SoDelxj99

Lately I have been having a rough idle and when I go to take off it wants buck and surge. It seems to go away after jeep warms up. But it is severe. Step on throttle and jeep either almost died and then take off then die and take off. I took throttle body off and cleaned it and air idle control. No engine codes are generated though. Any thoughts????

CCKen

I had a similar experience once. I changed my upstream O2S and it fixed it.

Tfox671

Mine does it if I don't let it warm up. I drive an 87 with a 5spd. She's cold-blooded.

SoDelxj99

Someone mentioned to change tps sensor???? Any chance it is that over upstream o2 sensor???

tjwalker

I agree with Ken that the upstream oxygen sensor is a suspect here.

Does it happen immediately when cold (and I mean immediately......) When completely cold, upon startup the engine uses default values for mixture and the oxygen sensors aren't in play, but the oxygen sensors do come into play very quickly as engine warms up. What I'm getting at is if the engine is not symptomatic for the first minute or so when you start a stone cold engine, but becomes symptomatic immediately after that.....that would make the oxygen sensor even more suspect. How many miles on your oxygen sensors? They have a finite lifespan of between 100k-150k on the 4.0 HO engine from my experience. If you end up replacing one, replace both of them and use NTK sensors; they are the OEM sensor and are the best choice for your 99.

Also whenever symptomatic, I always review tuneup hardware status. Plugs, wires, cap, rotor. How many miles on these items?

The TPS can be tested and I'll include how to do that below, but this really doesn't sound like a TPS issue to me.

I'd test fuel pressure though with a cold engine to make sure you don't have a weak fuel pump. For a 99, you should have 49 psi, plus or minus 5 psi. Fuel pressure gauge can be rented from some parts stores; call around.

Good luck and keep us updated!
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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 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 fault TPS can report values outside the denied 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 excessing misfires resulting in one or more cylinders being shut down to prevent engine and catalytic converter damage.

Lowrange2

Yup. My wifes did the same thing. O2 sensor fixed the issue.

SoDelxj99

Thanks. A lot of great info. The o2 have 101,000mls on them. The tune up was just done. All new oil, filter, plugs, wires, cap, rotor. Removed throttle body and cleaned with throttle body cleaner. So I think I will replace o2 sensors and see how it goes! Keep you posted.

SoDelxj99

Just installed new up and downstream sensors yesterday. Problem seems to be gone. Fingers crossed....gonna keep an eye on it over the weekend. But I think it's fixed. Thanks to everyone for the great info as usual.
Special thanks to TJWalker....awesome info and help!!!!!