Would that cause the spiking shown in these charts?

https://dodgeforum.com/forum/1st-gen...-2002-r-t.html

(sorry, I can't seem to get them to load here too)

 

The pre-cat O2, MAP, IAT, TPS... sensors, and PCM work together to regulate the air/fuel ratio by constantly oscillating between slightly rich and lean. On an O-scope this would look like a sine wave the average of which is supposed to be the target AF ratio. The charts you showed look more like a vehicle that has entered “closed loop” but the cat hasn’t fully “lit off”.
What I mean by closed loop, is the pre-cat O2 sensor must reach operating temp, until then the PCM maintains the AF ratio to a preset rich level or what‘s known as open loop usually at a higher idle speed. Cat light off occurs when the cat gets hot enough to start re-burning the exhaust gases into more water/ carbon dioxide and less of the bad stuff. The outlet temp of a “lit off” cat is actually hotter than the inlet. This also equalizes the hydrocarbon spikes, so, while in closed loop with a lit off cat, if you hook the post cat O2 sensor to an O-scope you would see more of a flat line and an oscillating (sine) wave from the pre cat o2 sensor.
If you’ve been through everything twice, injectors are good, not running too hot of a spark plug…. I would probably star looking at wires. A corroded, loose, or shorted wire from any of these sensors will change the input voltage to the PCM, making the PCM over or under compensate for the conditions.

 

In re reading your posts I noticed you had a universal cat installed and it was shorter. I looked up the part no. and found it is only 49 state legal on 1995 and earlier vehicles, this makes me think it was a 2-stage and not a 3-stage cat. A lot of performance cats are just 2-stage meaning they only take care of CO and HC’s. The third stage is for the oxides of nitrogen.

 

Two things suggest to me that it's probably a three stage. One, I found a couple on internet references to it as a 3 stage (meaningless, I know). Two, the curves with the stock cat and the new one are ******* duplicates, with the same spikes in the same places in the test (more significant).

Thanks guys, you've been a great help. Give me a few days to get the injectors checked, the upstream O2 sensor checked/replaced, better gas and new plugs. I be back to post the results.

Man, I love it when a plan comes together . . .

 

Yeah, I couldn't find a definite answer on the stages. The Magnaflow site just doesn't show it compatible with anything newer than '95. As to why the spikes are the same; that's kinda why I first posted checking out more of the upstream components. One more thing to be careful of with aftermarket cats is their core construction. Some of the high performance cats are a spiral wound metal, these do flow better, but they tend to melt down if the vehicle is running too rich. I'd stick with the OEM style ceramic. Best of luck to you. I'm curious as to what you find.

 

ZCR1 and hydra, you guys are too good!

The final chapter in the saga: As suggested, (and after completing another huge honey-do at the homestead) I checked all wiring, checked the O2 sensors, changed the plugs for new Champion copper, correctly gapped plugs and seafoamed her. No distinguishable difference on the dyno.

At that point I was eligible to take her into a state wizard shop and have them figure out what was wrong. Here's the story:

For whatever reason the injectors are running 7-10% lean and the ECM is compensating during most conditions. When the throttle is lifted (even to slow the rate of accelleration) the ECM drops its correction and recalculates. This was causing momentary leaning and the spikes noted in the graphs. Then he made a statement that the readings were just what he would expect if there were no cat at all in the system. I flashed back to the wizard ZCR1's comment that it looked like the cat hadn't lit off. So my new cat was not only undersized, but completely non-functional as well. Went to Meineke and dropped another $360 to have them cut the Magnaflow out and replace it with a factory spec unit. The charts speak for themselves. She's got tags!



HC GPM ____ 0.2082 _____ 0.2162 ____ 0.3325 _____ 1.2000

CO GPM ____ 5.0743 _____ 2.1490 ____ 0.6991 ____ 15.0000

CO2 GPM _ 630.9291 ___ 632.3327 __ 674.0432

NOx GPM ___ 2.5960 _____ 2.7806 ____ 0.2733 _____ 2.0000

5/20/2011 - Original Try
6/18/2011 - Magnaflow cat installed, plenum gasket replaced
7/14/2011 - Magnaflow cat replaced with factory spec cat

Given the dramatic difference in the readings after the latest cat, I can't help but wonder if the shorter (but not too short) cat might have been sufficient had it been functional. (??)

Thanks for your help guys, I'd still be standing around scratching my head without it.

 

Thanks for the update!


Seen it a few times where people put in smaller CAT's and had issues. The funny thing is that the Magnaflow ones were completely inoperable?

 

Seems so. At least that's what the numbers from the emissions station point to.