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Some time ago my check engine light came on intermittently. My gas cap appeared to be in good shape, except that the rubber washer was quite hard but not cracked. I bought a new washer from the MB dealer, but it did not fix the check engine light. I also noticed when I unscrewed the cap there was no sound of the vacuum being released, and I found gas in the flange below the gas cap. I tried replacing the gas cap, but the light stayed on, I got no vacuum and the gas continued to collect in the flange below the gas cap. I tried replacing the gas cap with one from Blau, but same results. For some reason I just can't get a gas cap to seal my fuel tank and I don't understand why. Is this a common problem? Any suggestions for what I can do to get a proper seal for my fuel tank?
 

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Premium Member 2013 SLK250
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After replacing the cap you need to start and stop the engine several times to reset the CEL. Drive it for a day or two should reset CEL or use a OBD2 reader with a reset.
 

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Super Moderator CA 2012 SLK55 AMG w/P30
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Some time ago my check engine light came on intermittently. My gas cap appeared to be in good shape, except that the rubber washer was quite hard but not cracked. Is this a common problem? Any suggestions for what I can do to get a proper seal for my fuel tank?
Try cleaning your purge valve.
So many parts, my mind boggles :surprise:

Found this that might help


DIY: Cleaning the Purge Valve - Mercedes-Benz Forum

PDF in the first post. Problem sounds exactly like yours. Let us know if it helps.
 

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Question!? Has anyone with an R170, ever experienced vacuum when removing the fuel cap? Iv'e had my SLK230 for 6 months and haven't heard anything when removing the gas cap.
 

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Administrator 2009 SLK 55 AMG/Founding Member 2006
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Question!? Has anyone with an R170, ever experienced vacuum when removing the fuel cap? Iv'e had my SLK230 for 6 months and haven't heard anything when removing the gas cap.
you should hear vacuum
check the seal around the cap
they go bad. get dried o
bad enuff you will get a CEL
 

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Did you pull any codes? Hopefully it's not Evap related such as( P0442) because it's a lot of work. You might have to clean purge/vent valve or replace the seals. Pellets might be causing a blockage due to bad carbon canister from overfilling. It's usually a small leak.

In addition, your purge valve might not be sealing properly, hence why you can't hear any pressure build up (vacuum). If you have pellets use compressed air to blow them out.

If your purge valve is OK and no charcoal pellets in the vacuum lines, then you have a vacuum leak. Remove engine cover use a spray bottle to spray the vacuum lines. At this point you should have a rough idle. Spray water on the vacuum lines, if you hear any distinct vacuum noise or an idle change, the leak should be around that area. You can use other tricks like a carb cleaner or propane, but it's not safe. I don't think that you will have to go this far though. I would start with the gas cap and work your way to the purge valve.

Did you disconnect the cables to reset the ECU? or use a scanner to reset ECU? Did you tighten the gas cap at least 1-3 clicks? P0440 indicates a big leak, which likely the gas cap issue and easy to locate.

The problem is not complicated as long as you are familiar with the evap system.

Slk in SLC? That's rare! Anyways I hope you find the problem.
 

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It all started here: :crying:
P2006: Intake Manf Runner Control Stuck Closed bank1 a)
P201D: Unknown code - More information may be available on the web
P200B: Unknown code - More information may be available on the web
P2036: Exhaust Gas Temperature Sensor Circuit High bank2 Sensor2

Then i replaced an O2 sensor downstream of #1 and then this:

P0101: Mass or Volume Air Flow Circuit Range/Performance Problem
P201D: Unknown code - More information may be available on the web
P200B: Unknown code - More information may be available on the web
P2023: Intake Manf Runner Position Sensor/Switch Circuit Intermittent bank2

Then replace the MAF cleared the ECU and again received another CEL:|

P0171: System too Lean (Bank 1)
P0410: Secondary Air Injection System

Keep in mind on a cold start-up the engine sounded like it had a VERY BAD exhaust leak, then 30sec later would become quiet..

I then replaced both components in PIC 3 and wire brush cleaned the metal tube and reassembly. Replaced fuel filter and checked operation of vacuum systems and associated one-way valves.
Problem solved? NOPE:surprise:

P0410: Secondary Air Injection System

At this point I've reviewed and checked operation of the MOT & check valve, associated vacuum hoses, reversed the rubber gasket on the fuel tank cap ordered a replacement fuel cap and reset my CEL.
All the while, I have never heard a pressure release when opening my fuel cap.
I'm currently on my third drive cycle after CEL reset.
 

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Premium Member 2005 SLK55 AMG; 2005 SLK350
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The only time you really hear a strong vacuum is if you open the cap during or just after one of the computers periodic leak tests. Ordinarily, the check valve will only allow a very slight vacuum to develop; but during a leak test much higher vacuum develops. I wouldn't worry about the absence of pronounced vacuum in your tank (until the evap codes return at least).
 

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In conclusion. Most of the above trouble codes cleared after a replacement of a Mass Air flow sensor, both O2 sensors, secondary air system components and a minor vacuum leak on the intake manifold. A brief overview of the emissions readiness test with my scan tool revealed the evaporative system passed. This took many drive cycles. I can't even remember how many but it was couple of weeks of daily cold and hot starts. Uhh.
What remains is P201D or (PO171 generic) system too lean, and a rough idle. Indicating a faulty injector. With 170,000 miles on factory injectors I'm thinking so. Ironically I just replaced the fuel filter, I must have dislodged something during replacement.


Sent from my Nexus 9 using Tapatalk
 

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Discussion Starter #11
In conclusion. Most of the above trouble codes cleared after a replacement of a Mass Air flow sensor, both O2 sensors, secondary air system components and a minor vacuum leak on the intake manifold. A brief overview of the emissions readiness test with my scan tool revealed the evaporative system passed. This took many drive cycles. I can't even remember how many but it was couple of weeks of daily cold and hot starts. Uhh.
What remains is P201D or (PO171 generic) system too lean, and a rough idle. Indicating a faulty injector. With 170,000 miles on factory injectors I'm thinking so. Ironically I just replaced the fuel filter, I must have dislodged something during replacement.


Sent from my Nexus 9 using Tapatalk
Thanks to all who responded to my post. Ultimately the problem turned out to be the oxygen sensor. I replaced it and the check engine light went off and stayed off.
 
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