Help on Emissions Failure Analysis ....

[stephenson]

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!

[david s poole]

the first test shows that you had way too much fuel [no brainer] and this was keeping the nox down.you don't show o2 readings and these could be helpful.the things that you did to the engine after the first test leaned it down considerably[and i''ll bet increased the available o2 in the exhaust] and so lots of nox.your co needs to come up to about .05-.07 and that should keep the nox under control if the cat is working ok.

[stephenson]

Thanks, David ....

My goal was simply to get this thing running and sell it as a project car for someone who enjoys this particular model car, but this emissions thing is pretty interesting :-)

I am going to replace the O2 sensor since it looked it was installed by a butcher ... I metered the sensor and it appeared to be generating cyclic input for engine computer ... but, that was only at idle and with it being sooty this seemed like best place to start - plus, I don't think I knew enough at the time to chart how big the rich and lean input deltas were.

Since the cat "appears" clean on exhaust side, and with a new O2 sensor, what if I swap cat ends?

Appeciate the advice!

[david s poole]

my guess is that you are right.o2 sensor can have big effect on leaning down mixture and if it's bad or going bad[sending too low a voltage]this could well be the last problem with this project.

[stephenson]

David - it just passed. I had been out of town for awhile.

So, something, or a combo of things made the difference BETWEEN second and last test.

(1) retarded base timing to 4 degrees BTDC (from about 6) - spec is 5 +/-2
(2) new (universal) O2 sensor (faster movement but old one tested OK)
(3) swapped catalytic converter (universal type) end to end (reversed)

FINALLY!

Appreciate your help.
+++++++++++++++++++
Three tests over two months - test number shown following parens:

HC (limit 92/15 mph) (1) 501, (2) 69, (3) 86
HC (limit 100/25 mph) (1) 501, (2) 100, (3) 58

CO (limit 0.37%/15 mph) (1) 1.6%, (2) 0.02%, (3) 0.04%
CO (limit 0.53%/25 mph) (1) 1.6%, (2) 0.04%, (3) 0.13%

NO (limit 1200 ppm/15 mph) (1) 123, (2) 1386, (3) 645
NO (limit 1167 ppm/25 mph) (1) 123, (2) 893, (3) 136

[david s poole]

that is the problem when you do more than one thing when you are looking for info.my gut tells me that the o2 sensor did most of the fixing because the timing retard would have lowered hc and co.your hc and co numbers rose as i suggested and it had the effect of keeping down the available o2 which then kept the nox under control.i am always pleased when a plan comes together.

[stephenson]

Hmmmm .... this started out as simply wanting to get past emissions test, but got more interesting :-)


So, in summaary, between test 2 and 3, after new O2 sensor, retarding timing a couple of degrees, and end for ending the cat:

HC rose at 15mph, but went down quite a bit at 25mph
CO rose at both speeds/rpm.
NOx down at both speeds, but way down at 25mph.

OK, trying to figure this out, but not getting far .... looking at only the 25mph test, then: since HC went down means running leaner, but CO rose which means richer - opposite effects?

Some texts note that the cat provides a reduction in Nox when the CO increases...

Probably interrelated play, then? Faster sensor cycle (primary), maybe the cat (secondary), and maybe some effect from the timing (tertiary)?

Thanks very much for your assistance, David ... now am sell this thing (although it does have a nice sound between 6-7000 RPM and spins very quickly. Gotta get to the next project - rebuilding the 1972 BSA 500!