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
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
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
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