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Discussion Starter · #1 ·
So I have searched the internet until my eyes started bleeding over this:
How does one test - and I mean definitively test - an IAC and a TPS on a 1999 (late model) 4.0L - those threads on renix 4.0L's are all wrong for me - not the right wire count, different sensors, etc.

I don't want to throw parts at the car, and id rather not go to the junk yard to pick up $2 sensors in the hopes they are good and I can use them to "test" the system. I could start swapping stuff with my brother's 4.0 but he actually has a drivability issue of his own hahaha.

I have a multi-meter, and the more than basic know-how (used to turn wrenches for a living) - but cant seem to find values or anything like that for either sensor. TPS I wish I had an oscilloscope for - but I can move it slow enough to check voltage for dead spots I suppose - is the only way process of elimination by confirming all other sensors that are possible culprits are good?

Or perhaps someone has the exact issue I have - since there seem to be an infinite variety of issues:

So, the basic issue is hunting idle - it has been getting more frequent and increasingly rough over the last few weeks. It doesn't seem to matter if the engine is cold or hot, and it doesn't stall. Normal idle for me is 750, this dips to maybe 550 and recoups and overcompensates to maybe 800. In neutral the issue all but goes away, and if I catch the idle when it dips as I accelerate it will result in a momentary stumble, but only if I accelerate at the low point of the idle. It *almost* seems like a front O2 sensor, with the exception that a) it doesn't stall and b) its not as erratic. I've had no sensors changed yet - I have cleaned the IAC and throttle body somewhat recently. The funny thing is, the idle doesn't move that much yet the result is profound. A lot of vibration at stoplights. No codes - no light, and none on the tool. Not really hard to start either - no long crank, no start then stall.
 

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Real Jeeps have dents
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Test procedures and expected test results are in the FSM. $7.95 at www.PacificCoastManuals.com. Simply from the symptoms described, I would strongly suspect the Idle Air Controller. TPS issues usually have more symptoms. A faulty TPS can cause poor idle control, the TPS is used by the ECU to determine if the throttle is closed and the car should be using the Idle Air Control Valve exclusively for idle control. A fault TPS sensor can confuse the ECU causing the idle to be erratic or "hunting".

You can try cleaning the IAC, but I would just buy a new one. Be sure to buy genuine Jeep sensors.

TPS TEST

The TPS is mounted on the throttle body. The TPS is a variable resistor that provides the Powertrain Control Module (PCM) with an input signal (voltage) that represents throttle blade position. The sensor is connected to the throttle blade shaft. As the position of the throttle blade changes, the resistance of the TPS changes. Along with inputs from other sensors, the PCM uses the TPS input to determine current engine operating conditions. In response to engine operating conditions, the PCM will adjust fuel injector pulse width and ignition timing.

The PCM supplies approximately 5 volts to the TPS. The TPS output voltage (input signal to the PCM) represents the throttle blade position. The PCM receives an input signal voltage from the TPS. It is best to use an analog meter (not digital) to see if the transition from idle to WOT is smooth with no dead spots. With your meter set for volts, put the black probe on a good ground like your negative battery terminal. With the key on, engine not running, test with the red probe of your meter (install a paper clip into the back of the plug of the TPS) to see which wire has the 5volts. This will vary in an approximate range of from .25 volts at minimum throttle opening (idle), to 4.8 volts at WOT wide open throttle.

Perform the test procedure again and wiggle and/or tap on the TPS while you watch the meter. If you notice any flat spots or abrupt changes in the meter readings, replace the TPS.

The TPS is sensitive to heat, moisture, and vibration, leading to the failure of some units. The sensor is a sealed unit and cannot be repaired only replaced. A TPS may fail gradually leading to a number of symptoms which can include one or more of the following:

 

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Discussion Starter · #3 ·
Well that didn't take long at all - thank you - I will be purchasing the FSM! Also, I am in agreement I think it is the IAC, but I hate to just throw parts at a car. That .25v to 4.8v on the TPS is exactly the type of info I was trying to get for both sensors.
 

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You may want to check your O2 sensors or the O2 sensor heater fuses that are under the hood before buying the IAC component.
This was my issue that I finally figured out after about a year. The rear O2 sensor wires were chafed, then shorted out. I replaced the sensor but didn't know it also had a fused heater circuit as well. The fuse was toast. It caused the CEL, idle hunting, stumbling, poor mileage a host of undesirable behavior.
BTW, I thought it was a TPS and IAC issue also and have the parts as spares now. I had the same approach as you have now.
I hope this helps and good luck.
 
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