Hey all!!
I was poking around under the car with the trans removed and noticed one of my O2 sensors was unplugged..
This made me question the cars rich idle condition..
The car is new to me and the assumption was that there was no tuning done after things like, injectors, exhaust, TB, bump in compression, cam were done..
As I tried to start researching about how many O2 sensors were original to the car I also read there are 3 .. 2 upstream and one post cat..
My exhaust is not original and there is no 3rd/post O2 sensor ..
I did find a 4 pin connector tied up next to the starter.. I have yet to determine if this was the post O2..
so between the 3rd missing O2 and the 1 upstream being unplugged I am wondering about the running condition..
I think I read the Post O2 would be just a CEL thing and the upstream would effect closed and open loop..
One of my questions is, I am assuming both banks need to be reporting happy numbers not just one ,
Is this correct?
I am also assuming open and closed loop conditions effect both banks simultaneously, not individually?
Some of the stuff I was finding on the web was not GM specific , so figured some here could help clear me up..
The few times I had it running/idling I didnt recall the SES light..
guess its time to start thinking about getting a laptop on this thing (once the trans is back in )
Thanks in advance
Chris

 

I did a little more poking around and found some info in the manual..
So this 94 LT1 has 3 .O2 sensors
bank1 front left
bank 2 front right
bank 2 rear right
So what I have is two harness side O2 plugs disconnected on either side of the starter..
Figured I'd use the manual to find the color codes of the wire to be able to tell which connector is front and which is rear but they both use the same color wires..
Using the manual and a continuity test I could unplug at the pcm to determine which plug is the front bank 2..\
Looking at the "branch" of the harness if you will that comes down to the starter that holds both bank 2 O2 plugs, one is at the end with the starter leads and one is a little higher up /closer to the firewall... I am guessing the longest one out /oil pan side is the front O2..
Continuity test will confirm.

 

Okay, so as you've found, your car was supposed to have two upstream and one downstream O2 sensor. The downstream sensor is only there to help the onboard diagnostics (OBD) assess the health of the catalytic converter. I will not affect how the car runs at all, but should throw a CEL (but not a flashing one). That test/reporting can be turned off in the PCM calibration if you have tuning software like Tunercat if you so desire.

The upstream sensors are required in order for the PCM to function properly in closed-loop mode. They PCM uses their information to manage fuel trims. If either is not functional and/or is not connected, it will probably also throw a CEL, but I'm not actually sure about that in a 94 (it definitely will in a 96, but that PCM has additional function because it's full OBD2). The PCM does not reference the O2 sensors at all in open-loop mode, so a bad/unplugged O2 sensor will not impair the performance of the car when it's not yet warmed up. After it warms up, the PCM enters closed-loop mode (for both banks - the mode is not separated between banks) and starts reading the O2 sensors and adjusting fuel based on their data. That can definitely make it run rich/badly.

I don't know which of the two Bank 2 plugs is upstream connector, but your idea to probe for continuity to the PCM connector is a good one. Let us know how it turns out.

 

Thanks for the info !
I understand all but want to be clear on one part.. Will an unplugged upstream prevent it from going into closed loop?

So .... I ended up checking the connector at pcm to verify which plug was the upstream plug.... It ends up being the farthest one out(with the starter leads ) and hangs between the starter and oil pan..

The interesting part, or maybe not, is what I found when I was pulling the "clear" connector I was looking for on the PCM for my O2 pinouts...
The socket marked clear had a transparent(black or dark grey) connector plugged into it... not the part of the harness I was looking for.. Flip the pcm over and found the clear connector plugged into a black or grey socket..
Not sure if these colors are supposed to match up or does this tell me this is not an original pcm???
There is no label on the PCM..
Red and blue seem to match up fine but the clear and transparent grey/black definitely do not match the sockets.. Physically they plug in just fine and do not appear to be hacked up.
I grabbed a pic, not sure if it is clear enough, but if you notice the case is marked "Grey" but the connector that fits clear..


thanks again

 

Without one bank reading, you were probably in open loop all along hence running rich when warmed up. No damage done except to maybe catalytic which could explain the disconnected post sensor.

 

Actually it probably will. I was just looking at my 96 Factory Service Manual and it says one of the conditions that has to be met for closed loop to commence is "Both O2 sensors have varying voltage output, showing that they are hot enough to operate properly." So a bad/slow/failing sensor won't prevent closed loop from starting, but an unplugged one probably will.

Not sure I understand the question here. The "Grey" port on the PCM in your picture should take the clear plug from the wiring harness. So that clear connector is correct. The "Black" port on the PCM should be diagonally opposed from the gray port, such that if the Grey port is on the lower right corner as you face the ports, the Black port should be on the upper left. Red should be above Grey and Blue should be below Black. The plugs from the harness should be color coded to match, except that the clear plug goes into the Grey port (not sure why they did it this way). There are some good pics here. The colors Grey/Blue/Black/Red should be cast into the casing above the ports. Does yours have other colors than this?

All the O2 sensor signals go through the Grey/Clear port/connector. If I'm missing the problem, let me know.

 

Got ya!! now I understand when the previous owners upgraded the exhaust there was no provision for that post o2..
thanks!!

So it seems all is well, I got tripped up between the clear connector in the grey socket, especially when the FSM called it "Clear".. so when I found it in the Grey socket I started questioning everything.. and the connector in the black socket could have passed for a dark grey lol..
Thanks for helping out..