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Hey everyone. A few questions. About 2 weeks ago I installed a cold air intake on my 2004 SLK 230. Yesterday the check engine light came on after driving about 20 miles. The codes were: P0131 and P0171. Low voltage on the O2 sensor and Bank 1 lean. Could this be because of the CAI? I have not had any programming done. If so, I can go back stock. If not, could it be the O2 sensor going bad? Still learning this car as I have been a Mustang guy all my life. Any help and/or insights would be greatly appreciated. Thanks!
 

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Hey everyone. A few questions. About 2 weeks ago I installed a cold air intake on my 2004 SLK 230. Yesterday the check engine light came on after driving about 20 miles. The codes were: P0131 and P0171. Low voltage on the O2 sensor and Bank 1 lean. Could this be because of the CAI? I have not had any programming done. If so, I can go back stock. If not, could it be the O2 sensor going bad? Still learning this car as I have been a Mustang guy all my life. Any help and/or insights would be greatly appreciated. Thanks!
CAI- Is it oiled filter? (I don't recommend) If oiled filter is used it had better be oiled lightly and evenly. If not, could be the problem and you will need to clean MASF properly. If oiled properly or is a dry filter then most likely the codes are telling you correct. I have a sense that you used an oiled filter and maybe applied too much oil, these MASF sensors are not as forgiving as Ford's.
 

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The filter came pre-oiled, although it did not seem to be oiled very much at all. What is the best way to clean the MAF? I might try that first and go back to the stock setup and see if it happens again.
 

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Administrator 2009 SLK 55 AMG/Founding Member 2006
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The filter came pre-oiled, although it did not seem to be oiled very much at all. What is the best way to clean the MAF? I might try that first and go back to the stock setup and see if it happens again.
Ya I have never bought one pre-oiled, chances are it was oiled too much. I really advise getting a dry filter like aFE pro-dry. Make sure you clear codes after you clean. What kind of filter did you get?
 

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Ya I have never bought one pre-oiled, chances are it was oiled too much. I really advise getting a dry filter like aFE pro-dry. Make sure you clear codes after you clean. What kind of filter did you get?
Cleared the code(s). This time it only showed a P0131 O2 sensor code. I believe the filter is a Predator, I'll have to check. What I did find yesterday after I got home was after taking the tube from the intercooler to the TB was a somewhat wet coating of what appeared to be engine oil. If it was from the filter, it would have to have been literally dripping with oil. It was not filter oil as those are usually a reddish color. I think I have a separator problem. No build-up that I could see on the MAFS. This morning it threw another P0131. I am beginning to believe that it is actually the O2 sensor now. I am going to try putting the stock filter back on, as the check engine light has been coming on after only about 20 miles of driving. If I can find what the ohm reading should be, I will try checking the O2 sensor. If it does it after putting the stock filter back on I will replace the O2 sensor. I still need to look into the oil separator. Is this a common problem with these cars? Thanks everyone!
 

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Cleared the code(s). This time it only showed a P0131 O2 sensor code. I believe the filter is a Predator, I'll have to check. What I did find yesterday after I got home was after taking the tube from the intercooler to the TB was a somewhat wet coating of what appeared to be engine oil. If it was from the filter, it would have to have been literally dripping with oil. It was not filter oil as those are usually a reddish color. I think I have a separator problem. No build-up that I could see on the MAFS. This morning it threw another P0131. I am beginning to believe that it is actually the O2 sensor now. I am going to try putting the stock filter back on, as the check engine light has been coming on after only about 20 miles of driving. If I can find what the ohm reading should be, I will try checking the O2 sensor. If it does it after putting the stock filter back on I will replace the O2 sensor. I still need to look into the oil separator. Is this a common problem with these cars? Thanks everyone!
Was the filter color red? I have had blue b4, maybe have look inside air filter to see if oil in there as well. If so most likely filter oil. If not, I see you have the supercharged version so ya I think you have something going on with it 4 sure since you have oil in TB, maybe whats triggering O2 sensor. I would look into oil problem first and forget O2 right now, worry about it after.
http://www.slkworld.com/slk-r170-class-diy/24957-oil-separator-change-all-hoses-r170-2001-a.html
 

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Was the filter color red? I have had blue b4, maybe have look inside air filter to see if oil in there as well. If so most likely filter oil. If not, I see you have the supercharged version so ya I think you have something going on with it 4 sure since you have oil in TB, maybe whats triggering O2 sensor. I would look into oil problem first and forget O2 right now, worry about it after.
http://www.slkworld.com/slk-r170-class-diy/24957-oil-separator-change-all-hoses-r170-2001-a.html
Definitely engine oil. Filter is black can't even feel any oil on it. Filter is a Specter brand. Took CAI off yesterday and put stock unit back on. No check engine light this morning on the way to work. I'll run the stock unit for a bit and see if it comes on again. Maybe CAI lets in TOO much air. Odd that it took almost 2 weeks for problem to show up. You would think that the ECU would compensate. We'll see what happens!
 

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From my experience, any code remotely tied to an O2 sensor, is the O2 sensor. I'd start there, especially since you got 2 related codes initially for the same location.
The code does not mean that Bank 1 IS lean, just that it MEASURED lean, by the O2 sensor. Since it also gave a low voltage code, just for kicks, unplug and clean & reseat its connector.
 

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Definitely engine oil. Filter is black can't even feel any oil on it. Filter is a Specter brand. Took CAI off yesterday and put stock unit back on. No check engine light this morning on the way to work. I'll run the stock unit for a bit and see if it comes on again. Maybe CAI lets in TOO much air. Odd that it took almost 2 weeks for problem to show up. You would think that the ECU would compensate. We'll see what happens!
Ok good its not filter oil then. I don't see CAI causing issue then, must just be intermittent sensor failure. If it does it again, clean it as Pioneer suggested then maybe replace if still issue. I would get that oil seperator issue fixed in the meantime.
 

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Cleaning the oil separator is definitely the next order of business. I'll work on that one afternoon this week. Will also try cleaning the O2 sensor connector. FYI I got to work this morning (drove a little harder than normal) 80+ MPH on I-485 and no check engine light. This is with the stock filter re-installed. Will stay with stock one for a few days and then try the CAI again.
 

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Interesting that this didn't start until you put CAI on which would lead one to believe it has to be a filter oil issue. I find it hard to believe that more air flow would cause this unless for some reason the ECU is not compensating for it. I am interested to see if it does it again after a while with stock filter on. At this point I think maybe O2 sensor just needs cleaning, or just replace it.
 

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Interesting that this didn't start until you put CAI on which would lead one to believe it has to be a filter oil issue. I find it hard to believe that more air flow would cause this unless for some reason the ECU is not compensating for it. I am interested to see if it does it again after a while with stock filter on. At this point I think maybe O2 sensor just needs cleaning, or just replace it.
I thought the same thing. This is now day 2 with the stock filter back on and still no check engine light. I'll run the stock filter for the rest of the week, then put the CAI back on and see what happens. I did take the O2 sensor connector apart and spray it really good with contact cleaner.
 

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OK Day 3 and still no check engine light. I'll put the CAI on tonight when I get home and see what happens. Very strange!
 

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Well, I guess this car just doesn't want a CAI on it. I ran the stock filter for 5 days. I changed it out for the CAI on Saturday evening. Drove it on Sunday to the store and back, about 16 miles. went to work yesterday, 22 miles and then halfway home, ding!, on comes the check engine light. I guess the ECU just can't compensate for the increased airflow or maybe the lower inlet air temp or both. Thoughts and opinions anyone?
 

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I don't know if you'll get meaningful results, but is it worth using something like Torque on your OBD port and logging engine readings as you travel with and without the CAI fitted? You may get a clue what's changing - or what the ECU is seeing then be able to sort out what to do.
 

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Maybe disconnect the battery for 30 min or so to force a relearn on the engine? (if you do, make sure the wheel is centered first, or do the right/left/right thing after.
 

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Discussion Starter #18
savcom, I do have the Torque and scanmaster apps on my phone. I have the stock unit on now, so I may try that on the way home from work today. Depending on what I find out from that with and without the CAI, I will then try Pioneer4x4's suggestion and see if that does anything. Thanks folks!
 

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Definitely engine oil. Filter is black can't even feel any oil on it. Filter is a Specter brand. Took CAI off yesterday and put stock unit back on. No check engine light this morning on the way to work. I'll run the stock unit for a bit and see if it comes on again. Maybe CAI lets in TOO much air. Odd that it took almost 2 weeks for problem to show up. You would think that the ECU would compensate. We'll see what happens!
It's a common thing with the SLK230K to get engine oil on the maf. Clean or replace your PCV valve under the intake manifold and don't put too much engine oil. Use MAS cleaner that's plastic safe. You have to clean the whole intake plumbing.

I didn't have time to clean the intercooler, but I cleaned the air/oil separator, supercharger plumbing, and the intake plumbing on the right side of the intercooler. Make sure the vacuum hose from supercharger to separator is fully connected or replace when worn out. I had to replace a few little hoses, replace the pcv that Mr. mechanic forgot to put back and bam....I can finally floor my gas pedal all day without contaminating my MAS unit with motor oil. A dirty MAS will not send an accurate reading back to the ECU regarding proper amount of fuel. The factory ECU and Injector should be able to compensate for the added air flow from CAI. It's not that much flow compared to an upgraded pulley. P0131? Maybe your 02 sensor got contaminated ? Is your cold air intake installed properly without vacuum leaks? This is assuming your MAS is clean & working properly. P0171 is running lean, but mostly due to vacuum leaks. I remember when my MAS was dirty, I was pulling 6 codes all day until I cleaned it.

I forgot to mention that your car will go through this limp mode or weak mode phase where your car feels like it only has 60hp. Either you disconnect the battery for a while or use an OBD2 Scanner to reset it. I suggest to use the Scanner to save time.

After you fixed it, but you still have rough idle every now and then, you probably need a new MAS. Good Luck!
 
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