bagelluvr

I have a 1996 Cherokee Classic that has issues starting sometimes. Too it to the shop that installed the fuel pump and he said the fuel pressure was fine and started and stopped the Jeep all day never had an issue. A week later we got a "check engine" light and brought it back. The one code was for miss fire in cyl # 6 and another was for random miss fire. Suggested a tune up including plugs, wires cap and coil and said it should fix the problem. Well it didnt. My daughter told me the mechanic said to pump the gas pedal if it doesnt start! I didnt think fuel injection systems needed to do that! He said it moves the position of something (I do not remember) and he said the next time you try and start it is in different position and it will start. What sensor could it be???

XJ-92

Throttle position sensor

mschi772

Or the IAC. Annoying that your mechanic would suggest pedal-pumping instead of checking the TPS and IAC and replacing the culprit.

Bustedback

Stop taking it to that mechanic, he will cost you more money and do more damage to the Jeep.

sycoglitch

Bwahahahah thats job security right there at its finest.

bagelluvr

Thanks or the suggestions but how do I test to see what needs to be replaced? And you are 100% right Bustedback!

mschi772

TPS TESTING

You may have one or more of these Physical Symptoms:

1. The engine loses power and is stalling.
2. The engine will idle, but may die as soon as you press the gas pedal. When driving, it seems as if all power is gone.
3. Sometimes it feels as if the transmission is failed or isn't shifting properly, if at all. If you quickly jump on the gas you might be able to get the transmission to shift, but it won’t shift properly by itself. Shifting manually, the transmission goes through all the gears.

NOTE: The throttle position sensor is also DIRECTLY involved with transmission shifting characteristics. The TPS function should be verified early in the troubleshooting process, when a transmission issue is suspected.

TPS TEST

You should have 5 volts going into the Throttle Position Sensor (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 5 volts. At idle, TPS output voltage should be greater than .26 volts but less than .95 volts. Move the throttle and look for smooth meter response up to the 4.49 at WOT. The other wire will be the ground and should show no voltage.

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:

• 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".
• High Idle Speed: The TPS may report faulty values causing the engine idle speed to be increased above normal. This is normally found in conjunction with a slow engine return to idle speed symptom.
• Slow engine return to idle: A failing TPS can report the minimum throttle position values incorrectly which can stop the engine entering idle mode when the throttle is closed. Normally when the throttle is closed the engine fuel injectors will be deactivated until a defined engine RPM speed is reached and the engine brought smoothly to idle speed. When failing a TPS will not report the throttle closed and fueling will continue causing the engine to return to idle very slowly.
• Engine Hesitation on Throttle Application: The TPS is also used by the ECU to determine if the driver has applied the throttle quicker than the Manifold Air Pressure sensor can read. The fueling is adjusted accordingly to cope with the sudden increase in air volume, however a faulty sensor can cause the ECU to ignore this data and the engine will "hesitate" when applying the throttle. In extreme cases with the engine at idle, a sudden application of full throttle can stall the engine.
• Engine Misfire: A faulty TPS can report values outside the acceptable range causing the ECU to incorrectly fuel the engine. This is noticeable as a slight misfire and can trigger the misfire detection software and/or Malfunction Indicator Lamp (MIL) light on the dashboard. Extreme cases can cause excessive misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.

mschi772

Replace with Mopar TPS if it is faulty. If the TPS is fine, take the IAC off and clean it thoroughly with sensor or throttle body cleaner and put it back on. If symptoms continue to exist, replace the IAC with a new Mopar IAC.

mschi772

If things still aren't better, check your CPS.




TESTING PROCEDURE 1991 – 2001 4.0L H.O. engines

1. Near the rear of intake manifold, disconnect sensor pigtail harness connector from main wiring harness.
2. Place an ohmmeter across terminals B and C (SEE Image). Ohmmeter should be set to 1K-to-1OK scale for this test.
3. The meter reading should be open (infinite resistance). Replace sensor if a low resistance is indicated.

mschi772

Wait, so you're still misfiring after all the tune-up parts?

bagelluvr

misfire is goine and it runs better (and it should) but still has random start issues.

CCKen

Don't overlook the Camshaft Position Sensor in the Distributor. A faulty Cam Sensor can cause starting issues, and misfires.

xjmcb

Where is the IAC located?