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Old 07-07-2002, 01:18 PM
Posts: n/a
Failed Smog - Help!

Hi everyone,
I finally decided to pick up a 1987 260E for a reasonable price. So I went to my nearby smog center and had it tested. It took quite a while so I knew that it did not pass, however, it was not much. It only failed HC. Here are the readings:

15 MPH MEASURED = 131 MAX = 110 FAIL

Now for the variables: I did not change anything before the test. The only thing I did was to pull the R16 reference resistor to see if I get more power ( it felt like it anyway). I had so far put it back in and removed/replaced the battery cable to reset the computer (just for GP). I also changed the spark plugs (no sign of rich or oily mixture), wires, and distributor cap/rotor, oil and filter. When I replaced the air filter, there was a pool of oil right where the breather tube exits into the air cleaner (Is this normal?). Based on previous thread readings, I noticed that the plug for the Lambda tower is missing or removed (I did not touch this at all). There's also a muffler leak that I patched and will replace as soon as I get it smogged and registered on my name.

In addition to the steps above, any suggestions from the forum to get my HC reading down? By the way, the tires are 195-65-15 (correction: they're 205-65-15s) which makes the speedometer register about 10 MPH slower than actual speed. Will this make a difference also?

Thanks in advance.


Last edited by denism6; 07-07-2002 at 02:41 PM.
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Old 07-07-2002, 01:41 PM
MikeTangas's Avatar
Senior Member
Join Date: Apr 2000
Location: So. Cal
Posts: 4,430
You're in San Diego and if I'm not mistaken you have the Dyno smog test (Smog check II). The dyno should read correct speed I would think. Should still be doing the higher speed test first, followed by the lower speed test (non dyno area testing is high idle [2500rpm] followed by low idle [800rpm])

A bad, or aging O2 sensor will not allow the mixture to normalize quick enough to pass the smog test. Possible your O2 sensor needs replacement.

Also good practice to drive for an extended period before taking it in for the test, to have the Cat good and hot and fully up to operating temp. This will help get a marginal Cat through one last test as well. I usually drive any car I'm taking in for smog at least 1 to 1.5 hours then leave it running while waiting for the test. Have not had one retest in the 17 years I've been in California.
Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

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Old 07-07-2002, 01:45 PM
pmizell's Avatar
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Join Date: Jun 2001
Location: Houston, Texas
Posts: 838
If I were you I would run by my local GM dealer and pick up a can of "top end cleaner". Follow the directions on the can exactly as described and clean out the carbon deposits that are on your pistons.

As Steve B. pointed out in a previous post, find a secluded area to run the car after the soak period, or else your neighbors may phone the EPA

After that, give it an El Wapo (Italian) tuneup by going out on an open freeway and open the engine up a bit.

Besides that, you may want to diagnostically check your O2 sensor and lambda, to make sure your fuel/air mixture is optimized.

Good luck!


'91 300E, 214K miles
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Old 07-07-2002, 03:33 PM
public enemy
Posts: n/a
Take your car for a drive of at least HALF HOUR immediately before the test and try to keep the RPM between 3 and 4 thousand during the drive (keep it on 3rd gear for that). Also try to floor the gas as often as you can (dont brake the speed limits just try to find an uphill and floor it). The idea is to keep the catalyst HOT. Then take it back for the test and let it idle all the time before the test so it will stay warm. You will be surprised to find out that you will pass.

My car failed both HC and NOX and the test guy told me to do as above and bring it back. I did and it passed with flying colors, the measurements were actually cut IN HALF of the previous ones. HUGE difference. These cars need to be warmed up good to pass.
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Old 07-07-2002, 11:08 PM
zr one's Avatar
Registered User
Join Date: Dec 2001
Location: toronto canada
Posts: 85
if all else fails...retard the timing (by turning back the distributor...if it has one) it might run like a dog,but it ll pass.then return to original setting. it works on domestic and jap cars
1992 500E silver
1990 300ce AMG
1993 500e smoke silver-sold
1992 500e sold pearl black-sold
1966 mustang converible 289
1969 camaro x11
1991 ZR-1
1972 corvette roadster
2000 dodge cummins 4x4 sold
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1987 300E (my first benz)
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Old 07-08-2002, 08:45 PM
sixto's Avatar
smoke gets in your eyes
Join Date: Jun 2000
Location: Eastern TN
Posts: 20,841
You can't retard the base timing on an M103. Even if you could, you'll fail the part of the smog check in which they check base timing.

Search the archives for how to tweak the idle mixture. I don't know if this will have any effect at 15mph. NOTE: it might be illegal to do this without a proper license!

FWIW, in CA, it's the seller's responsibility to have the car pass smog even in an as is sale. Keep this in mind if you decide you'd rather not keep the car.

91 300SE
87 300SDL
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Old 07-09-2002, 03:28 PM
Posts: n/a
hey denism6,

your numbers indicate either an intermittent misfire or a failing O2 sensor, check all wires, plugs, vac lines, egr, PCV, primary and secondary timing,scope the O2.
I strongly discourage retarding the timing.
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Old 07-09-2002, 11:48 PM
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Update: Thanks to all your inputs guys. I did all of the above mentioned steps (got close to getting a ticket!) but car still failed. I finally had it diagnosed and the techs suggested a cat change. Yep...ouch $$$$ is the proper word. I went ahead with it because after searching through the glove compartment, I found out the car had rough idle due to leaky valve seals. The car has 170K mi so the cat is probably gone. All in all, I'm still happy with the car. It's my daily driver, gets me around in comfort, and is comparable if not cheaper in price compared to some of the used Hondas or Toyotas. Now I'm really ready to play and start tweaking it ( I used to tweak Mustangs).
Next stop is that R16 resistor that is said to adjust timing. I looked at it and it looks relatively easy to solder new resistors from Radio Shack and have a complete set of say -- 1 up and 1 down. I'll keep you guys posted. Happy driving---or MBZing!


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