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1993 XJ Using lots of fuel/O2 sensor problems

Old 03-17-2018, 09:54 PM
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Default 1993 XJ Using lots of fuel/O2 sensor problems

Hey guys! Been a while but I'm still here and happy with my Jeep. Well kinda.

So I noticed my Jeep has been terrible with gas lately (12mpg). I have no engine light on and I know it works cause it came on for a bad alternator a few weeks ago. I checked codes and I got a 21 and 51. Rich fuel mixture detection. So. Here's the weird part, back when I first bought my Jeep, it had no 02 sensor, plug was hanging, yet the engine light wasn't on. So I installed one. Did a few other things and been driving for the past few years averaging around 15-18 mpg (now 12 mpg). I've read that a missing 02 will set a MIL on. I unplugged it to see if it would and it still doesnt. Another thing is that no additional codes are set with it unplugged. This doesn't seem normal to me.

Anyways. So basically i got codes 21 and 51, bad mpg, and the weird situation above. Can anybody lead me in the right direction as to what I should check. I don't want to throw parts at it but if someone could give a few tests to do or lead me to a good place to start checking . . .

Also, within the last 2 years/20k Miles, it has had new new 02 sensor, new map sensor, and fuel injector upgrade. Just figured I'd mention it.

Thanks guys, appreciate it.
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Old 03-18-2018, 06:42 AM
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Wouldn’t hurt to check air filter and clean out Throttle body and IAC as well. I chased down a similar issue, I ended up having a bad wire in the o2 sensor as well as a blown fuse for the o2 sensor as well.
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Old 03-18-2018, 12:44 PM
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Originally Posted by PoorMansWrangler
Wouldn’t hurt to check air filter and clean out Throttle body and IAC as well. I chased down a similar issue, I ended up having a bad wire in the o2 sensor as well as a blown fuse for the o2 sensor as well.
The air filter is fairly new and I cleaned the throttle body and IAC a few months ago as well. I will check my fuses. That's seems like a good place to start. Thanks
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Old 03-18-2018, 08:07 PM
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Ok. I check fuses and they're all good. I also checked wiring for anything obvious or broken. And all looks well. any other ideas? Thanks
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Old 03-19-2018, 06:18 AM
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Could be your throttle position sensor acting up? What brand o2 sensors did you use? I had issues with the Bosh ones. Most recommend the NTK brand. I would also try seafoaming the intake and see if that helps it any. In my case I went through the o2 sensors, Seafoam, throttlebody and IAC, air filter and TPS before something stuck
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Old 03-19-2018, 06:20 AM
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Also checked for a clogged CAT, that can throw a code for the o2 sensors
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Old 03-19-2018, 07:54 AM
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The 02 sensor is a Bosch so maybe I should try a different brand. Forgot to mention but Cat is new. Any test I can do to verify a bad tps
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Old 03-19-2018, 11:21 AM
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Concentrate on the 21, for the throttle position sensor.

The throttle position sensor can be tested and you should do that. Info below.

------------------------------------------------------------------------------------
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.
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Old 03-19-2018, 12:28 PM
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Ok. Ill test it. but is code 21 for the TPS. I thought it was for 02 sensor. Here's what I was goin by. Correct me if I'm wrong cause I thought I had an 02 sensor problem this whole time. Lol. Also might mention that the Jeep runs great, no hesitation, quickthrottle response, etc. just terrible mpg
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Old 03-19-2018, 01:13 PM
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Originally Posted by 93XJeeper View Post
Ok. Ill test it. but is code 21 for the TPS. I thought it was for 02 sensor.
My mistake, 21 is for oxygen sensor. The 2 digit OBDI code for the throttle position sensor is code 24.

NGK is the preferred brand of oxygen sensor for the 4.0 engine, it is what your XJ came with off the factory line. Some 4.0s are finicky and don't play well with Bosch oxygen sensors.
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Old 03-19-2018, 03:04 PM
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Originally Posted by tjwalker
My mistake, 21 is for oxygen sensor. The 2 digit OBDI code for the throttle position sensor is code 24.

NGK is the preferred brand of oxygen sensor for the 4.0 engine, it is what your XJ came with off the factory line. Some 4.0s are finicky and don't play well with Bosch oxygen sensors.
Alright. Let me try a sensor and see what happens
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Old 03-19-2018, 03:27 PM
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Originally Posted by 93XJeeper View Post
Ok. I check fuses and they're all good. I also checked wiring for anything obvious or broken. And all looks well.
"Looks well" isn't enough with wiring. It's a good starting point. Now you need to do a continuity test.
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