I recently bought a car for sub challenge money. It was most likely a poor decision as I don't like the car all that much. Its a 1990 Mercedes 300SL. It has 79k on it and I am trying to get through the CA Smog test. I failed yesterday with an HC reading of 141 at 1800 RPM passing would be 102. All other readings are good. I have attached the report. What do I need to do to pass? I changed plugs today ( they were resistor plugs) and I have an air filter on the way.
What did the old plugs look like? From what I remember ignition is usually the cause of high HC levels, so you might've taken care of it. Maybe wires as well? What does the rest of the ignition system look like?
Given the balance of all the emissions, the ignition looks to be a good source- that's a good suggestion.
Start with ignition. But the dirt cheap way is to run good gas it it get is really hot when testing and sea foam the E36 M3 out of the engine. I have taken cheap cars from there times over the limit to passing with just sea foam, new plugs and a distributor cap and timing check.
In reply to Mr_Asa :
The plugs looks decent to me. 6 is on the left:
Bummer. Happened to my 2001 Tacoma. Catalyic changed, all o2 changed. Did Toyota drive cycle reset (annoying as heck). After 1 month, I went to a "special shop" and had it passed.
You are close, it shouldn’t take much to bring you under the limit. Do your tune up, every little bit will help. But perhaps more importantly, make sure the cat is nice and hot before you bring it in for a retest. Don’t bring a cold car to the inspection station, flog it for a bit and take it on a nice long drive first. You had worse readings under the lighter load.
In reply to Steelpig :
Check the rest of the ignition system- wires, cap, etc.
does it have an O2 sensor? If it did, it would be a cheap one wire version, so that would be cheap to change.
I don't suspect the catalyst with the rest of the results.
Every 2 years I went through this with my E36. I understand the test's value but man is it a pain on older stuff. I swore off anything between 1975 and 2000 just because there's no way to tell if it will pass on the dyno until the money is spent.
PO should have smogged it, I believe they're on the hook financially for it not passing should you decide to pursue that.
Do the tune up and take it for a long drive, like 50 miles if you can, and then take it in. Driving it like it you stole it seemed to make things worse for me. You want the car to stay in closed loop and get the cats working well before you take it. A nice cruise on the freeway should do that. A fresh tank of good gas might help too. Good luck
In reply to spandak :
At least here in Portland you don't pay for the test unless you pass. Makes troubleshooting a bit less painful. Sucks that Cali isn't that way though.
CIS injection? (Or whatever Benz calls it.) Or better, does it have one of these? The VW version has a place to adust the idle mixture. I don't know about the Benz version. If not, change the oil , replace the air filter and dump a bottle of drygas in the tank. HC is unburnt fuel. The oil change will remove residual fuel from the oil and the drygas will lean the car out just a tad. And the air filter will insure that all the clean air the engine needs will get in. Let us know what happens. If you car do not have a fuel distributor (the part in the huge picture) the same can still apply. Change oil, air filter and add drygas.
Rule of thumb is that high HC with non-zero O2 means the cat isn't doing its job. You have zero O2, so it's done all it can, meaning it's running rich.
On an EFI car I'd look at the sensors that inform the mixture choice. O2 sensor, MAF/MAP/flapper door sensors, air intake temp sensor, coolant temp sensor, etc. No idea which or any of those exist on a mechanical fuel injection system if that's what this has.
In reply to codrus :
Because CO and NOx are both quite low, I don't see the catalyst as the big culprit. Not perfect, but it seems to be working. A new one will still show problems.
Is there a possibility in a 30 year old car there's a leaky injector o-ring?
Think through it logically. Anything that would cause it to run rich. The simple things could be FPR, Airflow sensor, O2 sensors, but that is likely not the issue. Look for vacuum leaks or exhaust leaks. Anything that would trick the O2 into thinking that things are less than stoich.
I had a BMW E30 that didn’t pass, (I wish I could remember if it was HC, but I can’t) unless I had driven it on the freeway for at least 30 minutes. I tried it once after driving for 15 min and it was not enough, had to be 30. It was surprising. The cats in German cars take forever to warm up because they are so far back.
Try it again after warming it up for 30 min on the freeway before digging further.
Many times I’ve sat ringing my hands waiting for the smog report to come back.
I checked the ignition and I have one plug wire that had way more resistance than the others, so that gets replaced when the wires, cap, and rotor arrive. I get a free check since I didn't pass, so I will replace everything and check it again. I read that if your car is 30+ years old they will allow you to pay a fee to register it even though you don't pass smog. I found receipts for new cats 1 year ago, so I'm hoping those are still good. If this doesn't work I will check adjusting the mixture. Thanks for the suggestions, I will report back once I retest.
I heard rumors about the pay once to pass thing but I didn't know it actually happened. Maybe we can buy 80s stuff again!
The previous owner is legally required to smog it. Doesn't mean you can't agree to do it yourself but they are responsible.
I third getting it super hot first.
also make sure you have fresh gas going through it. My e36 failed on old gas. New gas and it passed no prob.
Never heard of pay to pass, that sounds unlikely to me.
And yeah, the seller is legally mandated to smog it. I always ask them to so I don’t run into this, but that’s not helpful unless you want to reach out to them.
spandak said:
PO should have smogged it, I believe they're on the hook financially for it not passing should you decide to pursue that.
This
Also, did not see where you said you changed the oil, which makes more of a difference than you would think
Round 2 came close, but no cigar. 
That was after plugs, wires, cap and rotor. Im going to pull the intake apart and look for leaks. I believe I can adjust the mixture as well. I'll also look at o2 sensors as well. I should have had the PO smog it, but I was feeling lucky. He was asking 3600 for it, and after I left he called and said 2k would take it. I figured I had some room to fix things and he had replaced the cats at the last smog. Its a really good looking car and exactly the kind of car I would never buy for myself. Its a 4000 pound 12/10 scale miata.
At this point, I'd change the O2 sensors. They can be simple Bosch universal ones- could be 1 or 3 wires.
It's so close to working, if it's not the ignition, that would be the next cheapest fix.
If it's rich, it's not very rich- the CO isn't all that high. If it were solidly rich all the time, it would be a CO and HC problem. But a lazy O2 sensor could lead to elevated amounts.
I'll replace the O2 sensor just for the heck of it. It's a 3 wire and the universals are cheap enough. Thanks!
Your cat is good. Don't change it. Running rich won't be good for it, though. I would be looking for faulty sensors, especially coolant temp and TPS. If you have KE-Jetronic, check your fuel injectors for good patterns and leaking after shutoff. If it's L-Jet or later, have your injectors cleaned. RC engineering is cheap and located in Torrance, CA. They are a GRM advertiser. They'll get you some injector o-rings too. You could have a vacuum (30yo injector o-ring?) leak causing misfire and showing up as a "rich" condition because the mixture's not lighting.
Unfortunately, we're out of books for that model at the moment. You might find the procedure on the web for blinking out fault codes on the dash. I'm thinking vacuum leak, bad sensor, partially clogged fuel injector.
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