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Old 01-21-2006, 07:25 PM
JAJ JAJ is offline
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Join Date: Jan 2006
Location: Vancouver, BC
Posts: 15
Emissions issue went away - how did it happen?

I'm new to the board - second posting.

I just picked up a 95 E420 with 86,000 miles on it. Beautiful shape, well looked after with complete service records - every service is stamped from PDI to today.

Of course it had problems with the wire harness, now replaced, and a problem with the ETA and engine thermostat, both now replaced as well.

Here in BC we have "AirCare" emission testing, and once the repairs were done I took it in for an overdue inspection. It failed in a spectacular way:

HC - measured 1.4 vs. limit of 0.5 (grams per km) - way over! FAIL!
CO - measured 7.7 vs. limit of 9.3 (grams per km) - passed, but only just
NOx - measured 0.8 vs. limit of 1.2 (grams per km) - passed, some margin

The test is a "rolling road" dyno test, ranging from stopped to 55MPH, with speeding up and slowing down through a complex test cycle that runs for 4 minutes. The faster it went, the higher the HC and CO, and even NOx was spiking during acceleration (if it flunks, they print a scan of readings vs. time including speed). The tests are done by Government certified labs that don't do anything else, no mechanics, no advice - just testing.

Since the car had been sitting on the dealer's lot for a couple of months, and then had been through the shop with a bunch of troubleshooting and new parts, I figured a "good run in the country" might help.

The next day, I took it for a run on the highway, and drove for about 150 miles or so, and then took it in hot for a retest. The tech ran it for 1 minute or so at 40MPH on the dyno to heat up the cat (very thoughtful of him) and then did the cycle. It failed again, but not by much:

HC - measured 0.6 vs. limit of 0.5 (grams per km) - still over - still FAIL
CO - measured 1.9 vs. limit of 9.3 (grams per km) - passed, much lower!
NOx - measured 0.4 vs. limit of 1.2 (grams per km) - passed, much lower

Armed with the new readings, I went back on the highway and notched it into third gear and drove another 100 miles at 70 MPH or so.

The last test results were a PASS:

HC - measured 0.2 vs. limit of 0.5 (grams per km) - Clean PASS!
CO - measured 0.9 vs. limit of 9.3 (grams per km) - a big drop!
NOx - measured 0.3 vs. limit of 1.2 (grams per km) - another big drop!

Can any of the experts here help me understand what was happening?

I did the fast driving because of an intuitive belief that it was the right thing to do. Ok, and it was. But why?

Was this the LH module "learning" the new components? Or was it fuel saturation in the whole system (the LH sensor was inoperative from the wiring harness and it was running super rich for the last few weeks, if not months).

The improvement in emissions was startling - HC down by 84%, CO down by 88% and Nox down by 65% from the initial readings. I went from expecting to spend big $$ on fixing emissions to getting it all done for the price of a tank of gas and a day cruising the freeway and listening to a great stereo.

Any insight from the experts would be welcome!

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Old 01-22-2006, 08:53 AM
Registered User
Join Date: Dec 2003
Location: Fribourg, Switzerland
Posts: 277
I am no expert, but still would like to guess.
The car has been sitting around for a couple of months so the inside of the cat developed some oxidation that reduced its activity. Running extended time at operating temperature restored most of its original activity.
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Old 01-22-2006, 11:28 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
Bruno's concept is probably significant. Also the O2 sensor was probably similarly inactive and slow or poorly reading. Beyond that the car would readapt over time (the early HFM cars were real slow), to new conditions.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 01-22-2006, 11:38 AM
Registered User
Join Date: Mar 2003
Location: Southern California
Posts: 2,281
I can offer no rational explanation other than Italian tuneups may be as effective today (for emissions) as they were in the old days.

Given this car's relative low mileage for its age it may have have seen a lot of short trips, and it's possible that habitual short trips can temporarily foul the catalyst due to rich operation during warmup.

For a modern Italian tuneup avoid WOT so the engine stays in closed loop operation, but use aggressive acceleration and revs with no more than 3/4 throttle travel.

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Old 01-22-2006, 09:03 PM
JAJ JAJ is offline
Registered User
Join Date: Jan 2006
Location: Vancouver, BC
Posts: 15
Thanks for the feedback - much appreciated!

It didn't make any sense to me that it worked, but I figured it would be a useful addition to the "knowledge base" on the board.

Thanks again!

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