Statement made in Haynes manual - JeepForum.com
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post #1 of 18 Old 02-20-2020, 06:22 PM Thread Starter
CodemanLivs
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Statement made in Haynes manual

I know there's countless threads on the topic of O2 sensors and I have read them all !
My 95 XJ, 4.0 flashes codes 12 , 21 , 55. (No CEL, just has a miss at idle) This is besides the point.

Reading the small section in the Haynes manual that describes the procedure for testing the O2 sensor and circuit. At the end of the paragraph explaining how to use volt meter, it states and I quote... (I'm getting to my point now)
"If the meter reading doesnt fluctuate, the sensor is probably bad (although a fuel system problem could be the cause) "

So my question is... What the hell kinda open ended statement is that ??
Can someone tell me please what in my fuel system could be causing an 02 sensor to not function properly?

This is the fourth new sensor I've put in it and "I know" .... yes, It's an OEM , NTK sensor.

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post #2 of 18 Old 02-20-2020, 06:57 PM
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A normally functioning o2 sensor will have an output signal between zero and one volt in response to the amount of oxygen that it senses. When the engine is in closed loop operation and using the o2 sensor(s) to adjust the fuel trim, the o2 sensor signal should constantly switch back and forth from high to low roughly once every second or so if things are working like they should. In other words, the engine computer is constantly adjusting the injector firing pulse width to achieve the correct mixture (normally stoic or roughly 14.7:1 AFR) and the results of that burned mixture are picked up by the o2 sensor which then feeds back into the computer which adjusts the injector firing and on and on ad infinitum.

If your o2 sensor signal is in the correct zero to one volt range but it doesn't switch hi/lo/hi like it should, that can be caused by a problem with the sensor signal wire such as a short or open, or it could be caused by a fault in the fuel system causing the engine to run too lean or too rich to correct for such as low fuel pressure, restricted fuel flow, leaking injectors, injectors sized much too large, etc. Another problem that can cause an o2 sensor to switch too slow or give a false signal would be an exhaust leak, which would allow extra oxygen in from the ambient air to mix with the exhaust gasses.

If however your o2 sensor signal is showing above one volt after changing the sensor multiple times, then most likely the sensor heater wires are shorted to the signal or sensor ground wires.

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post #3 of 18 Old 02-20-2020, 07:34 PM Thread Starter
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MukLuk,
Thank You....That was exactly the answer I needed !

So if I stick a pressure gauge on my fuel rail, shut the engine off and the pressure very slowly drops is that a leaking injector?
Also , I seem to get mixed results in regards to the amount of pressure I have, some times 35 and 40 ( 40 with the regulator unhooked) and other times its lower. Could be operator error too.
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post #4 of 18 Old 02-20-2020, 08:35 PM
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Before doing anything else, I'd start by using a scanner to find what the actual P0xxx code is being thrown that's causing the 21 MIL code. That will narrow down whether the fault is with the upstream or downstream sensor circuit and should tell us what the fault is, whether that be high/low voltage, slow response, etc. From there we can troubleshoot what is actually causing the code. Alternatively, you can grab your voltmeter and start checking voltages from the sensors and at the harness plugs to manually see what the fault may be.

Keep in mind fault codes indicate a problem with a circuit, not necessarily a component on that circuit.

Checking the fuel rail pressure after shut down will only tell us if there is a leak down problem or not; the source of any leak down could be from an injector, yes, but it could just as easily be from a faulty check valve in the pump or regulator.

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post #5 of 18 Old 02-21-2020, 12:23 AM Thread Starter
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95 XJ's only have the one O2 sensor and with OBD1 I don't know of a way to connect my ScanTool to it. (Scratching my head) Am I missing something?
I've checked the voltage at the sensor and everything appears ok.
I'm not getting a check engine light like I did with the Bosch sensors. Just has a miss especially after a cold start. Makes me wish I could adjust the timing.
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post #6 of 18 Old 02-21-2020, 01:40 AM
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Sorry, thought by '95 they had OBD2. Does the Jeep still give you a 21 MIL code after clearing the stored codes or is the only issue now an idle misfire?

Thoughts for misfire at idle only:
- Verify serviceability of ignition components, read spark plugs for indications of problems.
- Perform a compression check to see if you have a low cylinder.
- If compression is low, perform a leak down check to determine source of loss.
- Use a noid light to see if all injectors are firing consistently.
- Connect a vacuum gauge and run the engine at idle, observe the needle to determine possible causes. Vacuum Gauge how to
- Check for lazy/leaking valve lifters.
- Check for leaking crankcase breather hoses and loose fittings.

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post #7 of 18 Old 02-21-2020, 10:07 AM Thread Starter
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Ok . Ready??
Here goes....
The miss appeared after changing the 02 sensor , do to the original one went bad.
So I went to crapzone and got a Bosch , it ran so so for a day then the CEL lit up again. So I returned it for another Bosch, same results. All the while I was on the forum and finally came across enough evidence to get me to try a different sensor.
Went to Napa and picked up an NTK. Soon as I started it the CEL would light up. Code was 21. I was like WTH ?!? So I let it run for a bit. Then I cleared it and restarted it and the light stayed off. But the miss was there like it was with the other 2 sensors.
Over the course of the next few weeks every day when I started it the CEL would light instantaneously., If I quickly shut it off and cleared it, It would come back on as soon as I started it. But if I let it warm up and then cleared it , it would stay off. So I thought it may be the heater. (In the sensor)
Before I had a chance to swap it out I drove it a few times and one day the CEL stopped coming on. I thought the bulb went out...haha.
But the miss remained.
Out of curiosity I check the codes and with NO CEL it was the same as it had been 12 , 21 , 55.
Exchanged the sensor for another one (same exact sensor). Started up, miss still there , NO CEL, codes still the same. Drove it around a cpl days, NO CEL, code still remains as does the miss.

There is a whole other story that supercedes this one. But it's not the cause of all this. This started the day I removed the old sensor. Up til then this was my woman's daily driver, never had problem with it. Well , nothing but normal wear and tear.

MukLuk...I sincerely appreciate you helping me out here. I was about to start banging my head on the wall pretty soon.
I'm a pretty good mechanic been wrenchin on everything since I could pick up a wrench. But this one has almost whipped me! It's very helpful just being able to bounce ideas off of someone else . So I can't thank you emough for takin the time to help me.
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post #8 of 18 Old 02-21-2020, 11:09 AM
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Haynes and Chilton's manuals are for oil changes and brake jobs. Once you actually start fixing things, you need a real service manual with proper instructions and accurate testing / diagnostic procedures.


Both manuals have been long ago proven to be too generic in coverage, full of inaccuracies, and short on instructions. Haynes and Chilton's omit lots of helpful/necessary steps, often assuming you know what you are doing already, and other useful information to save paper and printing costs. A genuine Jeep service manual has complete disassembly/assembly and testing procedures. Try www.pacificcoastmanuals.com.

I find Haynes and Chilton's manuals work best to soak up oil spills, or start campfires.

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post #9 of 18 Old 02-21-2020, 02:09 PM
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Does the idle misfire go away if you leave the o2 sensor unplugged and run the Jeep? It's safe to do this short term for troubleshooting, the PCM will just stay in open loop operation, though you can expect the MIL to illuminate because the PCM isn't seeing the o2 sensor signal.

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post #10 of 18 Old 02-22-2020, 03:55 AM Thread Starter
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CJ7 Tim, the Haynes came with the XJ, I definitely agree with you and I generally only use it for specs, values etc..
I wasnt at all surprised by the authors lack of info by any means. Just sure would have been nice if they could of at least pointed me in some kind of direction .
They sell enough of their books , hopefully by now they have invested some of the profits in a better editor.
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post #11 of 18 Old 02-22-2020, 04:20 AM Thread Starter
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MukLuk,
I did run it with the O2 disconnected, I guess I was more interested in the getting it to work that I didnt pay much attention to the idle and it was over a month ago. I will try it this afternoon and report my findings.

And regarding your previous response...
I rebuilt the motor six months or so ago. The head was replace a year ago with a practically new head, then I lapped valves and installed new stem seals when I had the motor out. Also, it has all new gaskets, belts, hoses along with all new ignition components.
The motor does not leak a single drop of oil, nor are there any leaks in on or around the intake or the exaust manifolds.

I'm not perfect by any means, but I'm always very thorough and meticulous. I stand behind my work and cutting corners only makes 2 more !!
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post #12 of 18 Old 02-23-2020, 08:43 AM
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Quote:
Out of curiosity I check the codes and with NO CEL it was the same as it had been 12 , 21 , 55.

I'm surprised that nobody told you to clear the memory out of the pcm. Even if you replace parts it stays there. Also it's wise to clear the pcm so it can learn(adjust parameters)to the new parts.


21 Oxygen sensor signal doesn't change (stays at 4.3-4.5V). Probably bad oxygen sensor



Not to familiar with the OBD1's but with this code the pcm will probably run off of a default setting not what it is getting from the 02(OBD2 does). First thing you need to do is clear the pcm, Disconnect the battery and turn the headlights on(or touch the two battery cables together). this will clear the pcm so start the engine and let it idle for a wile before taking out for a ride. See what happens, If the 21 comes back then you need to hook up a scanner or phone app that reads live data so you can see what the 02 reading is. It may not be the 02 but the wiring to it.



Quote:
I find they work best to soak up oil spills, or start campfires.
I think the Chiltons has a bit more info but both are useless for any rear repairs.
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post #13 of 18 Old 02-24-2020, 03:41 PM
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Quote:
Originally Posted by mukluk View Post
Does the Jeep still give you a 21 MIL code after clearing the stored codes or is the only issue now an idle misfire?
Thought it was pretty clear that one should clear the PCM fault memory then see if the fault recurs.

@CodemanLivs
Any updates? Have you run the Jeep with the o2 sensor disconnected and paid attention to whether the misfire stops when doing so?

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post #14 of 18 Old 02-25-2020, 05:08 AM Thread Starter
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Ok.... I ran it with the sensor disconnected, it does not make one bit of difference. If I rev it , it's smooths out and it's not even a consistent miss at idle it's more like one of those kinda popping like when your popcorn is almost done in the microwave... Haha

Yes I clear the codes often, getting really tired of reprogramming the dang radio !!

Any way, as far as hooking any scan tool up to it for live data, not gonna happen.
I have an old Snap-On Brick, and I know I've tried in the past to connect that awful spagetti looking adapter, I threw in towel !

And there's no points in the dizzy so my old dwell /tach meter won't work for anything.

Like I said before this is the 4th O2 sensor and the 2nd NTK brand ive put in it. I'm 99% sure this one is working as it should.

Will an exaust leak down stream of the O2 cause any problems?

What about a leak where the throttle body plate rod be pulley connects to throttle lever ( Sorry, for the life of me I can't remember what it's called. ) Ya. Know opposite side of the TPS.
There is no MAF sensor so don't think it would effect it
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post #15 of 18 Old 02-25-2020, 09:25 AM
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Running the engine with the sensor disconnected and that having no effect on the issue rules out the sensor itself as being the cause.

Based on the information provided thus far, this would be my to do list:
  1. Load test the battery, ensure it's fully charged.
  2. Verify battery cables are good, clean and tighten all connections at both ends of the cables.
  3. Verify you're getting 12v or better through both the ASD and fuel pump relays when energised.
  4. Verify the alternator is charging correctly at idle.
  5. Perform a resistance check of the four o2 sensor wires -- the BK/TN can be checked to a chassis ground, the other three should be done end to end. Resistance should be no more than 5 ohms and there should be no continuity from wire to wire.
  6. With the engine running or fuel pump relay energised, verify you're getting 12v to the o2 sensor connector at the DG/WT wire (sensor heater power).

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