A commom method of trying to eliminate the EGR System on modern engines is by tricking the ECM into thinking that the air outside (or the engine itself) is colder than it really is. Some EGR systems do not acitvate until the air temp is at 5 deg C.

     This section explains why the resistor on the Intake Air Temp (IAT) sensor wire is not a good idea on modern engines. The example used is the 4JJ1 Engine as found in Holden Rodeo & RC Colorado as well as the Isuzu D-Max.

KNOWNS :

The 4JJ1-TC (turbo charged) has an engine capacity of 2.99L (0.00299m3 or 2999000mm3).

Air makeup (Sea Level) : 78.09% Nitrogen 20.95% Oxygen & a few other small bits or Argon, Carbon Dioxide etc.

At Standard Atmospheric Pressure (Sea Level) the density of air :

   TEMP (C)            Density (kg/ m3)

                       0                            1.293

                      20                           1.204

                      30                           1.165

 

We will analyze this issue in grams of air as it is directly related thru Stoichiometry to the number of moles of the combustible element Oxygen.

So in 2.99L of air at Sea Level :

          TEMP (C)       Density (kg/ m3)               Grams of air

                 0                        1.293                                3.877

                20                       1.204                                3.611

                30                       1.165                                3.49

 

Since the air is only 20.95% Oxygen the table develops into :

           TEMP (C)         Density (kg/ m3)            Grams of air               Grams of Oxygen

                 0                           1.293                             3.877                             0.8122

                20                          1.204                             3.611                             0.756

                30                          1.165                             3.49                                0.731

If the engine thinks it’s getting 0 deg C air it thinks it is getting 0.81g of Oxygen to work with. But at 20 deg C the air only contains 0.75g of Oxygen. This is an ERROR of 7%.

As the air temp increases the error increases (30 deg C air only has 0.73g = 10% ERROR).

All this of course gets put thru the turbo where it compresses & the intercooler to cool it back down but the amount of oxygen that was originally sucked thru the air box (over the IAT) hasn’t changed. Lets assume you get 15psi of boost (that’s gauge pressure which is about 2atm of absolute pressure). So you have just packed in about twice the amount of Oxygen into that same volume.

 

The engine thinks its 0 deg C air coming in, 0.8122g O2 x 2 = 1.6244g, should be in there, but the air is 20 deg C & therefore only has 0.756g O2 x 2 = 1.512g. Still 7% ERROR.

 

Everything stated above is fact. It is based on numbers & principles that have been around since before engines & are used to design engines to this day.

 

Note this does not account for losses in the turbo system etc. That is why the engine manufacturer put a BARO sensor on the output of the intercooler. This reads the actual pressure coming from the intercooler just before it gets put into the throttle body.

 

The BARO reading gives the pressure achieved in the turbo charger system.

The Intake Air Temperature (IAT) gives the amount of Oxygen available.

The Mass Air Flow gives the volumetric flow of this air.

The ECM takes the BARO & IAT & MAF & calculates the amount of fuel for the most efficient combustion. If one of those #’s is skewed the combustion can not be as intended – this is called a misfire.

The worst part about a skewed IAT is that it is not only misfiring at idle, it is misfiring thru the entire rev range. An engine with a skewed IAT can not develop its full potential of power.

These engines have very complex Engine Control Modules to achieve the most efficient fuel consumption possible. Thousands of very smart engineers design every aspect of these engines, from the sump plug to the tiny angle of the cam shaft, every component on the engine serves a purpose. Tricking an essential sensor into sending false signals to the ECM can only result in negative effects.

 

DO you think ISUZU engineers would accept 10% error in combustion thru the entire rev range??