With many of the MQB based vehicles, VW Auto Group adopted the standard that so many Japanese cars used for decades. There are better versions available worldwide than in the US. In this case, some cars had multi-port injection, but in this specific case, some had Flex Fuel. We’ve gone ahead and added that feature to the US cars so you can enjoy the benefits of ethanol in your car without needing to move overseas.
Enabling Flex Fuel
To enable flex fuel on your map will take touching (or double-checking) a few different tables, but is a simple 1 or 0 effort.
Flex Fuel Sensor Enable needs to be set to 0 to allow the sensor to read ethanol.
Master Switch (Flex Fuel) changed to 1 in order to enable flex fuel calculations
Verify Manual Ethanol Content is set to 0 and the Manual Ethanol Content Switch is disabled (0). Either of these can interfere with the system calculating.
The Factory Fuel System
Utilizing existing pins in the ECU, the ethanol content sensor sends an update of the fuel’s chemistry from the feed line to the high pressure fuel pump, straight to the ECU. Because this is a returnless fuel system stock, it’s important to remember that there is always going to be some amount of fuel between the sensor and the engine. While not an issue once the ethanol has mixed into your tank of fuel, as the fuel goes through the process of mixing and evening out, you may have a fairly different ethanol content going through the engine than what is being read at the sensor. Most of the time if you’re targeting a similar ethanol content with the fuel being added to the car, this won’t be much of an issue. However if you’re going from 0% ethanol to 85%, it may be a good idea to give the ethanol content some time to stabilize.
Fuelling Adjustments
Due to a change in it’s ideal air to fuel ratio, Ethanol requires an increase of ~30% fuel, however compensation for that extra volume and compensation for startup in cold weather is built into background calculations in the system, so the only changes a basic car will need (assuming you’re starting from a good pump gas tune) is if you want to adjust the fuel ratio to make additional power.
Flex Fuel AFR Addition Table
This table allows you to adjust the target air fuel ratio for the vehicle when using flex fuel. The changes are made in AFR and only positive changes are allowed (which lean the mixture out). Typically you’re able to gain additional power on ethanol by leaning the air fuel ratio out slightly so this allows you to do that based on airflow and RPM.
This table serves as a base which is adjusted by the Flex Fuel AFR Ethanol Blend Percentage table. This allows you to use a different percentage of the AFR addition table depending on the Ethanol content. In most cases you’d want to use a higher percentage of the adjustment as ethanol increases.
Flex Fuel Torque Adder
The MK7 and MK7.5 vehicles have engine management systems that are based around torque requests and torque output. This means that in an ideal (and simplified) world the driver “asks” for torque with the accelerator pedal and the engine does what is required to achieve that output in terms of boost and ignition timing. However on the flip side, unless you ask for more torque,
Operating with a different value based on RPM and Gear, you can add torque to different portions of the range and speeds. This table gets multipled against two different tables
With Flex Fuel Torque Ethanol Blend Percentage you can adjust the torque based on ethanol content similar to the AFR adder and Ignition adder have.
To get the final torque adder you’ll have both the Ethanol content and throttle position blend percentages multiplied against the torque adder.
Torque Adder Table x Ethanol Blend Percentage x Throttle Blend Percentage = Final Torque Adder
Boost Control and Flex Fuel
Ignition Adjustments
One of the benefits of ethanol is a higher knock resistance (referenced as the octane in the US) compared to gasoline. Pump gas his fairly easy to understand and has options from 87-93 commonly available depending on where you live. Ethanol has an effective octane rating of ~107, making it a pretty good proposition for many people (especially given the prices of high octane race gas). This means in order to optimize you’ll likely want to adjust the ignition timing as ethanol increases.
The Flex Fuel Ignition Adder table establishes an amount to add to the ignition timing of the vehicle based on RPM and airflow. It has a single blending table that blends in a percentage of the ignition adder based on ethanol content.
Misc Flex Fuel Tables and Toggles
Enable Ethanol Sensor Diagnostics is a toggle found under the Advanced Engine Parameters heading in the software. This toggle allows you to enable the DTC options for the ethanol content sensor, assisting with troubleshooting during inital setup, or will help the driver be more aware of hardware failures ahead of time. The DTCs that are enabled can be found here: COBB MQB MK7/7.5 Flex Fuel DTC List
Maximum Ethanol Content (DTC) sets a limit. Upon exceeding this limit of ethanol content you will get a check engine light on the dash as well as the identifier _ _ _ _ . This will only be enabled if you check the toggle for Enable Ethanol Sensor Diagnostics. Alternatively setting a value of 101% would prevent this from being set in the event that you don’t want a DTC for ethanol limit but want the diagnostics enabled for the other benefits it offers.
Threshold to Update MFF Value sets an amount by which the ethanol content needs to change before the ethanol content the system is using will update. For example, if you set the threshold to 2, and the ethanol content changes from 30% to 32% the system will continue using 30% ethanol. However as soon as it breaches that 2% threshold (let’s say 32.1%) it will now change the calculations to be based off of 32.1%.
Flex Fuel Ethanol Hold Content Boost
Once boost pressure exceeds this limit (or the hold content for RPM) the ECU will stop updating the ethanol content and utilize the last read value for all of it’s calculations. This can be helpful to avoid the computer making rapid swings in compensation for ethanol content which can happen under many circumstances (if the fuel in your tank isn’t mixed well, is contaminated or poor quality).
Flex Fuel Ethanol Hold Content RPM
Once engine RPM exceeds this limit (or the hold content for Boost) the ECU will stop updating the ethanol content and utilize the last read value for all of it’s calculations. This can be helpful to avoid the computer making rapid swings in compensation for ethanol content which can happen under many circumstances (if the fuel in your tank isn’t mixed well, is contaminated or poor quality).
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