Intake Air Temperature (IAT) Mod


But First A Word From Our Sponsor....

History:  This information was posted numerous times, for numerous reasons.  It is posted here for edification and further justification.

(Note:  This is the cheapest and most beneficial Mod ever devised.  It's simplicity is incredible and if I knew who came up with the idea, I would erect a statue in honor of; and dedicate an entire web-page in homage to; said individual.  As I am unable to determine who came up with the idea, I offer the following in hopes that it was posted for public knowledge.)

What is it???

Theory:  The Intake Air Temperature (IAT) sensor (or Air Charge Temperature [ACT] sensor prior to 1992) is nothing more than an Inverted Thermistor.  It's output is Increasing Resistance for a Lower Temperature (voltage drops w/ temp).  As air temperature decreases, the risk of detonation (spark knock or pinging) also decreases.  Therefore, a more aggressive timing curve can be used without risk of detonation.  It also tells the computer that the inbound air is more dense and the ECU adds more fuel (Gasoline or E85) to the mix (A/F Ratio is enriched).

Air-Fuel Ratio Lesson:  The relationship between thermal efficiency, air-fuel ratio, and power is complex.  Without oversimplifying, "Stoichiometric Combustion" (air-fuel ratio = 14.7:1 for a typical non-oxygenated gasoline and 10:1 for E85) is neither maximum power - which occurs with a Rich mixture (12-13 parts Air to 1 part Gasoline, 8 or 9:1 E85), nor maximum thermal efficiency (AKA: Economy) - which occurs with a Lean mixture (16-18 parts Air to 1 part Gasoline, 12 or more for E85).  The air-fuel ratio is controlled at part throttle by a closed loop system using the oxygen sensor(s) in the exhaust.  Conventionally, enrichment for maximum power air-fuel ratio is used during full throttle operation to reduce knocking while providing better driveability.

Oxygenated Fuels:  To reduce "Green House Gases" and smog, the EPA has ordered some areas to use MtBE (Methyl-tertiary-Butyl-Ether) to oxygenate fuels during the winter months.  MtBE is recognized by the State of California as a carcinogen.  Up until September of 2000, you could find articles about this all over the Internet.   What happened?   Before I start on one of my rants, I'll just say that it creates a Lean mixture when burned.  If you are using it on warm days, it can cause detonation.

Note:  Detonation is serious, it is a result of excessive load on an engine that is being fed: insufficient octane; hot air; oxygenated fuels; or has too much initial timing (advance).  Left unchecked, detonation can lead to "Pre-Ignition". This is the point where the fuel "Auto-Ignites" (fuel burns on it's own, before spark is added.   This happens before TDC, creating EXTREME cylinder pressures) during compression and leads to serious damage...

O.K., What Do I Do With It??

Application: By adding resistance to the IAT sensor wiring (inline, like a fuse, on either wire), we can "lie" to the ECU and make it believe the incoming air is (much) colder than it actually is.

Now What? Well, it's experimentation time! If you have a '94 Ranger 3.0L and your ambient air temperature is over 95 degrees and you are using 87 octane fuel, start with a 5.6K ohm, 1/2 watt (due to the thicker leads than on the 1/4 watt) resistor, if it pings, reduce the value of the resistor until the pinging stops. That's it.
You have just maximized the computers timing curve FOR 87 OCTANE FUEL.

Just do the same for each fuel you use and keep a record. That lets you swap resistance to get the most out of whatever fuel you switch to.

1986 to '92 Aerostars and Rangers

Located in the upper intake (plenum) on drivers side.
Prime candidate for relocating!

Photo by author © 2000

1993 and '94 Rangers

Located on the lower half of the Air Box facing the engine
(MAF and Air Intake removed for clarity)

Photo by author © 2000

1998 and Later Ranger and B3000

It is the 2-wire sensor in the very bottom left corner.  
On K&N FIPK or KKM "True-Rev" installations, it is in the Intake Tube similar to above (If you have a .jpg picture of a KKM/FIPK install, send it to me please!)

This photo courtesy of A. Schessler © 2000, Thanks!

More Things To Ponder...

You can add a 2P3T switch (picture soon) to allow 3 distinct settings.  I used:
Up: 89 Octane = 10K
Cntr: Cheap Fuel or Heavy Load = No Resistor
Dn: 87 Octane = 5.6K

Still not enough for you?
Add a rotary switch for 5, 10 or more settings!!

Make it cumulative, 5.6K + 2K + ...

Move it!
(my latest configuration)
Cut the existing end off leaving ~ 4" of pigtail on the connector.  Remove the IAT from the airbox (Upper Intake Manifold/Intake Tube) and move it to the FRONT of the radiator support brace.  Plug the hole with a 1/2"NPT Plug.   Now wire it back together.  It isn't polarized, so it doesn't matter if you get the wires swapped.

and finally:

For racing with high octane fuel (94+), unplug the connector and bridge a 100K ohm resistor across the 2 terminals of the plug.  Secure the assembly with electrical tape.  This tells the ECU it is 32°f outside.  REMEMBER, the sensor is now bypassed and can't adjust for temperature changes.

5.0L Mustang owners are reporting .1 sec quicker and 1 mph faster in the quarter mile over racing fuel alone.   That's almost 5 HP and 12 ft/lbs. of torque!

**Reset the ECU before beginning for maximum benefit.
**Aftermarket chip users Beware: Your timing curves are already close to maximum.   Start with a 5K resistor and work up from there.   Always drop back 1K at the first hint of pinging!

For Those People Who Need Numbers...

Temps and Numbers:
At 104°f the sensor has 16.15K ohms resistance
At 86°f the sensor has 24.27K ohms resistance
At 68°f the sensor has 37.30K ohms resistance
At 50°f the sensor has 58.75K ohms resistance
At 32°f the sensor has almost 100K ohms resistance

Each 10K ohms resistance = ~20° cooler.

If, while performing this mod, the "Check Engine" light comes on, Check the sensor wires.   An "open" (broken wire, unhooked sensor plug, etc.) in the IAT loop will trip the light!

Codes are:
112=Intake Air Temp Sensor shorted to ground (too little resistance).   IAT reporting 245°f outside temp.
113=Intake Air Temp Sensor open (too much resistance).   IAT reporting -40°f outside temp.