What is the fuel stumble?

 

The fuel stumble is an issue that primarily plagues the 2008+ STi.  Subaru made some changes to the fuel pressure regulator setup that caused a strong stumble or hesitation in the 2700-3200 rpm range.  Subaru attempted to remedy this with some load based compensation tables but this is honestly an incorrect approach to fix the issue they created.  The fuel pressure regulator design itself is unique for the 2008 and newer STi.  It attempts to feed fuel to the rails in both directions which does not work well in practice.  The regulator also references the vacuum/boost directly off the cylinder four intake runner.  Cobb Tuning did some testing and proved this is also a big part of the issue.  Using one cylinder for reference leads to a fluctuating signal which can cause fluctuation fuel pressure.  As you change parts, injectors and fuel types on the 2008+ STi, its easy to notice the stumbles more and more and they can change rpm as well.  The best approach is a mechanical change to the fuel pressure regulator and reference line.  With this done, the tuning becomes more normal rather than having to use the load based compensation tables that should have little to no use anyway.

 

 

What do you need?

 

We offer a plug and play solution that we have tested and used extensively and found to almost completely remove all stumbles on most cars.  Some cars might still have a very slight stumble but we can generally tune this last bit out very easily.  Our kit comes with a nice compact fuel pressure regular as well as the gauge and barbed fittings for install.  How you choose to mount it is up to you but easy to handle and we can supply some install pictures for ideas.

 

Check it out here: Fuelab mini FPR kit

 

​

 

​