Quick question...my 96 Ram just threw a P0141 code=lean condition in cylinder bank 1.

So....

1. Im guessing that's a bad 02 sensor?
2. If so, as soon as the CEL came on, it started running rough and dumping black smoke out the pipe, so Im assuming the computer is compensating the lean condition by richening up the mixture, and as a result my gas mileage dropped to about 10 mpg.

yes

bad O2s can cause horrible fuel mileage in closed loop mode.

that's when the computer is reading what the O2 sensors are seeing, and then compensates the fuel mix by adjusting injector pulse width to give you perfect stoich 14.7 AFR at part throttle cruise.

However, "lean in bank 1" does not automatically mean bad O2. I would think that if the O2 was bad, then the computer wouldn't know that the left bank was lean.

thanks Tom

Since Google is my friend, I just double checked the code and apparently "bank 1" doesn't mean "cylinder bank 1" I guess it means the O2 bank.

You have a similar Dakota, are the O2 sensors at the catalytic convertor the only ones on these trucks? I know some vehicles have 4 or more.

Check fuel pressure as well.

Wish I could give you a definitive answer on that Dan.

not all the Dakotas are the same, so I won't assume that the Rams are like the Dakotas.

example, my 3.9V6 Dakota had 2 O2 sensors, one pre-cat, and one post cat. it was a 98 truck.

my current 98 was a factory 5.2 truck, also only had 2 O2s.

98 R/Ts had the 5.9, and they had 4 O2 sensors. one left, one right, (each in the down tubes just behind the manifolds) one pre cat (at the Y going into the cat) and one post cat

99 R/Ts had 2 O2 sensors like the rest of the trucks.

around 00 or 01, all the trucks ended up with 4 sensors.

the left/right O2s are the ones that control the engine tune. or the single O2 that's before the cat.

all the O2s that are behind the cat have to remain plugged into the wire harness, but do not have to be in the exhaust. Guys who remove their cat when they put in a performance exhaust will often leave the post Cat O2 zip tied to the frame rail. computer sees engine exhaust in the first O2 sensor, uses it to adjust the tune, then sees clean air at the rear O2 and assumes your cat is working VERY well

leaving it in the exhaust, without a cat between it and the front sensors, will tell the computer that your cat has failed, and will trip a CEL.

If you have access to a wideband stand alone O2 sensor, it may help you troubleshoot, and figure out if you have a bad O2 sensor, or a good exhaust reading from something else going wrong.

Could be that lower intake cover gasket, too. It had been a lot of trouble. There should be a code thrown for the O2 being out of range if it was the problem. It is easy to test if you pull it out.

This problem will take a little more investigation to pin down.

In order for your truck to tell which bank it is it has to have four 02 sensors or it could not verify which bank is lean if it did not and only had 2 sensors. I dont think its a high fuel pressure problem as that would effect both banks. It sure could be a bad 02 sensor but without taking readings on a scanner you will be guessing. Many throw an 02 sensor in because they think the code means the sensor is bad but it does not it just means the PCM sees a problem in that circuit as it could be any part of that circuit. If you have a vacum leak the PCM will adjust addaptive fuel and 02 readings to make up for it as long as its not to big of a vacum leak. Thats the thing about the electronic fuel injecton as with a small vacum leak the PCM can add fuel and keep the vehicle running good so if the check eng lite did not come on the cust may not know there is a problem but he would have much worse fuel milage. One thing to remember is the 02 sensor is just a fine tuner as by itself is can not control alot of fuel adjustment. What happens is when the eng needs more fuel then the 02 can adjust then the addaptive fuel memory takes over and starts changing the addaptive fuel cells to add fuel and once it changes enough to make the 02 start moving again then the 02 fine tunes the final adjustment. Also the downstream 02 sensors are more important then many think as it helps adjust the upstream 02 sensors switching goal which is the point it switches form lean to rich. The normal switching point is about .5 but it can tell by the downstream 02 sensor if its on the rich or lean side as the down stream looks at oxygen in the exh after the cat. It can move the point at which the upstream 02 decides to switch from lean to rich to help fine tune the fuel mixture. I have seen some with the switching point down at .42 and up at .64 and so on. Of course the downstream 02 also has its switching speed and amount checked by the PCM to determine how well the cat is. So the downstream 02 is more important then some think.
Your best bet of course is to have it scanned first to determine the problem but you can do like many and throw an upstream 02 in it and you may get lucky and fix it as that fixes that problem many times. Also Bank 1 means the bank the #1 cyl is on. Ron