Not on one of my MBs, but the Mitsubishi forum hasn't been abe to diagnose the issue.

Bought 1992 3000GT knowing it had spun bearing - kinda as something different to do. Rebuilt the engine (made me long for my MBs) and have reinstalled it in car and run it about 1000 miles without mechanical issues. (it does, btw, run like a scalded dog with a very nice sound - my first manual trans in a few years - big smile!!)

One month ago with 200 miles on engine .... fails HC and CO, passes NOx
HC limit was 92 at 15 mph and reading was 501 - FAIL
HC limit was 100 at 25 mph and reading was 501 - FAIL
-
CO limit was .37% at 15 mph and reading was 1.6% - FAIL
CO limit was .53% at 25 mph and reading was 1.6% - FAIL
-
NO limit was 1200 at 15 mph and reading was 123 - PASS
NO limit was 1167 at 25 mph and reading was 123 - PASS
-
Dilution reading at 15 mph was 13.6 and received "valid"
Dilution reading at 25 mph was 13.6 and received "valid"

So, I checked and reset normal sorts of things in a baseline OBD system - O2 sensor may have been disconnected (the plug ends were almost separated - was not providing a code, though), TPS adjusted (there was a TPS code that has not returned after I adjusted it), ISC/IAC cleaned and works appropriately when A/C selected, etc and idles pretty well. I did NOT test the coolant temp sensor as everything else seemed to work OK. On start it fires instantly, idles high then slowly comes down and appears to enter close loop as temperature comes up. It runs very, very well - scary almost with 2X legal speeds possible in 4th gear at 6000 RPM - then returns to idle smoothly at a traffic light. Good MPG - simply no issues.

Two days ago - barely passes HC, easy pass CO, and now fails NOx ...
HC limit was 92 at 15 mph and reading was 69 - PASS
HC limit was 100 at 25 mph and reading was 100 - BARELY PASS
-
CO% limit was .37 at 15 mph and reading was 0.02 - EASY PASS
CO% limit was .53 at 25 mph and reading was 0.04 =- EASY PASS
-
NO limit was 1200 at 15 mph and reading was 1386 - FAIL (passed first time)
NO limit was 1167 at 25 mph and reading was 893 - PASSED BUT WAY HIGH
-
Dilution reading at 15 mph was 14.7 and reading was "valid"
Dilution reading at 25 mph was 14.8 and reading was "valid"


So, after lots of reading it seems like the cat or O2 sensor causing it to run rich?

Removed cat (17" universal fit) - intake pipe lip was very sooty - exit pipe lip also sooty. Intake side honeycomb was dark colored but not plugged looking - exhaust side honeycomb was clean and white. Main muffler exhaust pipes also very sooty.

Removed O2 sensor (4 wire heated type) - also sooty looking.

Would seem then, that it is running rich, which is supported by high (but not failing) HC, but low CO, right? Does this explain the huge differential in NOx readings between first and last test?

I have seen so many discussions where folks just start replacing sensors, cats, etc that I am now interested in a better understanding of the relationships as they relate to real world pass-fail scenarios.

What do you guys think?

Appreciate the help on the non-MB 3KGT!

__________________
George Stephenson
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg