under the dash, by your right knee

 

That is a bit more difficult on a 1991 as its OBD I and not OBD II...

 

Why is that?

 

I'm betting on the o2 sensors. It should be throwing a code though.

 

Why is it ODB I or why did I say its a bit more difficult?

OBD I stopped in 95/96ish, then came OBD II. Your avatar says your truck is 1991.

It'll be a bit more difficult because its not as common as OBD II. OBD I wasn't really "universal" if I understood correctly, so not all diagnostic systems are the same from vehicle MFG to MFG.

 

LOL. I was asking Why is it more difficult.

 

I have a 2000 4.0. About every 10 second It clicks, idles up for about 10 seconds, then clicks and idles down. It Does it all day. Also he check engine light came on last week. Any kind of advice would be welcome. I am new to the jeep world.

 

Strategy for any problem that involves a check engine light is ALWAYS the same.

Pull the codes and post the EXACT code number here for comment.

Autozone will pull the codes for you for free; otherwise you will need a code reader or a scan tool or any shop can do this for you for a nominal cost.

 

I just got back with a code P1494 evap ventilation or mechanical fault. Possible cause open or short circuit condition, poor electrical condition, failed vent solenoid, and vacuum hose plugged. Any information for my next step would be very welcome. Sorry about stepping on your thread.

 

next step: start your own thread.

 

Ok, thanks to defiance I finally read my codes, I had to do the on, off, on, off, on then turned the ac on then off and finally got some codes, 12 & 21. I also don't have the plug under the dash.

 

12 is the battery has been unhooked in 50 starts and Ill look up 21 in the fsm tonight.

 

Well the battery hasn't been unhooked in about 6 months and I drive it almost every day so I don't know what that means.

 

12 is normal; means nothing.

Code 21 is the THROTTLE POSITION SENSOR circuit code.

That means you need to test and or replace your throttle position sensor. Here is more on it.
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The throttle position sensor is connected to the throttle shaft on the throttle body. It sends throttle valve angle information to the PCM. The PCM uses this information to determine how much fuel the engine needs. The TPS is really just a simple potentiometer with one end connected to 5 volts from the PCM and the other to ground. A third wire is connected to the PCM. As you move the accelerator pedal with your foot, the output of the TPS changes. At a closed throttle position, the output of the TPS is low, about a half a volt. As the throttle valve opens, the output increases so that, at wide open throttle, the output voltage should be above 3.9 volts. Testing can be performed with an electrical meter. Analog meter is best. You are looking for a smooth sweep of voltage throughout the entire throttle band. While slowly opening and closing the throttle, take note to the movement of the voltmeter needle. There should be a direct relationship between the needle motion to the motion of the throttle. If at anytime the needle moves abruptly or inconsistently with the movement of the throttle, the TPS is bad

You should have 5 volts going into the TPS. At idle, TPS output voltage must be greater than 200 millivolts. At wide open throttle (WOT), TPS output voltage must be less than 4.8 volts.. The best is 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. One of the other wires should show .26V (or so). The other wire will be the ground and should show no voltage. Move the throttle and look for smooth meter response up to the 4.49 at WOT.

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: -

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

 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 acordingly 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 fault TPS can report values outside the deined acceptable range causing the ECU to incorrectly fuel the engine. This is noticable 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 excessing misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.

 

ok, thanks. Coach