O2 Sensor Question/help

I replaced the o2 sensors in my car about 2 years ago when i put in a 2000 explorer long block. I have been dealing with an idle surge this summer and have been through the suring idle checklist many times. the only codes i get are egr and emmissions related codes and i have no egr and all emission stuff has been removed.
The surge and idle quality acts up when the engine hits operating temperature.

So, i finally decided to try disconnecting the o2 sensors and driving around to see if that would make a difference. Well, it did. It idles great with no surge at all temps when they are disconnected. It even sounds better witht he cam being more pronounced.

So, my question is can 02 sensors go bad that quick? I have pulled the codes many times and their are no o2 sensor codes.

A little more info, last year the driver side o2 sensor wires melted on the header and it blew the inline fuse. i soldered in some new wire and put a blade fuse holder where the inline fuse was. it idled fine for a few months after that but started acting up this summer.

Thanks and any help will be greatly appreciated.
 
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Testing the O2 sensors 87-93 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear.

Disconnect the O2 sensor from the harness and use the body side O2 sensor harness as the starting point for testing. Do not measure the resistance of the O2 sensor , you may damage it. Resistance measurements for the O2 sensor harness are made with one meter lead on the O2 sensor harness and the other meter lead on the computer wire or pin for the O2 sensor.

Backside view of the computer wiring connector:
a9x-series-computer-connector-wire-side-view-gif.71316


87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
The computer pins are 29 (LH O2 with a Gray/Lt blue wire) and 43 (RH O2 with a Red/Black wire). Use the ground next to the computer to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


Testing the O2 sensors 94-95 5.0 Mustangs
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a red/black wire) and 27 (RH O2 with a gray/lt blue wire). Use pin 32 (gray/red wire) to ground the voltmeter. The O2 sensor voltage should switch between .2-.9 volt at idle.


Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer. Using the Low Ohms range (usually 200 Ohms) you should see less than 1.5 Ohms.

87-90 5.0 Mustangs:
Computer pin 43 Dark blue/Lt green – LH O2 sensor
Computer pin 29 Dark Green/Pink – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Dark blue/Lt green wire in the LH O2 sensor harness and the Dark blue/Lt green wire on the computer pin 43
From the Dark Green/Pink wire on the RH Os sensor harness and the Dark Green/Pink wire on the computer pin 43

91-93 5.0 Mustangs:
Computer pin 43 Red/Black – LH O2 sensor
Computer pin 29 Gray/Lt blue – RH O2 sensor
Disconnect the connector from the O2 sensor and measure the resistance:
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 43
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 29

94-95 5.0 Mustangs:
Computer pin 29 Red/Black – LH O2 sensor
Computer pin 27 Gray/Lt blue – RH O2 sensor
From the Red/Black wire in the LH O2 sensor harness and the Red/Black wire on the computer pin 29
From the Dark Green/Pink Gray/Lt blue wire on the RH Os sensor harness and the Gray/Lt blue wire on the computer pin 27

There is a connector between the body harness and the O2 sensor harness. Make sure the connectors are mated together, the contacts and wiring are not damaged and the contacts are clean and not coated with oil.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts

Make sure you have the proper 3 wire O2 sensors. Only the 4 cylinder cars used a 4 wire sensor, which is not compatible with the V8 wiring harness.
 
ok, i tested the voltage on the 2 o2 sensor wires at the cpu when engine was warm. When i tested them, my voltmeter jumped from .o1 to .8 and it kept jumping around in that range. is that normal?

Thanks!
That sounds about right. The low of .01 seems a bit off, But I think that it should work.
 
ok, now i need to test the connectors. I am hoping some one can give me some guidance on this. my probes on my multimeter are not long enough to reach to the 02 connector and to the cpu to test the resistance.

How do you guys do this?

Thanks in advance!
 
would using a wire with an aligator clip as an exstension work? So, i would connect the alligator clip to the probe of multi meter and the other end of wire with alligator clip to whatever i use to probe the o2 connector.

would the readings still be accurate?

Thanks!
 
would using a wire with an aligator clip as an exstension work? So, i would connect the alligator clip to the probe of multi meter and the other end of wire with alligator clip to whatever i use to probe the o2 connector.

would the readings still be accurate?

Thanks!
That would work OK, just be sure that you have good, solid, clean connections.
 
In the instructions Above, the connectors to the o2 sensors are described ast "LH" and "RH". Can someone confirm if "LH" is passenger sid or driver side and if "RH" is passanger or driver side?

I aplogize for what is probaby a common sense question but i just want to make sure i did not switch the connectors when i installed the motor a while back and to make sure i am measuring the resistance between the correct wire and cpu pin.

Thanks!
 
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The point of reference is that you are sitting in the drivers seat facing forward. Right and left are viewed from that position.
 
so it seems the voltage test and connector test both passed. i thought one of these would fail for sure since the car idles great with the o2 sensors unplugged but surges when they are plugged in.

Any other ideas or things i can try?

Thanks!
 
Just a stock long block, or have you got a cam in it?

Some cams just won't idle properly until you bump it up. I see a lot of guys having issues with the alphabet cams. I tried everything I could to get my Comp XE274HR to idle at near stock RPM and the ECU was having none of it. I'm now running around 800-850RPM on the stock tach and it seems to like that a lot better. It all but eliminated my surge.

Some of my issue I'm certain has to do with it needing a tune, but for the time being, bumping the idle is the fix.
 
so it seems the voltage test and connector test both passed. i thought one of these would fail for sure since the car idles great with the o2 sensors unplugged but surges when they are plugged in.

Any other ideas or things i can try?

Thanks!

With the O2 sensors unplugged the computer goes into limp mode, and runs on a fixed set of parameters that insure safe operation. With the O2 sensors working, it is in closed lop using all the available sensor inputs. Some sensor may be on the edge of going bad, but not enough to trip a code.
 
i've been doing some research on o2 sensor harness and cpu's but am a little confused. when i did my maf conversion i used a maf cpu out of an auto tranny car. my car is a 5 speed manual. so, i am using the 5 speed manual o2 harness from my 87 with a maf auto cpu. Is this creating issues?

I can still pull codes.

Thanks!
 
i've been doing some research on o2 sensor harness and cpu's but am a little confused. when i did my maf conversion i used a maf cpu out of an auto tranny car. my car is a 5 speed manual. so, i am using the 5 speed manual o2 harness from my 87 with a maf auto cpu. Is this creating issues?

I can still pull codes.

Thanks!
No problems; I have run an A9P Auto trans Mass Air computer in my 89 5.0 5 speed. It works OK, and is my backup computer if I ever need one.