Q:  What is the 3,800 RPM hesitation problem?  How do I fix it?

Many FC's are affected by this hesitation problem.  It could be as minor as a slight hiccup under acceleration to a violent bucking right under 4,000 RPM.  Many times, the hesitation problem is not consistent, which makes troubleshooting very hard.

To understand what causes the problem, we need to understand some basic FC engine control systems.  At the root of the problem is the fuel injectors.  The FC uses four fuel injectors; two are primary fuel injectors, while the other two are secondary fuel injectors.  Under most driving conditions, only the primary fuel injectors (one for the front rotor, the other for the rear rotor) are firing and delivering fuel to the engine; the primary fuel injectors fire alternately into the engine - this is called "sequential" mode.  The secondary fuel injectors are silent until the engine revs over approximately 3,800 RPMs (+/- 300 RPM depending on load).  At this critical time, the ECU switches over from just firing the pair of primary fuel injectors to firing both the primary fuel injectors and secondary fuel injectors; all four fuel injectors are now firing in "batch" mode.  It is this important fuel injector crossover that's the heart of the 3,800 RPM hesitation problem.

Now that we know a little more on what's happening, let's look at potential causes of the hesitation.  Seeing that the secondary fuel injectors are not operating all the time, it's potentially possible that non-use would cause them to stick or jam with deposits.  Daily drivers that do not regularly rev over 4,000 RPM occasionally could cause this condition.  This is one of the reasons why we recommend to redline the engine regularly!  Regularly revving the engine beyond 4,000 RPM would force the engine to use the secondary fuel injectors all the time.  If the problem of sticking secondary fuel injectors is the minor problem, using a good fuel injector cleaner will help.  We recommend using Redline fuel injector cleaner.  If Redline products are not readily available, people have had good results using Chevron Techron in the plastic bottle, as these can be bought from any Chevron gas station.  Read the directions and run through at least two full tanks of gas.  Another good fuel injectors cleaner is the BG brand of chemicals.  This fuel injectors cleaner is not easily available to the general public, but your local full service station might carry it.  If these fuel injectors cleaners don't work, you might need to pull the fuel injectors and get them professionally cleaned.
Click here for info on professional fuel injector cleaning!

Before you go worrying about pulling your fuel injectors, there's a good chance that the problem is a bad ground.  In fact, 90% of the time, fixing or adding ground wires will minimize or eliminate the hesitation!  We know that the fuel injectors crossover is the heart of the problem, so how does that related to bad grounding?  Due to the age and mileage of the vehicles, corrosion and wear&tear has taken it's toll on most of the chassis still running around.  When fuel injector crossover occurs, the amount of current required to fire the fuel injectors doubles!  The ECU actually fires a "pull down" signal; a "pull down" signal is a ground or negative voltage signal.  The fuel injectors are connected to the positive +12VDC side at all times when the engine is running.  Therefore, current is needed on the ECU side of the signal.  By making sure the ECU grounds are clean and secure, we keep current flow at it's most efficient.  Would it surprise you that the ECU main ground is secured on top of the engine?  Yes, it's true.  The main ECU ground snakes through a pair of black wires to a single round connected secured on the top of the engine.  This is not the most easiest area to access, as a 13BT engine requires the intercooler and upper intake manifold to be removed.  Most of the time, the ground lug is not the problems itself but the engine ground is.  The ECU ground on the top of the engine has to make it back to the battery; the engine must be grounded securely for this to work efficiently.  Adding several engine grounds from the engine to secure points to the chassis has worked for a lot of people!  This is a very easy thing to do for most people.  Make sure that the extra ground points on the chassis are to bare metal, so you'll need to scrape off any paint that is under the connector.  A more involved ground modification is to find all the ground wires (there are several) at the ECU and add splices to those ground wires and secure the new ground wire to points nearby.
Click here for more info on grounds!

There is a TSB on this hesitation problem by Mazda by adding an extra ground wire to the boost sensor.  This ground boost sensor problem is supposedly fixed on cars built after November '86; VIN should be after ...H0522332.

Questions?  Comments?  Send mail to:  reted@fc3spro.com