What’s In The Loop

Defining the differences between open loop and
closed loop on today’s automobiles.

Before computer processors took over the modern
automobile carburetors were the norm.  A cold engine
was started by first depressing the gas pedal down
far enough to not only give the engine a little squirt
of fuel, but also to flip the choke butterfly shut. 
Closing the choke allowed the engine to pull more
fuel and less air, allowing for a rich fuel condition,
and would stay that way until the choke thermostat (or hand operated choke lever) was eventually moved to allow the choke to stay open for normal operation.  That was then, now computer software does this by controlling the injector pulses.

Open Loop
A computer controlled system will go into open loop at start up on a cold engine. In open loop the PCM will ignore the input signals from the oxygen sensor (O2) and calculate the air/fuel ratio based on inputs from the coolant, intake air temp, and the MAF sensors, but mostly from a pre-programmed table in its memory.  The PCM will hold this rich condition until it starts to see a varying voltage output signal from the O2‘s, and a coolant temperature of 100-105F, or just wait out its internal timer after the engine has started.  The specific time and or temperatures vary with different engine styles and manufacturers.  Some engines will automatically start in open loop even after the engine has been previously warmed up, but as soon as it sees the coolant temp value it will most likely go into closed loop before too long.

Closed Loop
The O2 sensor is the major player in closed loop.  This sensor reads the amount of oxygen still present after combustion. It tells the PCM whether or not the exhaust gas is rich or lean based on the oxygen content.  IF the mixture is lean, the PCM extends the injector pulse widths to make it rich.  It does this in small graduated steps (short fuel trim).  Not only to keep from adding too much fuel at any given point but to allow the O2 sensor to be able to respond to the PCM injector pulses.  Once the mixture changes to rich the O2 sends that signal back to the PCM and the PCM shortens the injector pulse widths to adjust the mixture again. This process repeats itself over and over again for as long as the engine is running. This feedback control is why it’s called “closed loop".  Everything the PCM needs to know about the condition of the fuel/air mixture is monitored by one of the sensors in its system.  No outer source or “opening" is responsible for the decision making, strictly a closed system.

One of the main reasons for a closed loop system is for the efficient control and operation of the 3 way catalytic converter.  The converter chemically changes the exhaust gasses from a high CO, HC, and NOx to a much more cleaner exhaust.  The old 2 way converter only cleaned up two of the harmful gases, CO and HC.  NOx (Oxides of Nitrogen) was not part of the original design.  When HC combined with NOx it formed smog, and as some scientist will tell you also brought on acid rain. The problem with this is that NOx is formed when combustion goes over a certian temperature, so to reduce the NOx manufacturers lowered the combustion in those engines which kept the temperature down as well.  But, lower the combustion and the temperature also meant less power and efficiency.  (Some of the cars from the 70‘s and early 80‘s are perfect examples of sluggish, low powered “emission” friendly cars.)  The 3 way catalytic converter brought back the ability to ramp up the compression and still be able to remove around 98% of the harmful emissions. These converters like to run at high temperatures and once hot it likes to see the exhaust gas composition cycle right around the stoichiometric value of 14.7 to 1.  The O2 sensors on todays cars have the ability to stay within a fine window on either side of stoichiometric and perform this function quite well.

Case study - Open Loop-Closed Loop
2004 Mazda RX 8 
Customer complaint - Runs great for about 8 to 10 minutes and then shuts off.  It will not restart right away.  You have to let it set for about 20 - 30 minutes, then it will restart and run the 8 - 10 minutes again.

Prior work performed - Front O2 sensor was replaced by the owner.

Codes - P0038 Rear O2 heater circuit high and P0130 Front O2 sensor circuit problem

Yes, there was a problem with the circuit alright.  All four of the leads to the front O2 were wired together into one large clump.  The owner had purchased a “one size fits all” O2 sensor and tried to wire it up himself.  The rear O2 had an open heater circuit. 
The first problem was to replace the faulty parts and see what happens.  Chances are it’s already done some damage to the PCM but one turn of the key will prove that.  Sure enough, the car started just fine and ran exactly 8 to 10 minutes as described.  So, what is going on?  Pretty simple at this point.  The car was doing it’s best to keep to it’s initial programming.  It was running in open loop and when the timer inside the computer ran its alloted time it would start to look for that return voltage signal from the O2 sensor... it could no longer read it.  That entire section of the computer was one large blackened  mess inside.  A new computer was in order along with the necessary programming.
Just as it was programmed to do, run in open loop until such time as it either seen an O2 response or adequate coolant temperature and then switch over to closed loop... but when it got there... there wasn’t anything for it to read.  And, since the coolant temperature had increased just enough for the PCM to go into closed loop if you tried to restart it immediately the PCM would see the temperature and try to start up using closed loop responses.  After letting the engine cool down just enough it would go back into open loop status and this whole process would start all over again. 
In this case a good understanding of the differences and what makes those differences between open and closed loop was a real asset in diagnosing this problem. 

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