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Discussion Starter #21
Here are the fault codes I got:
P0171 01/03
P0171 P 02/03
P0174 P 03/03

Does this have to do with the Mass Airflow?
 

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The P0171 code means that, on the first bank of the engine, the fuel system is running weak or a vacuum leak exists near this side of the engine. A lean condition occurs when the engine either receives too little fuel or too much air. Could be a mass airflow sensor giving wrong readings, Common causes include:
- Intake or vacuum leaks
- Clogged or faulty Mass Air Flow sensor (MAF)
- Faulty PCV valve or other parts of PCV system
- Stuck open purge valve/solenoid
- Faulty oxygen sensor
- Low fuel pressure
- Clogged-up catalytic converter
With a good scan tool that shows the live data diagnosing the code P0171 is fairly easy. Without a scan tool, the only way to find the problem is to check the basics under the hood including: is the air filter box properly closed? Does the intake boot have any cracks? Are any of the vacuum lines or hoses disconnected?
If you have a scan tool, check the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) readings at idle and at higher rpms (Read more about fuel trims below). Normally, both the LTFT and STFT should be close to zero. If the STFT rises higher than plus 15-20% at idle, but drops at higher rpms, it's most likely you have a vacuum leak. If the STFT is close to zero at idle, but elevates to 15-20% at higher rpms, the problem could be caused a faulty mass air flow sensor, clogged catalytic converter or low fuel pressure. I would bet on the mass airflow sensor, as they get dirty after a bunch of miles and it is usually unlikely that you'll have a vacuum leak. But we are just guessing. Mine did the same thing after I cleaned the MAF yesterday, hiccuped at a red light today. BUT, I only had a little cleaner left in my can from the last three cleanings on other cars; however I did think I cleaned it pretty well. I may just spring the $80 for a new Bosch MAF on Amazon as it is a fairly good bet, and it's "only $80." If it still acts up I might get new Oxygen sensors. Not necessarily the best way to fix a problem but those 2 parts are the most likely culprits, the other one not mentioned is the fuel filter which I have hears id atop the gas tank and accessed under either the 2nd or 3rd row seats, and I'm not sure but I think that part costs about $250-300. I will research that shortly just to learn! I hope it is not an issue of clogged catalytic converters, However my ML320 I owned for several years quit running and the PO did not know what to do, sold it cheap and I found out those had melted and the molten metal clogged the catalytic converters and plugged up the exhaust, and that vehicle had 130K miles on it, so it is Possible that my cats may begin to try to clog! Time will tell...Maybe I'll call the exhaust shop that worked on my ML320 and ask them if they can measure the exhaust pressure if I bring it over to them. But, it runs so well that I have doubts that it's the cats going bad... But- possible!
 

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Discussion Starter #23
Thanks again Tonylinc. I really appreciate your timely responses. Helps me a lot.


Today, I removed the Mass Airflow (MAF) and cleaned it really well with the appropriate spray. After I installed it back into the car, I noticed that it is possible that I may not have inserted the air filter box properly. So I made sure it is properly fitted now. Next, I tried to erase the code but it failed to erase. I just stopped there because the car is still on the lift while I wait for the front struts. So I will try again after I finish with the suspension strut and ready to start the engine. If it continues, I will order the MAF. Like you said, $80 is not too much.
From what I read it looks like the MAF is the culprit. Fuel pump problems will make the car turn off or not start at all. My car starts and continues to run. Also it has all the symptoms of MAF - engine coughing and engine hiccups everytime I stop at a red light.

Thanks again and I hope your car keeps running smoothly.
 

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I talked with the Exhaust guys and they said it is most likely one of the O2 sensors is bad, giving faulty numbers every now and then on my car, and more often yet on yours. They have a scanner that is more in-depth than a generic code reader and will be able to tell which one of the sensors it is, and I should bring the car to them tomorrow. The O2 sensors read the exhaust and tailor fuel input, when one of them is worn out (my GL has 144K miles) it will start to give erroneous numbers. So that is where I am going tomorrow, I'll report back findings! Also, when one of these is making the car run too rich or too lean, it can be bad for the engine long term, because too lean can make it run hotter than it is supposed to. So it anyhow, is always best to chase down the culprit when you have a check engine light on! My ML320 had a bad O2 sensor almost the whole time I owned it, yet always ran like a swiss watch, however. Maybe it was just slightly out of parameter.
 

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I talked with the Exhaust guys and they said it is most likely one of the O2....
.... So that is where I am going tomorrow, I'll report back findings!
Looking forward to hear the good news... Your feedback will be greatly appreciated!
 

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Well they recommend replacing both upstream oxygen sensors. The cheapest price I found for original Bosch OEM units was 99 bucks each on amazon, I found them elsewhere for as much as $200 each... there's one on each side of the engine. The exhaust guys said they will replace those for free since it is easy to do they do it all the time, since while I am there I am having the two mufflers at the back of the vehicle removed and replaced with pipes, that will get probably another 35-40HP and the car will roar just a tiny bit. As if it needs more power! Those exhaust sensors are usually good maybe ten years? Mine look to be the original ones and with 144K miles on them, they are likely past their time... Thursday or friday I will have that done. It is acting up a little more often the more I drive it. Hiccuping at a red light... So it will be clear to see pretty soon after new sensors whether that takes care of the problem! I'll keep ya posted.
 

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Drove the GL today to take a friend to eat, then to the airport, the car never hiccupp'ed once, ran perfect. One upstream (between the engine and the cats) O2 sensor arrived today and the other is due to arrive tomorrow, then they both get replaced: after that it will be interesting to see if mileage goes up, and whether the engine will still hiccup once in a while or not. Time will tell!
 

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Discussion Starter #30
Is there a sticky on here how to change front and rear diff fluids for 2008 gl450?
Extremely easy job. Simple drain and fill.
I just changed the rear and front differential and transfer case oil,
For the differential I used Mobil 1 75W-90 and for the transfer case
I used Pentosin Transmission fluid. Could have cost me $100 each.
Check out Youtube videos and follow it - very simple.
 

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Discussion Starter #31
Yes having someone work off the clock or doing it yourself is always great. My independent isn’t available so I am doing this oil change myself. Getting everything from the dealers wholesale site for $60 is amazing. Oil for a little over $4 a liter is way better than even mobil 1 at Walmart.


Sent from my iPhone using Tapatalk
What website did you buy it from?
 

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Hope all is well, I got all my oils from Amazon. After replacing the two oxygen sensors, I drove 50 miles before the check engine light came back on. So, I ordered the MAF sensor, original Bosch unit same as factory issue. I also saw right next to that, at the back passenger side of the engine, a PCV Oil cover that has a diaphragm inside, and has little rubber gaskets that can leak sometimes, and a new one of those cost $14 so I got a new one of those, too. Upon taking it all apart, I saw that the hose connecting to the PCV oil cover was not connected. It goes to the air directing elbow that the connects the MAF to the throttle body. I took that elbow off and cleaned it; that elbow and the MAF just clip onto the throttle body with weird looking clips. I wondered if that hose being left disconnected might be causing the check engine light! Anyhow, since I was laying there on top of the engine, and I had new parts I put them in, the way those clip to the engine and the way the air box fits up to them takes a few times of on/off before you actually get it on there right! Some cars just gotta be a Bicth! Including that little wire clip that's supposed to hold the back of the airbox down. Some of this almost seems like drunk engineering to me, but once together, it all works. After all this, I highly doubt I'll see a check engine light for a while. The rubber gasket seal on that oil cover was not leaking but it was turned to goo, and due to leak at any minute. And its inner diaphragm was hardened and shriveled. So I feel good about replacing that! If the CEL comes back on, I'll let ya know!
 

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Discussion Starter #33
Hope all is well, I got all my oils from Amazon. After replacing the two oxygen sensors, I drove 50 miles before the check engine light came back on. So, I ordered the MAF sensor, original Bosch unit same as factory issue. I also saw right next to that, at the back passenger side of the engine, a PCV Oil cover that has a diaphragm inside, and has little rubber gaskets that can leak sometimes, and a new one of those cost $14 so I got a new one of those, too. Upon taking it all apart, I saw that the hose connecting to the PCV oil cover was not connected. It goes to the air directing elbow that the connects the MAF to the throttle body. I took that elbow off and cleaned it; that elbow and the MAF just clip onto the throttle body with weird looking clips. I wondered if that hose being left disconnected might be causing the check engine light! Anyhow, since I was laying there on top of the engine, and I had new parts I put them in, the way those clip to the engine and the way the air box fits up to them takes a few times of on/off before you actually get it on there right! Some cars just gotta be a Bicth! Including that little wire clip that's supposed to hold the back of the airbox down. Some of this almost seems like drunk engineering to me, but once together, it all works. After all this, I highly doubt I'll see a check engine light for a while. The rubber gasket seal on that oil cover was not leaking but it was turned to goo, and due to leak at any minute. And its inner diaphragm was hardened and shriveled. So I feel good about replacing that! If the CEL comes back on, I'll let ya know!
Excellent information and resource. Thanks again Tonylinc. I cleaned the MAF, cleared the codes and the check engine light went away but the engine still 'hiccups' when I drive. I just ordered the MAF. After I install it, I will let you know. Beside the engine coughing, the car drives like a dream after all the work I put in. I will scan the car today with my new MaxiPro MP808 and go from there.
Thanks again for sharing your experience - it helps and I really appreciate it.
 

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Ha! I got quotes for the transmission service from two shops, both about $500. I changed front differential fluid for $12.00 and transfer case fluid for $10, and spark plugs myself for $77. But the rear differential, the exhaust pipe was in the way so the plug couldn't be reached. My oil change was about $95 with a Mann oil filter and 9.5 quarts of Liqui Moly 5W40. Crazy to pay people to do things that you can do yourself! I learned that 15 years ago with my first Mercedes. An indy wanted to charge me $200 labor to change the spark plugs, I found it very easy with only 8 of them and simply remove two screws that hold the coil, then there's the plug. My check engine light was on before I changed the spark plugs but the engine ran perfectly. After changing the plugs, it came back on, I doubt it has anything to do with the plugs. I don't recall the numbers for the codes but they corresponded to rich mixture left and right banks, and errant data from the mass airflow meter. So, I believe that may need cleaning. There is a spray cleaner product made just for those, you don't want to clean them with any other product. That said, mine couldn't run any more perfectly. But soon I may tear into it and access that MAF and clean it. It's the part that the air cleaner housing mouth attaches to with its round mouth, once in there I think it is easy to remove, clean and put back. We will see !!! I don't have an SDS, all I have is a generic code reader I got as a Christmas gift! So I am guessing either MAF or possibly an O2 sensor?? I am glad my suspension seems to be in great shape. I once owned a 1996 W202 C220 that acted crazy, RPM all over the place, engine dying.... cleaned the MAF and it was Perfect after that. I still have the can of MAF cleaner that I used...
What kind of Benz do you have? I just bought a 2010 GL450 with 72K. I did my research and had an inspection done but, I'm a bit nervous.
 

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Congrats on your purchase.. That's very low miles. Mine is a 2008 GL450 with 144K miles, the car looks, runs and drives like new. I bought it on July 3rd.
 

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So I got new spark plugs, exact ones that came with the car and are specified to use in the owners manual, I got an new mass airflow meter, I got two new oxygen sensors, and reset the codes P0172 and P0175 meaning rich fuel mixture. Still the check engine light comes on and once in a while when waiting for a light to turn green, the engine will hiccup once in a while. It will Only do that when sititng at idle, with transmission in Drive. Car runs very well, accelerates like new, drives like new. Still wondering what the cause could be. Maybe a slightly plugged PCV valve? I could replace that, then replace the fuel filter, and go on and on.
 

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Discussion Starter #37
I finally took mine to the mechanic because it was the exact problem. He changed cylinder 5 spark plug and coil pack and voila! problem solved.
Before I took it to the mechanic, I changed the mass air flow sensor and the problem continued. When I scanned it, AUTEL showed cylinder 5 had a misfire.
Do you have a scanner? If not, see if you can find someone with a good scanner.
 

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Well having done all the other things, I got new ignition coils, and now they are on the engine. I figured it's a good investment anyhow, the car should run like new for a very long time. I will be VERY surprised now, if that light comes back on!!
 
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