Throttle Position Sensor (TPS)


The job of the TPS is to tell the computer what the position of the throttle is.  This sensor is vital in helping the computer determine if the throttle is closed or open; or how fast the throttle is opened or closed.  The throttle position sensor is a simple potentiometer that uses ground and 5-volt reference inputs to produce a varying output signal depending on the position of its detection arm or shaft.  At rest, this sensor outputs a relatively low voltage signal; as the arm/shaft is turned (as it would when the throttle opens), the output voltage increases.  If this sensor is out of adjustment or is failing, the result could be stalling, idle surge, flat throttle response, hesitation, or erratic engine operation.  Most of the TP sensors you find on stock Fiero engines are the adjustable type while those found on newer GM engines are usually non-adjustable.  If the TPS on your engine is adjustable, it must be set correctly in order for the computer to function normally. 


Some throttle position sensors are adjustable.  In order to adjust the TPS, you will need a scan tool (or laptop/PC running scan tool software) or a digital volt meter.  You will also need the proper tool to loosen the retaining screws.  If you have a scan tool, all you need to do is pull up the TPS voltage data.  If you are using a digital volt meter, then voltage will need to be measured across the blue and black wires going to the TPS.  I suggest this measurement be made by going directly to the connectors at the ECM (located between the seats)  so the weather seal is not broken in the wiring/connection out in the engine compartment.  If this seal is broken, moisture can corrode the connections to the TPS and create all sorts of problems.  Once you get your TPS voltage display up, turn the ignition on but do not start the engine.  The voltage with closed throttle should be between 0.400 and 0.625 volts for most applications (check appropriate reference guides for your application’s specified voltage range).  If the voltage you get is not within spec, loosen the TPS retaining screws and adjust as necessary.  Normal voltage reading for wide open throttle (WOT) is above 4.00 volts.


There are trouble codes that are associated with the TPS.  One code will set if the TPS voltage is too high when the computer expects to see it lower.  Another code will set if the TPS voltage is lower than the computer expects to see.  The TPS code for low voltage is the most common and will usually set if the TPS is out of adjustment or the sensor has failed.  The first thing you should do when you get a TPS code is to check adjustment and signal output of the TP sensor before replacing it.  Be sure to wiggle all connections while watching scan data/voltage readout to make sure the problem is not a loose or bad connection.


There are circumstances that could occur with a failing throttle position sensor that may not set a trouble code.  One of the most common symptoms of a failing TPS would be a tip-in hesitation or stumble when you apply throttle to take off from a stop.  This can be caused by a dead spot in the TP sensor’s internal circuitry, which usually causes the output voltage signal to not change (or it drops out) when the throttle opens.  Unfortunately this type of failure is not easy to diagnose without the proper tool – a digital waveform scope.  Most digital volt meters and scan tool displays will not respond fast enough to show this type of a glitch; but some may.  If you do find this fault, then the obvious fix is to replace the TP sensor.

Idle Air Control Valve (IAC)

The IAC valve GM uses for most of their engines is a stepper motor actuated valve.  A stepper motor is a device that moves a predetermined amount per electrical signal it receives by whatever device is controlling it.  The GM IAC has a pintle that extends or retracts into or out of an idle (bypass) air passage in a throttle body or intake manifold attached to the engine.  Typically as the IAC’s pintle extends, air flow to the engine is restricted as the air passage is shut off.  As it retracts, air flow to the engine is increased as the air passage is opened up.  This air passage is a simple bypass for incoming air to take around the throttle blade.


Diagnosing problems associated with the IAC aren’t simple.  There are many other causes that can make a IAC valve appear faulty.  In many cases, there may only one or two trouble codes in the ECM/PCM assigned to the IAC, but these trouble codes can set for a variety of reasons.  Basically, this trouble code sets if the ECM cannot make the engine idle at a set (desired) speed by control of the IAC valve’s position.  There are limits set up within the ECM that only allow it to move the IAC in and out of the idle air passage so far.  The IAC valve position is referred to as IAC counts.  Scan tool data indicating “0” IAC counts means the IAC valve is fully extended (shutting off idle airflow to the engine); and “255” IAC counts means the IAC valve is fully retracted, allowing as much air to enter the engine via the throttle bypass passage as possible.  Some ECMs may never allow the IAC to reach 255 counts.  Generally, anything you see over 160 counts should be considered to be a near- or fully-open idle air passage.


On a normal operating engine, it is typical to see high IAC counts (100 or more) when the engine is idling cold and during the warm-up cycle.  As the engine warms, the IAC counts should decrease.  By the time the engine reaches operating temp (fully warmed up), the IAC position should drop into the range of about 20-50 counts in park or neutral.  If the scan data you get reports counts lower than this, then that can indicate one or more of the following problems exists:


·        There is a vacuum leak allowing unmetered air to enter the engine

·        The throttle stop screw is adjusted incorrectly (throttle being held open too far; more on this later)

·        There is a problem with the throttle cable or cruise control system that isn’t allowing the throttle to close all of the way

·        The IAC valve itself is faulty



Now if you see the scan data reporting IAC position higher than 20-50 counts on a fully warmed up engine, this could indicate one or more of the following problems exist:


·        There is carbon buildup on the IAC pintle, or in the idle air passage restricting air flow

·        There is carbon buildup on the throttle blade or throttle body bore

·        The throttle stop screw is adjusted incorrectly (throttle resting closed too much)

·        There is a mechanical problem with the engine resulting in lower than expected vacuum levels at idle (this will require the IAC to open further so the engine gets the required amount of air to maintain the preset idle speed)

·        There is increased load on the engine (such as what would occur if the automatic transmission was shifted into gear)

·        The IAC valve itself is faulty


Any one of the above issues can cause a an IAC fault code to set in the ECM.  As you can see, there are many issues other than a faulty IAC valve that can cause a code to set.  So before replacing the IAC valve, you should check all of these possible issues first.


            The IAC valve cannot be tested using conventional electrical testing means.  There are special tools available that are designed to test GM IAC valves, but I have discovered most shops don’t have these tools anymore.  To be quite honest, you don’t really see many IAC valves fail.  When they do, they usually freeze up or get stuck in a fixed position.


            The ECM “resets” the IAC valve when the car is operated at normal road speeds (35mph or more).  During this time, the IAC valve is typically extended out all the way (IAC counts = 0), thus closing off the idle air passage.  This helps the ECM “learn” the position of the IAC valve.  Any time the IAC valve is replaced, this “learn” procedure should be performed.



Throttle Stop Screw (minimum air setting)


            The throttle stop screw’s primary function is to prevent the throttle blade from closing too far and getting wedged/stuck in the throttle bore.  However, it serves as a secondary function to adjust the minimum air setting.  The “minimum air setting” is what is used to describe the amount of air that is allowed to enter the engine thru a “closed” throttle.  Because the throttle valve cannot be allowed to completely close (because this would result in it getting wedged/stuck closed in the throttle bore), some air will always be allowed to enter the engine around the throttle valve.


            On a 100% factory stock engine, you should never need to adjust the throttle stop screw.  This is the reason why GM installs a tamper-proof plug over the throttle stop screw on the throttle body.  But there are times when the adjustment of this screw is necessary.  One example of this is when the engine is modified, or a different throttle body is being used than what originally came with the engine.


            Larger displacement engines require more air to maintain a set idle speed.  Aftermarket camshafts with lots of duration or lots of overlap tend to lower the amount of vacuum an engine can generate at idle.  Lower vacuum levels translate to less pressure differential between the intake manifold and outside (ambient) air.  This means there isn’t as much pressure difference to force air into the engine around the throttle blade or thru the IAC passage at idle.  And either the IAC needs to open up or the throttle blade must be opened more to allow more air to enter the engine.  Engines that have higher compression or are new/rebuilt can have higher internal loads/friction which can also result in a drop of idle vacuum levels.  Basically any condition that increases load on the engine will result in the vacuum level to drop at idle, which will require the IAC or throttle blade to be opened up to compensate.


            Setting the throttle stop screw can be  accomplished a couple of different ways.  If you have a scan tool, I recommend allowing the engine to warm up to operating temperature and then adjust the throttle stop screw in or out until the observed IAC position counts come to rest within the spec range I  provided earlier.  After adjusting the throttle stop screw, it may be necessary to adjust the Throttle Position Sensor, which we discussed in my Jan/Feb 2008 segment.  On cars that don’t have an adjustable throttle position sensor, the ECM automatically learns the “closed throttle” voltage when the ignition is keyed on (after the key has been off for at least 10 seconds).  If you don’t have a scan tool and you are working with a pre 1994 model year ECM, you should be able to adjust the minimum air setting by doing the following steps…


1)      With the IAC valve connected, ground the diagnostic (ALDL) terminal (same as you would do to flash trouble codes thru the check engine light).

2)      Turn ON the ignition, but do NOT start the engine.  Wait at least 30 seconds.

3)      With the ignition still on, disconnect the IAC electrical connector.

4)      Remove grounding of the diagnostic (ALDL) connector and start the engine.  Allow the engine to fully warm up and go into closed loop.

5)      Adjust the idle stop screw so the engine idle speed obtains 550rpm in drive (auto trans) or 650rpm in neutral (manual trans).

6)      Turn the ignition off.  Disconnect power from the ECM for at least 10 seconds to clear codes (in case any are present) and reconnect the IAC electrical connector.


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