Failed CA Smog Test
 

I have a 1979 450SL. I had to take my car to test only smog station. I drove it on the freeway for 1/2 an hour, checked the air filter, etc. It failed the NO. The max No at 15 mph is 1277 mine measured 1932. At 25mph the max is 1137 mine measured 1706. Any ideas out there on what caused the high readings and failure?

I don't know what caused the high reading, but I was wondering if CA had a "classic" or "antique" classification that would allow you to bypass the smog test? I registered my 81 380sl as a "classic" in PA, since it was over 25 yrs. old, and I don't have to pass the emissions test.

The warmer the car is, the higher the NO readings you'll get. How did you do on the HC and CO2? Mine did great on those, but was borderline on the NO readings. I passed, but I took it back later and had them do a test-only (no report) on the car without doing the long, hard freeway trip I'd done first. I was just at operating temperature, and I didn't let it idle while I waited for him to hook it up. I passed with flying colors on everything. The NO readings were just over 1/2 what they had been.

BTW - I've got a 78 450SL. California edition.

Failing nox is usually something to do with the egr valve, plugged ?
High internal engine temperature.
Regards,
aam.

CA used to have a 30 year rolling exemption, but they froze it in 2005. All cars 76 and later must pass smog.

Failed CA Smog Test
 

I passed the HC, CO tests. I was about 20 higher than the average, and way below the Max on these tests. Same was true for the %CO2 and %O2.

High NOx
 

If nothing is broken (i.e. bad valves or rings) then NOx rises with leaner mixture and both CO and HC fall with leaner mixture. If you fail by a lot on NOx and pass by a lot on CO then your mixture is simply too lean. A too hot engine may cause the mixture to lean a bit but let's not confuse cause and effect. The mixture is the key.

The CO adjustment screw will allow you to richen the mixture by a small amount to bring down the NOx. The smog station should have tried that. Since your car is '79 you may not have an oxygen sensor. I think the oxygen sensor started in '80. If you don't have an oxygen sensor then you'll need to adjust this screw with a CO meter. Any competent Benz shop can easily do this.

If you have an O2 sensor then you can make this adjustment using a dwell meter. The duty cycle of the lamba valve should be 50% when the mixture is correct. The lamda valve is a fuel injector thingy next to the air metering housing which makes small adjustments in the mixture using feedback from the oxygen sensor. The on-time vs off-time (dwell) of this valve adjusts the mixture.

The range of adjustability of the CO screw is small. If it doesn't adjust enough then you'll have to replace the Warmup-Regulator (WUR). It's very common for the calibration of the WUR to drift up over time.

I had a bung for an oxygen sensor welded into my exhaust pipe before the catalyst. My local muffler shop did it for $10. A one-wire oxygen sensor cost me $30 at the local parts store. This setup is a lot cheaper than a CO meter and a lot more accurate.

When i bought my 107 in 1998 the car would not pass
and so the seller had to fix the car. The problem was
the EGR valve. It took a week to obtain the part
and then it passed as it has ever since.

I think this is a cheep fix so give it a try

Steve '84 280sl 5 speed 191k:D

CA Smog Test
 

It sounds like the mixture could be too lean. One question I have is a year ago I had a high idle problem. A mechanic replaced leaky vacuum hose and the auxillary air valve. It still ran high idle when it was cold. Someone on the forum suggested tapping the center of the Aux. air valve. I did this and the high idle stopped. Now since I'm having a problem with high NO measurement could tapping on the A. A. Valve have effected the mixture so the engine runs leaner?

Get some maf cleaner spray take out and clean the valve.
That is not going to lean the mixture.
There other ways to do that.
Regards,
aam.

When was the last time you replaced the gas cap? Try the cheapest fix first, a new gas cap that has a new rubber seal. Happened on my car.

Check your mixture.
A comment on EGR valves....mine was seized. So I spent an hour cleaning it and freeing it up only to find the diaphragm pooched. Dealer only. $400.

If your CO and HC readings are in order and the EGR system is functioning properly then you're looking at a catalytic converter problem . The avereage lifespan for these critters is roughly 100-120k miles . Good luck