Fixing the 2.0TFSI overboost problem

"VAG 2.0TFSI engine keeps overboosting (P0234) after remapping or tuning."

Before starting, I would like to clear up something. The Volkswagen Group made a little mess with their engines’ denominations. This article focuses on two engines: the EA113 2.0TFSI, launched in 2004, and the EA888, unveiled in 2008 by the German holding.

The former was used to power many cars of the Volkswagen Group such as the VW Golf V GTI, Jetta V, EOS, Passat B7, the Audi A3 and A4, and the SEAT Leon, among others. The latter will also be a subject of interest in this article.

The Audi S3 8P, TT Quattro, and the SEAT Cupra used the base of the EA113 engine but with a bigger turbo and some extras.

When the EA888 engine was released, Volkswagen changed the engine’s commercial name to TSI while Audi kept using the TFSI acronym. Some models, like the Audi S3 8P, took longer to adopt the EA888 engine.

A few years later, in 2012, VAG launched the EA888 Gen 3 engine and in 2016 the Gen3B engine was released.

Volkswagen keeps calling them TSI, and Audi TFSI. Gen3 engines are pretty different; that’s why I’m talking about this. If you have a Gen 3, you will find some useful information, but not everything applies to your engine.

Now let’s begin!

How the P0234 error code happens

Some users report that their cars enter limp mode after they have remapped their ECUs or modified their engines.

When they research a little more or they plug a diagnostic tool into their vehicles, they find out the dreaded P0234 error code (Manifold Pressure / Boost Sensor - overboost condition - upper limit reached).

Or even worse, their cars enter into limp mode under certain driving conditions, but the failure doesn’t last long enough to be saved into the ECU’s fault memory.

As you may know, overboost happens when the turbocharger pumps more pressure into the engine than expected. Modern engines like the TFSI can detect overboost and put the engine in limp mode (or emergency mode) to avoid being damaged and to protect components like the turbocharger.

How is overboost detected?

How TFSI engines detect overboost is pretty simple; the ECU collects information from different sensors and sets the boost pressure that the turbo should deliver (specified boost pressure).

What is limp home mode?

When the ECU enters into limp mode, the power loss is significant and noticeable.

A MAP (Manifold Absolute Pressure) sensor located in the turbo inlet pipe monitors the air pressure after the turbocharger, before it enters into the throttle body. Scan tools show this value as “actual boost pressure”.

The ECU will put the car in limp mode and it will detect an overboost condition if one of these things happens:

  • The MAP reads a boost pressure (actual) much higher than the pressure specified by the ECU. Boost pressure peaks don’t usually trigger the overboost condition unless the difference between the two values is too high.
  • The MAP reads a boost pressure (actual) higher than the ECU’s specified pressure several times during the same drive.
  • The ECU detects a mild level of overboost for long periods.

Troubleshooting an Overboost condition

Some Considerations

If you are experiencing this problem in your 2.0TFSI engine after you reflashed your car’s ECU or if you made some upgrades that increased your engine’s boost, there are some considerations to make before starting to make tests, and I would like to give you some advice from my professional experience as a car tuner.

  • It’s not a good idea to remap any vehicle’s ECU without making a thorough inspection of the engine and its components before doing it, especially when it comes to turbocharged engines. Adding boost and power to an engine can expose or worsen failures that were already there.
  • If you are going to make major modifications such as installing a big turbo, or a high flow manifold, for example, make sure to install all the necessary hardware to avoid wrong boost pressure readings.
  • Test, test, and test again. Use a good diagnostic tool like Ross Tech VCDS to read some live data. Do some logs, and keep testing.

I know that you want to fix the problem right away, and it’s an annoying problem, but keep calm and don’t start changing parts without a good reason.

If you have any doubts about this point, just read the previous point again. Doing a thorough diagnosis takes time, but it will prevent your spending money on unnecessary repairs and parts.

Main reasons that can cause a 2.0TFSI to overboost

  • Bad software: if the fault appeared just right after you had your car’s ECU reflashed, tell the tuner who did the job. Well-known tuning companies and serious tuning garages will help you to find out what’s wrong, and they may flash your ECU back to stock to check if the problem persists. 2.0TFSI ECUs have hundreds of maps, a well-done map requires more than just adjusting the boost pressure, the fuel trim, and modifying a few parameters. A poorly calibrated ECU can cause great damage to an engine. If the tuner is reluctant to review your car when you come back or refuses to help you, I wouldn't trust that software to begin with.
  • Bad ECU: is the last thing that you should worry about, but it can happen.
  • Faulty or cracked MAP wires: 2.0TFSI’s MAPs have 4 wires and are powered by the ECU. Check that the MAP is receiving from 4.5 and 5 Volts. Check the ground and also the signal wire.
  • Faulty MAP sensor: 2.0TFSIs use the MAP sensor (VW calls it G31) to know how much air is entering into the engine. A faulty MAP sensor may give the ECU bad readings, making it assume that there is an overboost condition.
  • Incorrect MAP sensor for your tuning: most 2.0TFSI’s MAP sensors can read between 2.5 and 3 BAR. The air flow that goes into a turbo engine is the atmospheric pressure (1 BAR) plus the boost pressure. For example; if your turbo is charging 23PSI (1.58 BAR), the total air pressure going to your engine will be of 2.58 BAR.A properly tuned ECU for a stock 2.0TFSI engine shouldn’t make the turbo charge more than 18 to 22 PSI (1.24-1.51 BAR) into the engine. Adding more boost without further modifications would force the turbo and the engine to run wide of those accepted parameters.If you are using an aggressive software map, have installed a bigger turbo that can hold more boost, or installed other upgrades that raised the turbo’s boost peaks, check your MAP’s capacity.

    Overloading your MAP will trigger an overboost warning. There are 4 BAR MAPs available for the 2.0TFSI engines that will allow you to get insane amounts of boost without problems.

  • Faulty N75 valve: the N75 is a solenoid valve that gives the ECU more control over the boost pressure. I will give you a rough idea about how it works:The N75 has three air lines; one receives boost pressure straight from the supercharger, another delivers air pressure to the turbo’s internal wastegate actuator (when the valve is open), and the third one is a return hose to the turbo intake.Depending on the boost pressure, RPMs, throttle position, and other parameters, the ECU sends power to open or close the solenoid, allowing or avoiding boost pressure to feed the turbo’s wastegate actuator. You may have heard about the N75 duty cycle or frequency. This is because the ECU opens and closes the N75 so fast that the signal can be read as a frequency. A faulty N75 can definitively make your engine overboost.
  • Stuck Wastegate or actuator: 2.0TFSI’s have a turbo with an internal wastegate. It’s connected to an actuator that is connected to the N75. The actuator or the internal wastegate can stuck preventing the air from going out to the exhaust. That will cause definitively overboost.
  • Wrongly adjusted Wastegate rod: the rod that connects the wastegate actuator to the internal wastegate has to be perfectly adjusted and in a specific position. If you took your turbocharger to be repaired, or if you installed a new one, no matter if it’s original or bigger, check that the rod is in the right place. Failing to do so could cause an underboost or an overboost problem.
  • Broken or leaking hoses: broken or cracked hoses and leaking hoses joints can cause bad MAP or make the N75 valve fail, generating false readings or real pressure build-ups that may lead to an overboost condition. For example, a bad wastegate actuator signal hose will undoubtedly cause an overboost.
  • MAF Housing size: this is a long shot but it can happen. If you are running a much bigger turbo than stock, keeping the standard MAF housing may be a mistake. It will cause your engine to run too lean, which will cause the turbo to spin faster. The same goes for some cold air intakes and custom pipings with restrictive airflow.

Measuring boost levels

Before starting to do some tests (you can refer to the list above to use as a checklist), it would be great to check if your engine is really overboosting. The best way to do it is using a scan tool.

Choose the right block and read the specified boost pressure, the actual boost pressure.

Take your car to a test drive and try it in different conditions. For safety reasons, you should bring someone who drives the car while you are doing this or the other way around.

The ECU reads the absolute pressure in millibars. In order to know the actual boost pressure in BARs, you should deduct around 1000 millibars from the readings, and convert them to PSI if you are more comfortable working in that unit.

Just check that there is really an overboost condition, or at least confirm that your ECU is detecting it.

A boost gauge can be handy to measure the real pressure that is reaching the intake manifold. There aren’t many places where to install a boost line in the 2.0TFSI engines, the best solution is an adaptor called “Boost Tap”.

It allows you to connect a boost gauge into the intake manifold with much effort. It’s cheap and easy to install. You can use a long boost line to allow the gauge to reach into the car’s interior while you test-drive the car.

Conclusion

This article is an introduction to the overboost condition in the 2.0TFSI engines. Some things to keep in mind: it’s better to treat the software, electric, and mechanical problems separately. Find your way to work methodically and rationally.

Don’t replace parts just because. If you tried something and it didn’t work, go back to square one, rescan the vehicle, and re-measure.

Test parts before replacing them, there is much information available about these engines and how to check their sensors.

Use information from reliable sources like trustworthy blogs, manuals like the Bentley, VW service bulletins, etc, and don’t believe everything you read in car forums.

If you follow these guidelines you will be just fine. I hope that after reading this guide you can find more where to start looking for the source of the problem.

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