91 300E Failed Smog Test by 5000%
 

Just took my daily driven 300E into get smog checked and got a concerning test result back. It failed both HC and CO, NOx was well within range. Car had a headgasket done 2 years ago at 120K, it has 165K now. Runs like a raped ape. No stumbles, no hesitations..it pulls from 0-80mph redlining every gear perfectly. I did a full ignition tune-up when I did the headgasket, along with a new EGR valve, smog pump. No smoke of anykind, ever. Never any rich smell except for very first startup on very cold days, which has always been the same.

The car gets awesome highway mileage on the last long trip I took (November)...got rougly 26mpg highway. Cruising up-grade the entire way from NM > CO at 80mph. Haven't done a mileage check in awhile, but will do one on the next full tank.

The readings are in (GPM - or Grams Per Million). For HC I got 3.2 when the limit is 1.5. For CO I got 118.57 when the limit is 20. NOx was .024 when limit is 3.5.

How could I have failed BOTH HC and CO so terribly if the car has absoultely no driveability issues?? I mean it has POWER all the time, never bogs down in any downshift/rpm situation..not even off the line.

I really have a feeling that the results are skewed and the test was NOT run correctly. They give you some % numbers on how bad you failed. Well my HC is 4579% higher than a comparable vehicle, while my CO is 1504% higher than it should. WTF? This can't be right? Or CAN it?

I checked the distributor cap and a few other things and even pulled a few spark plugs. Everything is normal. As it should be. As I would expect it to be. I'm thinking Catalytic Converter? If the goons at the testing place didn't screw up...

EDIT: Just for S&G I pulled up the emissions test from 2 years ago. Mileage is 123K. HC GPM was 1.6, limit 2.5. CO GPM is 15.6, limit 20. NOx 0.17, limit 6. CO is a tad bit high, which could have been indicating beginnings of the cat having reduced efficiency. But nothing has been done to engine in the last 2 years other than oil changes. By my super accurate seat of the pants meter, and the sheer amount of seat time in this one car...I can say the engine has lost no power in the last 2 years.

Bad Cat?

I should also mention, that the car only failed emissions when accelerating. Cruise and Idle emissions readings were all in spec. What parts should I be looking at? What would cause ridiculously high CO, high HC and normal NOx? Without affecting drivability and mileage?

When was the O2 sensor last changed?

Give the cat a bang with your fist. If it sounds like a christmas tree it could be the culprit.

Not sure how much help this will be, but:

HC=unburnt fuel, caused by misfiring or excessively rich mixtures
CO=partially burnt fuel, usually an indicator of a too-rich mixture
NOx=excessive cylinder temperatures, usually from a too-lean mixture under load.

HC and CO being high point toward a rich air-fuel mixture. In this case, since there is not enough oxygen to support complete combustion, the reaction stops short at CO instead of CO2.

The air pump, if it's like my 560SEL, only operates on cold start before the O2 sensor starts working. The way the catalytic converter is set up on most modern cars is the reduction catalyst (for NOx) followed by the oxidation catalyst (for HC and CO). That way the O2 stripped off of NOx is available to oxidize HC and CO in the oxidation catalyst just downstream (or in the same shell). Also, it is necessary for the air-fuel mixture to go lean every now and then so the oxidation catalyst can operate when NOx is low (since a lean mixture will result in some O2 being present). That is the job of the O2 sensor, and if it does not switch and causes a rich mixture, the HC and CO will be high without the needed O2 in the exhaust. This will give the impression of a bad catalytic converter when in fact it just can't do its job. So, in your case, make sure the O2 sensor is operating as it should. If it is, then suspect a failed catalytic converter. If the O2 sensor is not operating, check the air-fuel adjustment at the air flow meter (refer to the shop manual for this). A rich mixture normally will not cause drivability problems or a significant loss of fuel economy unless it's ridiculously rich.

Current test:
HC: 3.2 Limit: 1.5
CO: 118.57 Limit: 20
NOx: 0.024 Limit: 3.5

Test from two years ago:
HC: 1.6 Limit: 2.5
CO: 15.6 Limit: 20
NOx: 0.17 Limit: 6

Looks like the limits for the HC and NOx have gotten more strict, your car two years ago would not have passed the current test if it performed exactly the same. :o

Well you can probably thank the stricter requirements to living in "Boulder County". All of the surrounding counties (excluding Denver corridor) to me have no emissions requirements!

But the O2 sensor appears to be failed. With the engine warmed, at idle....the voltage reading from the o2 was oscillating slowly between .05mv and .08mv and then it stopped oscillating and the reading stayed at .09mv. I purchased the generic Bosch replacement this evening ($40!) and will install it in the morning.

"Redlining every gear to 80mph".....the 300E is not even close to redlining 3rd at 80mph.....that happens around 118 or so....:D Not that I do that regularly or anything. :D ;)

The M103 runs extremely good after a real hard run.....seems all MB's love that. My sister's 190E 2.3 was stumbling a bit recently and stalled on her a couple times at idle....so I took it out on the highway for 15 minutes and whupped it.....hasn't stalled or stumbled since!

She drives about 25-40mph 4-6 miles at a time....poor car...always short trips.

Whoa! Amazing price of $40! Where did you get it if I may ask?

A 1990 Mustang GT uses the same sensor. I just bought one for $36. All you gotta do is splice it into your current connector.

Yes, Italian tune-ups do wonders!
Regard, Eric

Replaced the O2 this afternoon, took about an hour. First thing I noticed was that my rough idle immediately smoothed out. Can't even tell the engine is running at idle in drive, which is how it should be :D. I was having some real bad random shudders at idle, so bad the first few times It happened I thought I got lightly tapped from behind!

Now the voltage readings from the O2 sensor are in the proper range, just slightly below a stoich mixture. I don't think duty cycle needs adjustment. The PO had a re-manufactured Bosch fuel distributor installed at the dealer before I got the car :eek:...which was hellishly expensive. So AFR is something I ain't going to touch, because it's been adjusted somewhat recently. Unless it fails emissions when I take it back. But it better not, or it's owner is not going to be very happy with it.

After tooling around all afternoon running errrands there have been a lot of upgrades to the driving experience. As I stated before the idle is rock solid. Hot start is now smooth, it used to stumble for 3-5 seconds after startup.

Low end torque seems massively improved. Off the line performance is a lot better. There used to be a semi-flat spot between 1000-2500rpm in first gear. Also shifts are a ton smoother. The downshift from 2nd>1st at 25mph hooks up smooth and fast, whereas before the engine would hesitate a slight second when I kicked down and then JUMP forward somewhat violently.

This sounds very similar to what I noticed when I changed my O2 sensor about a week ago. I was suspecting that my injectors were showing their age, I had no idea that changing an oxygen sensor would make THIS much of a difference. I changed my oxygen sensor since I knew it was in the car for well beyond 60,000 miles and just wanted to replace it. My 208K mile old M103 just got a whole lot more respect in my eyes! :)

Please post the next smog test numbers.
Regards, Eric