Got a 94 xlt with 130,000 miles. I am getting EEC codes for HEGO sensor for both rich and lean conditions on the RH side. I have already replaced the fuel pressure regulator as well as spark plugs, wires and air filter. Heres my question. Does the O2 sensor adjust the mixture based on the readings its getting. Heres my thinking. A bad O2 sensor senses a false lean and tells the computer to richen up the mixture when not really needed, at which point it then senses a rich condition.
Is this possible. I don't know if the O2 sensor provides feedback in order to make the computer make nessesary adjustments, or if it just is there to sense an out of spec condition and then report it.
This might get complicated...... the O2 sensor doesn't know squat from 'rich' and 'lean'. I does, however, know little O2 and lots of O2.
The PCM has two fuel curves that interact.... Long Term Fuel trim is a learned or adaptive strategy - the pcm reverts to this learned strategy during open loop conditions, i.e. extended idle, WOT, or when the CEL is lit. LTFT is the "God" of fuel strategies. Short Term Fuel trim, is how he PCM reacts to information from a warmed up and functioning O2 sensor. If short term fuel trim is either adding fuel or subtracting fuel, on a predictable basis, LTFT will learn this behaviour and adjust to return STFT to midrange readings.
The O2 sensor does not adjust the mixture... the PCM reacts to O2 sensor readings. O2 sensors can fail (indeed, there is a recommended service interval) but the failure is usually manifested in slow or no 'switching'.... sorry, it's close to bedtime and I'm just not into the explanation of that one at this time. The code descriptions appear to be a matter of national security and I am leery of autozones code descriptions, anyway.
Since you have both rich and lean codes set for one bank, I would recommend concentrating on conditions that can affect one bank..... the O2 sensor itself, vacuum leaks, inappropriate spark plugs (single plats in a waste spark system are a waste of time and money) or other conditions. If there are no running problems (since these codes will likely be continuous memory codes) I would suggest clearing the codes, resetting the KAM (Keep Alive Memory) and driving the car to see if the codes will return.
Jim, Thanks for the indepth information. I have indeed reset the codes as well as reset the whole system (ie battery terminals). The same codes return everytime. The engine is also exibiting symptoms, ie running rough, sluggish, and blowing carbon. This does not happen untill the vehicle has been driven 50+ miles at one time. (Runs fine around town). This has been the case for a year or more.
I have replaced all the plug wires as well as the plugs (I used autolite platinum. didn't know this was a waste). The old plugs looked fine with no unusual wear or deposits. Also replaced the fuel pressure regulator (not nessesary I now know).
You seem very knowledgeable on the subject of troubleshooting these systems. If you (or others) could point me in the right direction on where to look next I would greatly appriciate it. Thanks
I had similar symptoms in a Nissan pickup, running rough, blowing black carbon, etc., finally would hardly run at all. Found out it was the thermostat stuck open, making the engine run too cool, which caused it to run too rich. Just a thought.
Dirty Mass AirFlow sensors are known to cause rough idle, stalling, and black exhaust, i.e., rich at idle. Is your engine setting oxygen sensor codes or fuel system codes? Do you know the code(s)?
I guess if you have more time than money you could switch bank 1 and bank 2 oxygen sensors, clear codes, and check if the fault moves to the other bank. Bank 1 should be the passenger side of the engine.
Switching sensors is acually a great idea. Thanks for the suggestion. Would it not also be possible to simply switch the leads rather then pulling both sensors??
The codes are for HEGO (o2 sensor) senses rich condition, and HEGO senses lean condition (codes are 176 and 177)
As far as the MAF goes, I replaced it about a year ago on the advise of a ford dealer and the problem didn't go away. I am thinking more and more that its the O2 sensor.
You do not want to simply switch the leads to the O2 sensors since the fuel system control strategy will adjust bank 1 fuel quantity based on bank 2 exhaust reading. It would be fun to see what would happen if you did that but my guess would be that the ECM would set fuel system faults within 10 seconds of engine start. It's kinda like if you were backing into a tight parking spot, and you were asking the guy standing in front of your car how close your rear bumper was to the car behind you, and he said "3 feet", but he was looking at how close your front bumper was to the car in front of you. Doesn't work. (Sorry...that's the best example I could think of at the moment)
The other test you could run is to hook up a DVM to the O2 sensor lead to the ECM and view the switching activity of each sensor. You would be looking for a smooth switching from say 0.2 to 0.8 volts about one cycle per second.
The hardest part of replacing them is probably going to be getting your hand(s) on the connector and unplugging the old ones and plugging in the new ones. On my 92 it's in an almost impossible to reach location.
MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.