Bad O2 sensor?

87, 4.0, AW4, Okay, here is my problem: I have a rough idle and sometimes stalling at idle while warming up. When the engie is warmed up, it's not as bad but still not real smooth. I took the Jeep to a shop and had them hook up a scanner and watch what everything was doing. The O2 sensor voltage was sitting at 4.95 and didn't move once. Obviously a problem, but at the time the engine wasn't totally warmed up so I may have been operating in open loop mode. So my question is this: is the O2 sensor voltage supposed to change when in open loop and the computer just ignores it or is it supposed to sit still like it was doing?

First things first - just so you know, the "Open Loop" mode is used during warmup - it's where the HEGO sensor isn't hot enough to give a reliable reading. This can go for anywhere from thrity seconds to about three minutes.

"Closed Loop" means that the HEGO sensor is at operating temperature, and should be giving out a reliable signal.

As far as you "steady" +5VDC, that's fairly normal for RENIX. RENIX uses a variable-resistance HEGO sensor, rather than the more common "voltage-generating" type. The HEGO is used as part of a voltage divider circuit, using a +5VDC reference supplied by the ECM. This return voltage is then read and compared to a list of "known values."

What you will want to look for is the voltage OUTPUT from the HEGO sensor - that should hover around +2.5VDC nominal, and vacillating nearly constantly (it's easier to read signals like this with an analogue meter, since it will respond faster than a digital. Or an o-scope.) I don't recall which signal is which offhand - I'd have to figure out where I put my RENIX fuel inejction book (damn thing's around here somewhere, I just know it!)

But that's the first distinction you'll need to make - the correct difference between "open loop" and "closed loop" operation. As I recall, if the HEGO sensor is failing (not switching or not supplying divided 5VDC reference signal,) the ECM will "fail" into "open loop" mode - which is similar to the OBD "Limp Home" mode. In this case, the HEGO output will be ignored entirely, so you don't get stuck. Open loop operation tends to be a little rich, so you'd notice it as the fuel mileage worsens.

Since you've got an 87, you're probably due for a new HEGO anyhow. OEMR service life is typically given as about 80kmiles, but I've seen them go anywhere from 60kmiles to over 200kmiles (I think mine finally wants replacing in the 88 - at 244k...)

You'll have to get the RENIX HEGO sensors, since I don't recall a lot of anyone else using the VR type - everything else (as far as I know,) is the voltage-generating sensor. Our HEGO sensors aren't too spendy, and I can usually find them locally. If you can't, check with Morris 4x4 Center (www.morris4x4center.com) - they sell OEM or Omix-Ada, and their prices are good.

5-90

5-90 said:

"...if the HEGO sensor is failing (not switching or not supplying divided 5VDC reference signal,) the ECM will "fail" into "open loop" mode - which is similar to the OBD "Limp Home" mode. In this case, the HEGO output will be ignored entirely, so you don't get stuck. Open loop operation tends to be a little rich, so you'd notice it as the fuel mileage worsens."
5-90

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I have a '92 XJ Sport.
I believe that it's using the original O2 sensor.
It's now at 108K miles and change.
The "maint reqd" light went on awhile ago, and I just recently learned what it's for.
How do I tell if my XJ is running in "open loop" mode (assuming that the system is the same)?
You mentioned that "fuel mileage worsens". How much?
I'm trying to gauge whether my 02 sensor needs an immediate replacement,
as unfortunately money is tight right now.
Is poor fuel economy the only symptom?
Thanks in advance.
-Reindeer

If it's a 91-up, it's the "voltage generating" titania ceramic element. Open Loop/Closed Loop operation is otherwise the same, but you'll measure DC voltage directly.

As I recall (I'd have to look...) but it should still hover around 2.5VDC - and again, use an analogue voltmeter if you have one (faster response.)

How far out of whack your fuel mixture will get usually depends upon the other sensors - the ECU will still try to monitir fuel mixture using the TPS, MAT, and MAP sensors - the difference is, that it won't have any response - it's "Open Loop" operation at that point. If the other sensors are in good shape, you may not even notice.

Also, while they say you "need" to change the HEGO every 60-80kmiles, it just ain't so. Granted, it will lose response over time, but there's no reason to keep buying one every couple years (if you drive anything like I do!) just because the EPA sez so. My 88's got the original HEGO sensor, and I just had it smogged a few months ago - passed, and better than most cars ten years newer!

If you've noticed a drop of about a mile per gallon, it's not really worth thinking about - if you've lost four or five, you've probably got sensor issues.

It's common for a lot of variable resistance sensors - where the resistance is varied by some mechanical method - to degrade and get "flat spots" in the resistance element - they will also want replacing over time. They can usually be checked with an analog ohmmeter - hook it up, and slowly run the sensor through it's movement range, heat it, or apply vacuum. The needle on the meter should move smoothly from one end of the range to the other - if it doesn't, you've got a "flat" spot and it will cause trouble for you.

5-90

5-90 said:

...My 88's got the original HEGO sensor, and I just had it smogged a few months ago - passed, and better than most cars ten years newer!

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I need to get the emissions checked yet again to renew my registration.
It passed two years ago, so I'm hopeful.

5-90 said:

...If you've noticed a drop of about a mile per gallon, it's not really worth thinking about - if you've lost four or five, you've probably got sensor issues.

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I have started recording mileage today. I'll know soon enough.
Thanks for your help!

This is a little offtopic, but anyway.

How much do you pay for thee O2 sensor?
I'm just asking, so I know if it's cheaper to order the part from US.
I'm living in Finland, greetings everyone!

JakeHammer said:

This is a little offtopic, but anyway.

How much do you pay for thee O2 sensor?
I'm just asking, so I know if it's cheaper to order the part from US.
I'm living in Finland, greetings everyone!

Click to expand...

$60.00 US +/- $5.00 Bosch.

$50 cuz I work in a parts store. Right around $60 is average.