Hello all. I just joined this forum and hoping for some help. I have a 1998 dodge caravan 2.4 liter 3 speed auto. I believe it is federal emissions as I only have one catalytic converter and no egr valve. it has 155k miles on it. Problem started summer of 2010. I noticed a hiccup when idling. very noticable from inside the vehicle. It feels just as if I put it in gear, that little bump. sometimes it comes in bursts 3-5 bumps. no codes active or pending. compression tests are very close to 180 in all cylinders. I just replaced the pre-cat oxygen sensor and no change. this hiccup does not happen in open loop (when cold). I connected the vehicle to my laptop using bowser electronics and monitored my oxygen sensors and see good waveforms for the precat, but they are mostly on the high side, meaning my engine is running rich. I was monitoring my second o2 sensor and see a big clue. immediately following every engine hiccup, the second (post-cat) oxygen sensor voltage dips very low. I would see it drop from .7 to .1 or .4 every single time. two things are troubling me. why is it running rich and why is it hiccuping? If the catalytic converter is clogging up, would that make me run rich? if the second o2 sensor reads low voltage, that means high oxygen content in the exhaust, which means I was running lean in those hiccup bursts, but I seem to have nice waveforms from the first o2 sensor on the RICH side. I'm confused.

 

If your van was running rich, i would think the ck engine lite would be on. And if the cat was plugged, the engine run poorly if it ran at all.You could have a sensor that could be going bad.TPS,Crank or Cam.

 

So I'm doing searches all over for proper o2 sensor voltage readings, here on this site and elsewhere, but I'm having trouble finding info. From my understanding and experience with other vehicles, the pre-cat o2 sensor should fluctuate between .1 and 1 and exhibit good waveforms. the POST cat o2 sensor should be a steady LOW voltage. this is how my other vehicles operate anyway. I just called the dealer and the tech there told me the post cat o2 sensor should follow the precat sensor. am I missing something? if the post cat sensor shows similar voltage fluctuations as the precat, as far as the car is concerned, there is a non-functioning cat (or gutted cat). back to my original post in this topic, my post-cat o2 sensor gives a close to steady .6-.7 volts for several seconds, then I feel the engine bump for a split second, and immediately after that bump, the postcat voltage dips way down to .1-.4. I guess my real question is, what is a properly functioning post catalytic converter oxygen sensor voltage? steady low, steady high or oscillating like the precat sensor? thanks for all info.