Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
2004 grand Cherokee 4.0 liter. Watching live data in park, revving engine a few times will peg out the downstream o2 sensors rich together and stay there until I rev it up again and they will peg out all the way lean and make engine idle go lower than usual and stay there until I rev it up again more slowly and they will go cycling like normal. According to Mitchell on demand the downstream sensors on this vehicle have a major control of fuel delivery. Does this sound like sticking o2s too you guys or am I missing something? While driving sometimes stopping at a traffic light or stop sign, the idle will go way down and I'm sure it's the downstream sensors making it do that. Ideas please.
Hey Jasondo! Do you recall where in Mitchell you read about the downstream fuel control on these engines? I had never seen that this generation of Cherokee used downstream fuel control, only newer years.
At first, this sounds more like an idle control issue than a sticking O2 problem. I find that the lower-than-normal idle after an engine rev or when coming to a stop is often the result of a dirty/sticking IAC valve, or a mislearned idle strategy. This can happen if the battery has been disconnected recently, and the throttle/IAC wasn't cleaned afterwards. Could be wrong! :silly: Just my experience with the 4.0L.
If you wanted to be absolutely sure that the downstream sensors are the problem, and not a symptom, you could try disconnecting both and driving again. If the low idle issue remains, then I'd suggest the downstream sensors aren't the problem. It's not unusual to see downstream sensors appear to stick rich or lean for several seconds or more at times, depending on the oxygen storage capacity of the catalyst, its operating temperature, and the mixture of feed gasses going in.
Thank you for your response. I also thought about the catalyst oxygen storage and watched a video about that from Paul, but they just stay there either rich or lean. I'm going too try unhooking them and driving around and see what happens. This doesn't sound like it could be a heater problem does it? But yes if I understood it right, under the description and operation of the downstream oxygen sensors they work with the upstream sensors for fuel control. Thanks again Tyler.
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