacuum pipes. Check to make sure all are fitting tightly and they are not cracked, broken or loose. Lastly there could be a problem with the intake manifold gasket.Lack of power could be related to the above, or if your vehicle is fitted with a distributer, the ignition timing may require setting. If required, the job must be carried out with a stroboscopic timing light and a bridge connecting T to E1 inside the diagnostic socket (usually located on the fire wall above the gear box side of the engine. Paint a spot a white paint on the timing cover on the TDC mark and another in the notch on the crankshaft pulley, Start the engine and leaving it idling at 650 rpm loosen and adjust the distributer by aiming th timing light at the front pulley, moving the distributer until the painted marks align with one another. Lack of power can also be due to poor service, a clogged fuel or air filter or a fuel pump becoming lazy (Though this is unlikely as they either work or fail) Have the engine cylinder compressions tested.and make sure it is with specifications. Hope this info is some help? Regards John

This sounds like the RPM may be a little high, but normal operation is called a "cold start routine". Like the old fashion Choke, the "cold start routine" is suppose to increase the RPM for several minutes and then decrease to running idle speed. This will engage in each "cold" start, but not on a warm engine. It is designed to do this.

Usually when an engine runs higher RPM than it is set for, it means extra air is entering the engine from somewhere. Using a spray water filled bottle, go around seams and also spray vacuum lines and the EGR pod while the engine is running. Any openings will draw in the water and affect RPM. Then you will have found the leak.

Now you can look up you exact engine on the Web and find out 2 things. If it has a 2 piece intake manifold and if it has intake Manifold runners. The 2 piece design often failed when the gasket holding the 2 halves started dissolving. The intake runners, could stick out of position and make RPM higher.

Now aside from these 2 items, you might get a Code showing that the engine is running lean. This would actually not be showing the lack of fuel, but the overabundance of air coming from the leaks.

Check the air intake sensor.

Check your power steering fluid level ?

Check engine light has reported that the cpu has found sensoes out of parametyers during the checks. Check fault codes for oxygen sensor, idle speed sensor and air flow sensor.

Hi Malcolm, We need to know what vehicle and what engine you are referring to. The earlier injection engines did have an air volume screw which increased or decreased the idle speed, but on later models the control is computerized and not adjustable. Have a diagnostic scan carried out as there may be a defective temperature sensor and if so it may be sending incorrect info the the management system, increasing the idle speed. Regards John

When engine is at idle does it run smooth until the transmission is engaged and you increase engine load it starts to stuble? If so you may want to check fuel pressure, possible fuel pump or fuel filter at fault.

Hi Dave the ECU receives messages from sensors, one of which is a temperature sensor. When the sensor responds to a temp higher than the normal operating temp the fans are then and only then supposed to operate. They will switch in for a brief period and then turn off again. The idle speed is in direct relation to the cold start up idle requirements and specifications. When the engine is cold the idle speed will automatically increase and as the engine achieves operating temp, the idle speed will decrease inthe way you have recorded. If you start the engine and allow it to idle until normal temp is achieved the fan should cut in. The way you've desribed all systems are A okay!. Regards John

remove and clean IAC ( idle air control ), this is what controls idling on to days cars. if this does not help a lot, check or replace coolant tempurature sensor. it may be bad, if it is, it will think the engine is always warm and send wrong info to pcm. good-day ! and may god bless. you can also have a vaccumn leak ! if check engine light is on, get it scanned to know what area is causing the problem.