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Discussion Starter #1 (Edited)
Hi all,
I more recently bought a 2018 GLA45 AMG and love the car so far. Unfortunately there is one weird quirky thing that it is doing that is driving me nuts. I was hoping someone here may have some knowledge. When the fuel tank gets down to one-quarter tank or so the peak torque displayed on the engine data will not go much above 310 ft-lbs. This is accompanied by slightly less pull from the engine, but the car otherwise runs well. I am doing these pulls in 4th gear at about 3/4-full throttle. There are no check engine lights or other noticable issues. Upon filling back up the tank and driving away from the gas station, within one or two pulls, the torque comes right back up to 354 ft-lbs and pegs the meter. It seems to stay that way again until the fuel tank gets between 1/2 and 1/4 a tank. My first instinct was bad gas and detonation with the car pulling timing, but I have now put 93 octane from 3 different gas stations into the car and the issue seems to be repeatable. I have no clue what's causing this, but it's definitely a little bizarre. Any thoughts? Thanks.

-Brett
 

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I never let my gas tank get below 50%, especially in Winter. Reason being that moisture in say a 1/4 tank is therefore that much more proportionately high. Low ambient temp condensation is one factor but more so is ethanol mixed into gasoline that naturally attracts moisture (I regularly use StaBil marine formula in my vehicles, boats and equipment for the mandated E10 in my area). I also only buy gas from "top tier" brands such as Mobil and Shell and 93 octane (since 91 not available). Lesser brands can often contain more water. In pandemic-reduced use of fuel stations condensate and ground intrusion moisture in their tanks has become even more of an issue.
 

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Discussion Starter #3
It could be an issue of moisture in the tank as it's getting low. I also have a boat and use enzyme fuel treatment. I can run the tank down low with low load driving, fill with fresh fuel and the treatment and see if that helps. I'm also wondering if there is anything in the ECU that resets when you fill the tank. You make a good point about low use of fuel during the pandemic and 93 octane is used even less during these times. Thanks for the advice. Anyone else have any thoughts?
 

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Beyond dirt and moisture most modern cars place the fuel pump in the tank and expect enough gas in that tank to help cool the pump. Pump failure is more likely to occur with a less full tank.

Combine all of these things and the computer may change the turbo boost curve.

Just for fun ... New gas may be cooler offering more energy.

It's best to fill early.
 

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Discussion Starter #5
Hi everyone. I wanted to follow up on this issue. Thank you to everyone for the advice. I am very convinced the issue was excessive water in the fuel which means the car was probably sitting for a while before I bought it. I ended up putting fuel stabilizer that I use for my boat (startron) into a full tank and running it through. One of the features of the fuel stabilizer is that it disperses the water throughout the fuel so that it can be safely burned through the engine. I noticed after filling the tank that the issue came back immediately which I believe was due to a dispersed concentration of water going through the engine vs the water concentration increasing as the tank level went down. I gently ran the tank all the way down until I had about a gallon left and filled with fresh fuel. I documented the issue with video footage to again show that the car went from making reduced torque to full torque immediately after the tank was filled. Since that time the car has run better than ever! The torque is now always pegged at 354 lbft and I can absolutely feel that the car is pulling harder. I am now through about two full thanks since that time and have had zero issues. Pretty crazy occurrence as I have never seen water cause this much issue with engine operation but I'm glad the issue seems to be gone. As always when you fix a problem another shows up. I took my car to the track and had some overheating issues with the 4matic, but thats a story I'll post in another thread. Thanks again to all.

-Brett
 

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Newer cars have more and more controls and sensors and over rides. The affect of computer compensation gets amplified in a small displacement highly boosted motor.

Glad you've found a resolution. Interested to hear about your 4matic.
 
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