I just bought a 2010 c300 sport 4Matic with 115K that looks and runs great, but is throwing a CEL (Autel says 1081, generic reader says P0456),
It is a small leak in the evap control system.

Previous owner knew it didn't smog, so I assume they have tried the obvious. I have the mystery of guessing what they did and why, while also fixing the problem. For instance, I know they changed the purge valve and the gas cap, because they were both new, but cheap after market parts. I replaced them both with Mercedes, the code didn't go away (I wasn't really expecting it to). I'm assuming I'll end up with something a little more difficult and expect it's going to be in the back, but I'm going through the engine first.

Which line is correct to use for a vacuum smoke test? I read in a post here to pull the vacuum line off of the brake booster, but other references say to pull a line from the purge valve. There are actually two vacuum lines that join in the middle and the middle was easiest to separate, so I injected smoke there, into the vacuum pump. Both of those vacuum lines look new, but the line to the vacuum pump is a part number that has been deprecated, but I assume that should not really matter.

A) I inserted smoke into the vacuum line going to the vacuum pump. Is that the right place to do the test and should I also do it somewhere else?
B) The result of the test was a lot of smoke out of the connection between the vacuum line and the vacuum pump. I'm assuming that is NOT correct and that connection should be air tight and I should replace that vacuum line (even though it is Mercedes and looks new). Is there any other place I should also smoke test? Will that also get smoke to the charcoal canister and valve by the back wheel for smoke testing connections back there? I'm still expecting to find a problem there, or with the fuel pump (though I do not smell gas in the car).

Any and all advice would be appreciated. This was supposed to be a Christmas present, so I hope it is not too tricky!

I've always have applied smoke into the intake if I am looking for a vacuum leak on the engine. If it's the evap control system, then I would say the purge valve.

Have you looked at any YouTube videos on the subject?

My old man just did it, I believe he punched a hole in a rubber glove and put the tube through there over the intake hole.

My first piece of advise would be to look up if your car qualifies for the extended warranty. My car had it since it has the PZEV engine. The warranty is 15yr/150,000 miles even if you are not the original owner. Unfortunately im over the 150,000 miles so no longer applies.

I say this because everything emissions related is covered, purge valves, gas tanks, spark plugs etc. Before mine went over 150,000 miles I had a cel for the purge valve, but it was less of a hassle to just replace it myself.

Other than that I dont know how to do a smoke test on the car. if it were me I would closely inspect all the hoses and elbows around the purge valve since they can get brittle and crack. Have you verified the purge valve is functioning, can you hear it ticking, even feel it with your finger?

Thank you so much for this! I had no idea that anything would be covered for that long. This car has the air filter behind the air filter, which I though was very odd, but I looked up the VIN and that is because this is indeed listed as "917 PZEV EMISSION USA". It was delivered in October of 2010 and only has 115K, so it should fall within the warranty. I've determined that the fuel filter is bad, and the fuel filter is one of the items covered under the warranty, so I may be in luck. The fuel filter is a whole lot more than the purge valve, so definitely worth taking it in.

I noticed your location so I figured its a California car which would most likely be the PZEV. The warranty info is in the back of the Service and warranty Information booklet that comes with the Operators manual. Be sure to go through everything. I recently had to replace one of the exhaust manifolds and it cost me alot. If I had caught it earlier before I went over the 150,000 miles it would have been covered.

The charcoal filter in the air box is supposed to catch fuel vapors from going out the air filter when the car is shut off.

The car is registered in California, but apparently was originally sold in New Jersey. Even so, it is PZEV. I called Mercedes and they verified my VIN qualified for the extended emissions warranty, and I have an appointment next week to have the fuel pump replaced under warranty.

Thanks again, so much. I had no idea about this warranty, and it will apply for almost another year!

Quick update:
As can be seen in the pdf that TimC300 posted, New Jersey PZEV are included in the full extended warranty. I'm a little confused from the Mercedes paperwork whether they actually installed a new pump or installed the old pump with a new seal, but they definitely replaced the pump seals with the gas tank on both the pump and auxiliary sides, and it was all covered under warranty.

I could not be happier with that outcome.

I have one last issue, an oil leak with the vacuum pump. I checked, and unfortunately I do not find that one covered in the extended warranty. Makes sense, because it is the brake booster, and not emissions. I created a separate thread for that issue.

Thank you one more time for the spectacularly good advice to look up that extended warranty.