1978 450 SL 186k miles California

got it back from a smog check and this is what it reported:

%CO2 %O2
15 MPH 13.6 1.2
25MPH 13.7 1.0

HC CO NO
MAX GP MEAS MAX GP MEAS MAX GP MEAS
191 384 [B]835[B/] 1.25 2.85 0.05 1277 2425 824
159 335 [B]554[B/] 1.05 2.65 0.03 1137 2225 716


Missed the fuel cap integrity test too. Would a bad fuel cap make the HCs put me in the gross polluter levels? I'll change it out of course, but don't want to shell out $70 for another smog check and advance my chances of taking it to a smog referee.

I checked lots of the old posts too, sounds like a bad cat or not hot enough before the test.

Am I on the right track?
Do I have a bad mass airflow sensor or do I hav one for that matter?

TIA

Hmmm...I don't have much information for something that old, but I'll take a stab at what I can get out of the readings. If I'm reading your post correctly, you failed because of high HC readings. HC is unburnt fuel. Because the CO (CO = carbon monoxide = indication of the air/fuel mixture) is low, these two readings together might indicate a lean misfire. Your catalytic converter does not appear to be functioning at this time. The catalytic converter may resume function once the HC is tamed. Or, the high HC could be hiding other problems.

__________________
1989 300 SEL that mostly works, but needs TLC

Look at the tuneup/emission label in the engine compartment and list the codes. In particular I am interested if it has an oxidizing catalyst (OC) or a three way catalyst (TWC). Also, does your car have O2 sensor(s) and EGR?

I expect your car is pre TWC/O2 sensor in which case it would have an OC and a full time air pump, no O2 sensor(s), and maybe EGR.

I need to understand the emission control architecture before I can offer a useful diagnosis of the test data.

I am assuming that this car is either a US or CA model (stated on the above referenced label) and NOT a gray market car. Is this correct?

Do you have any previous emission test data?

Does anything in the engine compartment appear to be jerry-rigged?

Fuel tank cap holding pressure has nothing to do with tailpipe HC readings.

My hunch is that the high HC readings with everything else okay are due to misfires, which points to the ignition system, but I'd like more information.

Duke

[QUOTE=Duke2.6]Look at the tuneup/emission label in the engine compartment and list the codes. In particular I am interested if it has an oxidizing catalyst (OC) or a three way catalyst (TWC). Also, does your car have O2 sensor(s) and EGR?
It has a full time smog pump and EGR. Where should i look for the O2 sensors? Would this be a model that has the cats close to the exhaust manifold?


I expect your car is pre TWC/O2 sensor in which case it would have an OC and a full time air pump, no O2 sensor(s), and maybe EGR.
Is this a better situation for diagnosing smog problems?


I need to understand the emission control architecture before I can offer a useful diagnosis of the test data.

Do you have any previous emission test data?
unfortunately no



I am assuming that this car is either a US or CA model (stated on the above referenced label) and NOT a gray market car. Is this correct?
Yes, to my knowledge


Does anything in the engine compartment appear to be jerry-rigged?
[I]no, just stock with a few new wires here and there[I/]


Fuel tank cap holding pressure has nothing to do with tailpipe HC readings.
Is there a rubber gasket that will fit the gas cap instead of buying a whole new gas cap?


My hunch is that the high HC readings with everything else okay are due to misfires, which points to the ignition system, but I'd like more information.

Duke

Hopefully this will be enough information. If it's not, please let me know if I can provide you with more. I'll try to look back for previous smog check data.Thanks in advance!

I checked for O2 sensors, there are none between exhaust manifold and cat.

The O2 reading seems low for a full time air pump/OC system, but since the CO is low the mixture is probably okay and the converter is getting overwhelmed with HC due to misfires, so the focus of your investigation should be on the ignition system.

Mercs have some differences in the ignition system architecture from most cars - non-resistor spark plugs with RFI resistors in the spark plug boots and distributor cap. If you use typical aftermarket parts - even is they are specified in the manufacturers catalogs they may cause problems.

You'll have to do some research on your ignition system specifics, then compare to the installed parts. Once you get the system back to OE spec with OE or EXACT OE equivalent parts, you might pass the test. Don't buy from McParts, buy from a Mercedes specialist like Fastlane.

Since your car has been tagged as a gross polluter you'll have to go through some hoops. Check www.smogcheck.ca.gov for additional info on certifying gross polluters.

Duke