Is a bad upstream O2 sensor causing my engine to run rich and idle low when warm? - JeepForum.com
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post #1 of 21 Old 09-11-2009, 07:40 PM Thread Starter
GeologistFelix
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Is a bad upstream O2 sensor causing my engine to run rich and idle low when warm?

Ok, so, for the past couple of weeks, I've been trying to diagnose and fix my engine troubles. Long story short, when it gets up to normal operating temps, it starts running super-nasty-smelly rich and bogging down at idle, and stumbling on acceleration from idle.

I've got more or less new plugs/wires/cap/rotor (500ish miles), and I've ruled out (and cleaned) the IAC, as well as the ballast resistor and latch relay. I haven't tested the TPS, but since I'm just having trouble with idle when warm, it seems like a waste of time to check it with the volt meter.

Anyway, today I did three quick tests...

Disconnecting the vac line to the MAP sensor made the engine run a little fast, and really...well, throaty, if that makes any sense.

Disconnecting the MAT/Air Charge sensor made no difference in how it runs.

Same for the downstream O2 sensor; I warmed up the engine, then shut it off and unplugged the sensor, and it ran the same plugged and unplugged.

So, should I go to the trouble of testing the O2 sensor with a volt meter, or was my test enough to tell me that it's gone bad, and needs replacing anyway? What about the MAT sensor? Even if I replace both, is it still possible my CPS or TPS is to blame?

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post #2 of 21 Old 09-11-2009, 07:54 PM
Lupitaboy
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It is the upstream O2 sensor that gives feedback to your cpu. The downstream one is there to to signal when the upstream one isn't doing its job, or so I've been led to believe. I just replaced mine and am waiting to see what, if any, effect it has on my mpg. I did this after new plugs, wires, distributor and rotor, oil, filter, T stat, waterpump, system flush and even an AC charge since I was on a roll. Before the O2 sensor replacement, my mpg was 17-17.5 on a 2500 mile trip I took. I am hoping to pick up between 2 and 3 mpg more after the O2. If I just pick up 2mpg more, I figure that within the next 10000 miles I drive I will have saved nearly $400 in actual gas which will definitely cover the $50 I spent on the sensor. Even a 1 mpg gain will net half that amount over the same distance.

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post #3 of 21 Old 09-11-2009, 07:57 PM
skyway0018746
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id say its a vacuum leak but that would make it run lean so it might be safe to say its the 02
a bad tps would give you transmission shifting issues (if you have a auto).

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post #4 of 21 Old 09-11-2009, 08:04 PM Thread Starter
GeologistFelix
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I have actually been having some shifting trouble, but only when I have the 'power/comfort' switch on the dash set to 'comfort.' It shifts fine when accelerating, but at certain speeds, it will switch between, say, 3rd and OD over and over, which is not what I'd describe as fun times, and I'm sure it's not good for the transmission. It doesn't do it in 'power,' however; it works just like it's supposed to.
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post #5 of 21 Old 09-11-2009, 09:07 PM
XJ Jay
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you need to figure out if the O2 is bad and telling the PCM to fatten up the fuel[o2 voltage would be low (.1v)indiacating a lean condition and calling for more fuel] OR if there is a fuel metering problem and its running rich[high o2 voltage (.9v)]


you need a DVOM and need to back prob the o2 sensor wire or a scanner with a data stream.

if the o2 voltage is low, "running lean", but the vhical is self is running rich, replace the o2

if the voltage is high and still running rich, creat at a vacuum leak(not the map sensor easther) o2 voltage sound drop a bit. if it does, you need to find what casuing a lack of air or too much fuel. also what is the map voltage at idle. what is it key on engine off. at idle it should be around 1v. key on engine off, it should be around 4.5v

also the rear o2 is really only there to ensure the cat is doing its job

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post #6 of 21 Old 09-12-2009, 12:36 AM
Mickey_D
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Um, your 1990 should not have a downstream O2 sensor. Are you talking about the wire going to your catalytic converter? Even in the FSM they state that the Renix system only uses one O2 sensor. And NAPA only lists one.

My 1990 XJ was doing much the same thing as yours when I got it. Turns out the vacuum line to the MAP sensor (the flat box looking thing on the firewall with the green wiring plug on it) was off. If unplugging yours only gives a slight difference in running condition I would say your MAP sensor is bad. Go to a pic-a-part and snag one for $5 to see. No sense in spending the full (almost $80) for a new one in case I'm wrong about this, but I don't think I am. I'm almost 100% sure your MAP sensor either is broken or isn't getting the vacuum.

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post #7 of 21 Old 09-12-2009, 01:23 AM Thread Starter
GeologistFelix
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It's the upstream O2, not the (non-existent) downstream.

Anyway, it ran noticeably weird with the MAP vacuum line removed. Almost the same idle, but it's like it was firing differently or something. It's hard to describe, but it was definitely different.

Anyway, I think I've pretty conclusively proven to myself that the O2 sensor is bad. If it was working correctly, the engine should run differently when it's unplugged compared to when it's hooked in, right?
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post #8 of 21 Old 09-12-2009, 01:33 AM
MaskedMallard
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Coolant temp sensor to the Computer?? I'd think if the sensor was still reading a "Cold" engine, it would continue to run rich, until the temp sensor showed a warm engine.

Do you have access to a computer scanner, where it shows whats going on with the engine management system?
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post #9 of 21 Old 09-12-2009, 03:50 AM
Boodyrider
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Wow, lotta guys don't understand the feedback loop thing huh?

6 cylinders
1 O2
slow stupid computer

LOT of opportunity to misunderstand the signals coming in, and make the wrong assumptions.

You NEED to know whether the INDIVIDUAL cylinders are running well. Pull the plugs, and google 'reading spark plugs'.

If all are aqually good/or bad, then you have most likely eliminated the HARDEST thing to find, a single cylinder problem that results in the computer making the wrong decision. for example, a single cylinder misfire dumps unburned fuel in the exhaust.. AND too much OXYGEN, because it didn't burn. Computer thinks it's LEAN and adds fuel to all the cylinders... making ALL cylinders rich... and ONE that doesn't fire due to the original problem, a single cylinder misfire.

Here's another ... one injector is partly clogged, resulting in a lean condition in ONE cylinder, causing the overall O2 to read lean.... so the computer adds fuel to ALL cylinders. results: one kinda lean, 5 too rich.

Because the problem occurs when it's hot, and ought to be in closed loop, I suspect you have a 'wrong decision' kind of issue. Start by reading codes (yup, bad O2 ought to throw a code). No codes? Likely all sensors are working fine, but you have another issue that's messing up the system.

A vac leak would do it... especially if it was unmetered air or leak into the exhaust. Bad TPS would do it too... and unplugging sensors without a complete understanding of what OUGHT to happen on a healthy engine isn't going to help you much.

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post #10 of 21 Old 09-12-2009, 03:56 AM
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Quote:
Originally Posted by GeologistFelix View Post
I have actually been having some shifting trouble, but only when I have the 'power/comfort' switch on the dash set to 'comfort.' It shifts fine when accelerating, but at certain speeds, it will switch between, say, 3rd and OD over and over, which is not what I'd describe as fun times, and I'm sure it's not good for the transmission. It doesn't do it in 'power,' however; it works just like it's supposed to.
Hmm...

Power is (overdrive off) and comfort is (softer shifts lower line pressure and overdrive available)


When the engine is not running right, or when certain sensors (like TPS) are flaky, the trans PCM has trouble figuring out what load the engine is under and deciding whether overdrive is appropriate. I don't think yours has lockup, but that's computer controlled too.

FWIW, even working perfectly lots of folks feel the AW4 seeks too much; poor gear choices not matched to the engine powerband as a result of Chrysler being damn near broke then and just trying to get some cars out the door with whatever was in the parts bin....

97 TJ Sport - 4.0/HP D30 w ARB/8.8 w LS/4.88s; 35s and flat fenders on 2.5 lift

84 CJ7; frame off restoration in progress.

The technical advice you get from me here is worth exactly what you paid me for it. Warranty issues will be reimbursed at double what you paid me for the advice.
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post #11 of 21 Old 09-12-2009, 11:47 AM Thread Starter
GeologistFelix
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It still goes to OD when it's switched to Power. Otherwise I couldn't be running at 2000 rpm at 60 miles an hour. From what I've read, and personal experience, the switch (which was only on early 4.0L engine models) controls the speed at which the TCU upshifts for each gear.
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post #12 of 21 Old 09-12-2009, 11:59 AM Thread Starter
GeologistFelix
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Quote:
Originally Posted by Boodyrider View Post
You NEED to know whether the INDIVIDUAL cylinders are running well. Pull the plugs, and google 'reading spark plugs'.

If all are aqually good/or bad, then you have most likely eliminated the HARDEST thing to find, a single cylinder problem that results in the computer making the wrong decision. for example, a single cylinder misfire dumps unburned fuel in the exhaust.. AND too much OXYGEN, because it didn't burn. Computer thinks it's LEAN and adds fuel to all the cylinders... making ALL cylinders rich... and ONE that doesn't fire due to the original problem, a single cylinder misfire.

Here's another ... one injector is partly clogged, resulting in a lean condition in ONE cylinder, causing the overall O2 to read lean.... so the computer adds fuel to ALL cylinders. results: one kinda lean, 5 too rich.
If I'm having trouble on any individual cylinders, it's not apparent from how the engine runs. I say this because the engine is doing the same crap it has been since before I did my tuneup. All of my old plugs showed similar signs of wear; there wasn't any single one that was significantly different from the others.

I also ran it with one totally broken spark wire before replacing the wires, and, with only five cylinders firing, it was running very differently from how it is now. Also, there's the matter of the engine running very smoothly and happily at speeds above idle.

Also, I really doubt my injectors are at fault, because, surprise surprise, it's been having this problem both before and after replacing my injectors last winter.

All of that said, I am willing to pull my plugs and see if any particular one of them is fouled. If one is, my apologies for doubting you.
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post #13 of 21 Old 09-13-2009, 06:40 PM Thread Starter
GeologistFelix
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Update.

I finally found a multimeter and tested a bunch of things.

- TPS is working flawlessly
- MAT/Air Charge sensor is working properly
- O2 sensor is dead

So, I guess I'm going to replace that sucker, and hope that fixes the problem. If it doesn't, I'm going to check the individual cylinder for fouled plugs etc.
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post #14 of 21 Old 09-14-2009, 11:31 AM
MaskedMallard
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Quote:
Originally Posted by GeologistFelix View Post
Update.

I finally found a multimeter and tested a bunch of things.

- TPS is working flawlessly
- MAT/Air Charge sensor is working properly
- O2 sensor is dead

So, I guess I'm going to replace that sucker, and hope that fixes the problem. If it doesn't, I'm going to check the individual cylinder for fouled plugs etc.
How did you test the O2? Do you have a 1 wire or 2 wire O2?
Are you aware the O2 produces its own voltage, once it is at operating temperture.
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post #15 of 21 Old 09-14-2009, 11:51 AM
Mickey_D
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Masked, maybe yours does, but ours are the ancient AMC ones. Ours are three wire. Two wires for the O2 sensor heater and the third has voltage applied to it from the Renix computer and the O2 sensor creates resistance to ground depending on the gas mixture it reads. So on ours you can take a resistance measurement according to the Factory Service Manual.

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