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post #21 of 30 (permalink) Old 08-12-2019, 02:13 PM Thread Starter
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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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post #22 of 30 (permalink) Old 08-12-2019, 05:54 PM
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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!

Last edited by tonylinc; 08-12-2019 at 05:57 PM.
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post #23 of 30 (permalink) Old 08-12-2019, 06:42 PM Thread Starter
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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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post #24 of 30 (permalink) Old 08-12-2019, 07:44 PM
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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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Last edited by tonylinc; 08-12-2019 at 07:47 PM.
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post #25 of 30 (permalink) Old 08-13-2019, 11:42 AM
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Originally Posted by tonylinc View Post
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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post #26 of 30 (permalink) Old 08-13-2019, 01:02 PM
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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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post #27 of 30 (permalink) Old Yesterday, 08:56 PM
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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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post #28 of 30 (permalink) Old Today, 03:13 AM
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Is there a sticky on here how to change front and rear diff fluids for 2008 gl450?
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post #29 of 30 (permalink) Old Today, 04:00 AM
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Pretty much a drain and fill.


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post #30 of 30 (permalink) Old Today, 06:14 AM Thread Starter
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Originally Posted by dkasht View Post
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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