Back in our January 2005 issue (see
Altering Closed Loop Mixtures) we covered how to modify
the air/fuel ratio of a car that’s always in closed loop. (‘Closed loop’ means
that the output of the oxygen sensor effectively controls the mixtures.) In that
case, we used the pictured Simple Voltage Switch kit to disconnect the oxygen
sensors once the load exceeded a preset threshold. The resulting mixtures were
then tuned with the Digital Fuel Adjuster kit.
But what if you’re dealing with a car that always runs in closed loop but
needs lots more fuel flow – for
example, you’ve turbo’d the engine? In that case, just disconnecting the oxy
sensors is unlikely to give you the result you want – you need a heap more fuel
flowing through those injectors. After many weeks of experimentation on a car
that has super intelligence in its engine control system (it’s a hybrid Toyota
Prius) and needed a lot more fuel at full load (it’s been turbocharged), we’ve
come up with an approach which we’re pretty confident will work on any car that
normally always stays in closed loop.
How Cars Work in Closed Loop
‘Closed loop’ refers to the ECU following a certain control approach. In
closed loop it allows the output of the oxy sensor(s) to correct the pulse
widths with which it is triggering the injectors. Most oxy sensors are narrow
band; that is, they are not designed to be able to
accurately measure air/fuel ratios of say, 12.5:1. The output of the sensor is
high when the AFR is richer than 14.7:1, and low when the AFR is leaner than
14.7:1. (For more on oxygen sensors, see
The Technology of Oxygen Sensors.)
When the output of the oxy sensor is high (eg 800 millivolts) the ECU leans
out the mixtures. When the output of the oxy sensor is low (eg 200 millivolts)
the ECU richens the mixture. The result is that in closed loop a car has
mixtures (and so an oxy sensor output) that rapidly rises and falls, a bit like
a sine wave. The average AFR, however, hovers around stoichiometric, the
chemically correct ratio of air to fuel that ensures best combustion. (And also
allows the cat converter to work most efficiently.)
The best way to see if your car works in closed loop all of the time, or only
at lower loads, is to fit a Mixture Meter – see
Smart Mixture Meter, Part 1. The dancing LEDs will soon
show you the ECU strategy – if they continually oscillate back and forth, the
car is in closed loop. If the LED remains steady, open loop is being used. (The
illuminated LED may slowly move, but once a constant load is reached, it will
For more on closed and open loop, see the breakout box at the end of this
Yes, we know that’s an awful lot of electronic kits mentioned in the first
five paras. However, they really are the cheapest and best way of achieving
Modifying Closed Loop Mixtures
A car that is always in closed loop cannot effectively have its mixtures
(Well, that’s not quite true. If it’s a car that uses a wide-band oxygen
sensor, and if the modifier is able to access the ‘target air/fuel ratio’ part
of the ECU software maps, the air/fuel ratio can be modified very well. And some
very experienced software re-writers can do just that – but for most of us,
that’s not a viable approach. Especially if we’re dealing with a one-off
By why can’t the closed loop car have its mixtures modified? Basically,
because the car will immediately ‘learn around’ any changes that you make.
Increase the fuel pressure and the car will run richer - but only until the ECU
learns from the oxy sensor(s) that there’s too much fuel going into the engine.
Reduce the output of the airflow meter and the car will run richer - but only
for as long as it takes the ECU to make a correction. (And in some cars, that
can be just seconds!).
So when in closed loop (ie the ECU is
using the input of the oxy sensor to fine-tune mixtures), the mixtures must
always remain as the ECU expects them – stoichiometric or about 14.7:1.
The above statement looks pretty obvious – but it isn’t. Take for example the
fitting of a rising rate fuel pressure regulator on a car that’s been turbo’d.
If you allow the fuel pressure to rise while the car is operating normally in
closed loop, the injector pulse widths will be pulled back. Why? Because the ECU
will see that the engine is running rich (because of the higher fuel pressure)
and so will trim the mixtures leaner. And as part of its short-term fuel trim
learning, all the injector pulse
widths will then be leaned out. (After all, the ECU figures that the fuel
pressure reg has just gone mad!)
Don’t see the point? OK, let’s take an example. Say you’re working with a car
that runs in closed loop at all revs and loads and a mixture check shows that
yes, the air/fuel ratio is at 14.7:1 all the time. You’ve just turbo’d this car
and desperately want mixtures much richer than 14.7: when on full boost. Say,
12.5:1 would be nice...
As part of the upgrade, you’ve fitted a rising rate fuel pressure reg that
increases fuel pressure disproportionately with boost pressure. (So, as boost
pressure rises by 7 psi, fuel pressure might rise by 20 psi – see The Vortech Fuel Management Unit for more on this topic.)
This increase in fuel pressure will force more fuel through the standard
injectors still being operated at their standard pulse widths, so enriching the
air/fuel ratio. You intend switching out the oxy sensors to force the ECU out of
closed loop whenever the load is high – and have set that switch threshold at 4
psi boost. (If you switch out of closed loop as soon as there’s any boost
pressure at all, you’ll dramatically drop in fuel economy – a well-matched turbo
will develop positive boost pressure at quite small throttle openings.)
So now we have the situation where even in closed loop, the fuel pressure is
rising. To take an example, say you’re driving on the flat then reach a
gradually steepening hill. Initially, the car is in closed loop, there’s no
turbo boost and everything is fine. Then you gradually start climbing the hill
and the boost pressure gauge shows 1 or 2 psi boost. In response to this, the
rising rate fuel pressure reg starts to increase fuel pressure – and so also as
a result of this, the ECU reduces pulse with of the injectors, to keep the
mixtures at 14.7:1 even at the heightened fuel pressure. The short term ECU fuel
trim then learns that the system is over-fuelling, and pulls back all injector
The hill steepens and you reach the point at which you switch out the oxy
sensors. But the ECU, which has been learning for the past few seconds that the
car is being over-fuelled, is now faced with a dilemma. It’s lost its feedback
loop from the oxy sensors, but last thing it knew, the mixtures were too rich.
It then decides that its ‘forced open loop‘ mixtures should be leaner than
Aaaagh. Your increased fuel pressure – even in forced open loop – has resulted
in mixtures that are now leaner than they were previously, and much leaner than
they would be if you went straight from no throttle to full throttle!
As we said above, the trick is that when in closed loop, the mixtures must always remain as the ECU
expects them – stoichiometric or about 14.7:1.
OK, so how do we keep the ECU seeing what it would normally see (and so
staying happy in fuel trims), and then suddenly change the fuel flow at just the
same time as we switch the oxy sensors out? The trick is to use two
independently adjustable fuel pressures.
Modifying Oxy Sensor Signals
But why all the hassle? Why not just modify the output of the oxygen
sensor(s) to indicate to the ECU that the car is running leaner than it really
is? Voila! One richer mixture...
Trouble is, modifying the sensor outputs is very difficult, even with a
programmable interceptor like the Digital Fuel Adjuster kit that can be
configured to work with oxy sensor voltages. Basically, because the sensor’s
chemistry makes it switch from high to low (and low to high) as the mixtures
pass through stoichiometric, altering the output signal doesn’t change the
switching point of the sensor. In practice, narrow band sensor signal
modification either results in no change to the mixtures, or the ECU thinks the
oxy sensor is stuffed and ignores it.
Wide band sensors? That’s another story.
Two Fuel Pressure Systems
Take this scenario. It’s much the same as the one above but with one critical
You’re driving on the flat then reach a gradually steepening hill. Initially,
the car is in closed loop, there’s no turbo boost and everything is fine. Then
you gradually start climbing the hill and the boost pressure gauge shows 1 or 2
psi boost. However, fuel pressure remains absolutely stock and the oxy sensor
closed loop system continues to work normally. In response to the increased
airflow, the ECU increases fuel injector pulse widths so as to keep the air/fuel
Then – WHAMMO!!
In one fell swoop you switch out the oxy sensor(s) and increase the fuel
pressure! The ECU doesn’t know anything about the increased fuel pressure and so
fires the injectors at normal (or even a little above normal) pulse widths. But
with your suddenly increased fuel pressure, you can squeeze a heap more fuel
through the standard injector opening times.
You have enriched mixtures (the amount of enrichment being best set by
adjusting the high load fuel pressure) and the ECU doesn’t know a thing about
it... and so cannot learn around
So how do you organise a fuel system that can dramatically switch from one
(regulated) fuel pressure to a much higher (also regulated) fuel pressure? We’ll
show you how next week...
More on Closed Loop
'Closed Loop' is the term given to the ECU behaviour when the oxygen sensor
signal is being used to largely control how much fuel the injectors are adding
to the intake. In most cars the ECU works in closed loop most of the time - when
the car is warmed up and idling, in constant throttle cruise - and so on. With
this control strategy the ECU watches the oxygen sensor output and if the
mixtures are getting a bit rich, it leans them off. If the mixtures are getting
a bit lean, it richens them up. This causes the mixtures to fluctuate rapidly
around 14.7:1 air/fuel ratio - what's called the stoichiometric ratio.
However, an air/fuel of 14.7 doesn't give the best power, so when you put
your foot down, in most cars the ECU forgets all about closed loop and goes
instead into an operating system called 'Open Loop'. This just means that it
ignores the output of the oxy sensor, instead picking the right amounts of fuel
from its internal memories. Typically, with increasing load, the air/fuel ratio
outside of closed loop might jump to 13:1, then 12:1 and then even richer still
at 10 or 11:1.
The final common operating approach is when the injectors are completely
stopped - yes, they're actually switched off sometimes even when the car is
driving along! This happens on the over-run - you're travelling along at 100
km/h, reach an 80 km/h sign and lift your foot. The ECU will then turn off the
injectors until either you reapply the accelerator or the engine speed drops to
near idle revs.
Like all things, these ideas apply to most cars - not all. Some Porsches, for
example, stay in closed loop all the time - even when the mixtures are richer
than 14.7:1. In other words, the oxygen sensor (a special one) is used to give
mixture feedback to the ECU in all operating conditions. Other cars have a 'lean
cruise' system, where on the open highway the mixtures will gradually lean out
to say 15 or 16:1, so saving fuel.
And as we have discussed in this story, some cars use constant closed loop to
keep their mixtures always at stoichiometric (except on the over-run). For
example, the Chrysler Turbo Prowler is a car that maintains closed loop 14.7:1
mixtures all of the time. (For more on this engine – Designing a Factory Turbo Engine).
Check Engine Lights?
In most cases switching the oxygen sensor(s) out will result in the setting
of a fault code in the ECU. Whether this lights the dashboard Check Engine light
depends on how often it occurs and the individual ECU fault coding.
Unfortunately, we don’t know of a way around this – simulating an oxy sensor
signal has proved to be unsuccessful in preventing the ECU knowing that it is
has lost the input of the real sensor.