failed emissions - suggestions?
 

93 300e 2.8 (m104) 70k

got it two years ago where it failed test at:

hc 162 (100) failed
co .40 (.50) pass

after new plugs:

hc 8 (100) pass
co 0 (.50) pass

this was what i considered the NORMAL OPERATING CONDITION

i had the head gasket & wiring harness replaced afterwards. put 20k miles on it over 2 years. this time around i did plugs again w/ coil connectors and failed the test:

hc 234 (100) failed (miserably)
co .50 (.50) pass

local indy spotted vacuum leak, fixed, then retested:

hc 160 (100) failed
co .52 (.50) failed

i assumed i was back to my NORMAL OPERATING CONDITION, but apparently not. my indy is closed this weekend so i've been driving around giving it an "italian tuneup". car runs great otherwise. the car was warmed up but not good and hot as recommended when i took the retest. it is almost like when i first got it when the plug change fixed everything up. but i had just changed the plugs. the only thing i did different was i left the plugs at the preset .32" where i opened it up to .34" the last time. i'm thinking of doing an oil change after the tank w/ techron in it, as i hear that helps.

anything obvious that i can check?

<bump>

come on guys, i've got between now and next saturday to take a retest. i've got a can of x-66, should i or shouldn't i? and how would i use that on an m104 anyway?

Plugs, what kind of plugs? Brand and model?

A friend with a 500SEL just went through the same thing. The car had hardly been used in 2 years. She failed the first time: engine wasn't very warm at the test.

She added a bottle of techron to a 1/2 tank fuel, changed plugs to what was oem recommeded, drove the car hard for several minutes and got the engine hot, went back and passed.

Haasman

Late eigthies/early nineties Mercs appear to be very sensitive to cat temperature. You MUST make sure that the car is thoroughly warmed up - say at least ten miles or 20 minutes of urban driving. Do not shut the car off prior to the test. In fact, run the engine at 1500-2000 revs while you wait.

The converters slowly degrade with age/miles, and loose margin. The hotter the converter, the more effective it is, and this becomes critical to passing field emission tests on older cars.

Duke

it' an m104 just like it says on my siggy ;). anyway i used standard bosch f8dc4's. they are fresh along with new coil to plug connectors (the beru's).

i have some techron in it now but maybe i'll take a run at it with a long drive beforehand. the tech said i just barely missed - but the last inspection two years ago, it was so clean with HC of 8 and CO of 0 (with fresh plugs)!

Okay.....one more time.
Do NOT compare to two years ago.
As converters " age ", they will become less effective at eating up HC & CO.
HC is normally elevated by ignition & mechanical problems, i.e. misfiring and oil consumption ( simplified ).
Are they testing idle & load ?
If they are, the load test should always be done first, as this will really heat the cat. converter.
Another problem with HC is, they are measuring in PARTS PER MILLION, and failing on a few parts, is just a rip-off.
Not being familiar with your state's regulations, my next question is, is there a " degradation " factor built in ?

i'm in NJ which has CA style emissions. for some reason, they can't or won't put my car on the dyno but they do test at 2500 rpm, so i guess that did heat up the cats.

question about the comment on not comparing against two years ago - would 2 yrs and about 20k mile age the cat converters significantly considering the car is only 10 yrs young? see how it went from 8 ppm to 160+ ppm and from 0% CO to .52% CO. i'm inclined to believe that there is some other failing component.

would an exhaust leak affect it that much? are leaks before the cat the only ones to worry about? we did have a harsh winter which is a killer on exhaust systems...

Okay, in the right order:

Testing @ 2500 rpm-no load, as compared to a dyno run, would result in a lower exhaust temperature.
Age & mileage will have a deteriorating effect on the converter.
Exhaust leaks would scew the readings ( diluted ), probably in your favour.
A large leak would be noticed by the analyzer & the operator.
In summation, a " lazy " Oxygen sensor could possibly be the main culprit of your problem.
Do you have the means to check the livelyness ( cross-counts, rich-lean swings ) of your sensor ?
If not,can you have someone do it for you ? ;)