A returning old poster here, but enough decades ago that my handle and account vanished. On my wife's '02 3.0 with 85k I have just replaced all 3 cats, all 4 O2es (Denso) and the front bank of coils/plugs (Denso as well) (and #4 was 6 years ago). I'm pretty ok using advanced search on other forums, but batting .000 here with the current rough idle and occasional hard start (hot or cold). It runs quite well except a lopey choppy idle.

I removed the IAC and it's surprisingly clean inside. I still sprayed in Gumout and let it set a spell = no change. But I want to check it for electrics but difficult finding the procedure for checking volt./ohms. Also, I'll be mirroring all above in turn with the TPS and MAF, so it'd be great having a diagnostic link or even stickie for these items (?and I just missed it?)

And for reference spraying Gumout all around the top end (and around PCV) showed NO change in revs.

 

You might want to check that the EGR valve is fully seated. Sometimes a piece of carbon can get jammed between the pintle and seat causing a constant exhaust gas into the engine .
Don't go looking for some complicated cause without doing all the basics first.
You only checked one area for vacuum leaks , there are others that you need to pinch off the line to make sure there isn't a hidden vacuum leak in other areas.
Did you check the PCV hoses and system? Carefully check the PCV curved hose. Sometimes they will crack and collapse and leak. This is all basic stuff.

 

Look at fuel trims to actually see if the engine has the proper fuel/air mix. A highly positive reading would verify a vacuum leak or a poor fuel delivery problem. Make sure your fuel pressure is at specs and the pressure holds well after shutdown. You can pull the connecting plug off your IAC to see if it is doing anything. The AIC actuator is very dependable, pintle and throttle body carboning is what causes IAC issues far more than the solenoid doing the actuating.. Usually when an EGR valve is even slightly stuck open, you will have an engine which barely will run at idle. I don't know how badly your is running but it's worth a check too. Don't go and start replacing items without actual testing. The TPS can easily be checked for dead spots with an ohm meter and a MAF sensor signal can be checked with a good scan tool which can also show you signals from other sensors too, such as your ECT.

 

Appreciate the leads and direction. I found upon once climbing underneath soon after starting that the 'just rather rusty' muffler had numerous pinholes, so had to wait to order and install a new one before working on it again in the garage. In the mean time I paid for the extra module that reads the O2s with my scanner app. The dashboard labels only O2 #1 and O2 #2, so it must not read the after cat-sensors at all. The forward reading would run the range from .2X to .8X idling and through the cycle, but the rear was reading a pretty steady .04 to .08 - no real cycling at all. I could get it to only .14 (even .19 once never higher) by revving, so a pretty strong indication of issue limited to the rear bank. And blipping the throttle even a little it ALWAYS went to perfect smooth running, and then back to a slight chop / fluctuating 500-700 low idle. Given basically the flatline reading my normal first assumption would be defective brand new DENSO sensor. Eliminating other variables without throwing parts at it I just removed the EGR to clean it and found the metal tube was not tight at the manifold end. EGR was already very clean and held when I sucked on it. After soaking in MAF cleaner anyway and reassembling, I hoped I had lucked out but exactly the same operation and readings front and rear.

All the above is tempered by something I spent a full day on when initially installing the new cats. After installing and final tightening / torquing both manifolds, I tried to connect the Y-pipe but it was "too long" by a 1/4 to mate both flange ends. I had tried bending the flange studs on both manifolds (all welded and very long and ) to align better, and even heated the pipe red at the 90* front end with 2 MAPPs trying to bend it even a little = no go. I had to take the advice of a You-Tube that when using the less expensive 'aftermarket design' (Walker in my case) v. 'hellaciously costly oem' it's better to loose assemble all components and scrinch down things from there. Both manifolds did mate to the heads as I cranked down but seemed to draw up by ~3/16" gap on the bottom of each.

It's currently 15F here, but climbing tomorrow so hitting it again then. First thing is install the 'old' upstream sensor I kept for the rear and see if the exact symptoms. After that I'll be egging out the bolt holes on one flange to relieve tension of manifolds against the heads - in case that's forcing a leak at the head.

Finally, there are NO codes, and fuel trims read all 0. I'm imagining that's due to not reaching whatever the requirement for establishing a full drive cycle ? I had the thing out for 45 minutes test drive right after finishing the first go-round trying to reach a full drive cycle, but have had the batt. disconnected since. Besides, I read somewhere it takes multiple start-stops together with a grand total time to reach the requirements.

Following up a couple suggestions above, I felt all around the PCV hosing and no softspots. I still have to cycle the TPS through rotation with a VOM and spray the MAF. I'm using VEEPEAK BLE+ and have to read up to see if I have to buy yet another add-on module to read fuel trims.

Since the thing ran 'perfect' up until getting the #5 Misfire code and soon after that puked the front cat into the intermediate one, my focus keeps running to what things were connected with just the actual parts I replaced and systems touched. After all those are checked then time to go after things like leaky intake.

 

"..........and fuel trims read all 0. I'm imagining that's due to not reaching whatever the requirement for establishing a full drive cycle ? I had the thing out for 45 minutes test drive right after finishing the first go-round trying to reach a full drive cycle, but have had the batt. disconnected since. Besides, I read somewhere it takes multiple start-stops together with a grand total time to reach the requirements......"

It turns out a case of my ignorance in equating fuel trims with O2 sensor readings Bank 1 & Bank 2. As far as data streams or anything else regarding fuel trims I get only the reply to the effect that, "Your vehicle doesn't support or include those parameters...."

But except for seeming peculiar O2 readings comparing the two banks with each other it is operating as well or better than before it puked a cat. I'm investigating now whether the dongles costing $100 more than the $30 I paid for the VEEPEAK one I posted above will provide more and better reporting - especially fuel trims.