IAT Sensor Mod Experiment

[Flatland2D]

This experiment is for the IAT sensor mod in which the signal from the IAT sensor is bypassed with a fixed value resistor. The value of the resistor is chosen such that the ECU reads a temperature hotter than the IAT sensor is reporting.

The theory: Fuel mix is determined by a very large number of factors and is directly controlled by the length of time the fuel injectors stay open. Each factor is supplied to the ECU through all the engine's sensors. The ECU then takes each value to a lookup table which will return a constant multiplier for a given input. For example, if a lean condition is detected from the oxygen sensor, the multiplier will be greater than 1, resulting in a richer mix next cycle. The same thing goes for the intake air temperature. If the sensor is reading a hot temperature (hotter than normal) then the lookup table for the IAT sensor will report a multiplier less than 1. By tricking the ECU into thinking it's always reading a very hot temperature from the intake, the ECU can be forced to reduce the length of the pulse controlling the fuel injector by manipulating one of the many factors controlling fuel mix.

I used the same resistors SVOBoy showed. The two 220 Ohm resistors measured 107.8 Ohms in parallel. This should produce a reading of approximately 240 to 250 degrees F. The coolant temp gauge read the same temp as before the mod. It does not appear that the leaning effects of this mod has enough effect to overheat the engine.
<br>
<table border="5" cellpadding="5">
<tr>
<th colspan="2">
<b>10/5/05 - IAT Sensor Mod Trial #1</b>
</th>
</tr>
<tr>
<td>Fuel economy:</td>
<td>35.3 mpg</td>
</tr>
<tr>
<td>Previous fuel economy:</td>
<td>34.0 mpg</td>
</tr>
<tr>
<td>Percent difference:</td>
<td>+3.82%</td>
</tr>
<tr>
<td>Trip:</td>
<td>79.35 miles</td>
</tr>
<tr>
<td>Fuel used:</td>
<td>2.245 gallons</td>
</tr>
<tr>
<td>Fuel grade:</td>
<td>Shell V-Power 93 octane</td>
</tr>
<tr>
<td>Driving type:</td>
<td>Highway</td>
</tr>
<tr>
<td>Target speed:</td>
<td>65 mph</td>
</tr>
<tr>
<td>Tire pressure:</td>
<td>40 psi</td>
</tr>
<tr>
<td>Temperature:</td>
<td>79 degrees</td>
</tr>
</table>
<br>

The previous fuel economy used was from Trial #4 of the tire pressure experiment. I'm averaging the temperatures for the trip there and back to get 79 degrees, which was tonight's temperature.

I don't know if I'll do any more trials because I really want to get started on acetone. It is possible that Honda uses different resistences for temperatures than other manufacturers. I'll try and research this to see if another trial is worthwhile.

Everyone else conducting this experiment, please post your results.

[Matt Timion]

Can you post exactly how to make this so I can make one? Including part numbers or schematics if possible.

[Flatland2D]

Well it's pretty easy. Go to Radio Shack and buy a pack of 220 Ohm resistors for $0.99. Take two resistors, place them side by side, and twist the leads together. Refer to SVOBoy's picture for exactly what it should look like. Next find the IAT sensor for your engine. Mine was sitting right on top of the intake maifold in a black plastic box. There should be a two wire connector coming out of the sensor. Remove the connector and stick the ends of the resistors into the female plug. Wrap some electrical tape around everything and you're done.

Twisting the resistors together in parallel will make the total resistence lower than the individual resistences.

1 / TotalResistance = (1 / R1) + (1 / R2)

For R1 = R2, total resistance equals R1 / 2. In this case, two 220 Ohm resistors in parallel would yeild a net resistance of 110 Ohms.

[Matt Timion]

Very cool

This will be on my list of things to test when my acetone test is done.

I have a few questions, but I'll address them elsewhere and leave this for posting results.

[SVOboy]

My only concerns with this that I think are worth mentioning here is that I think flatland should've figured out the resistance crap for honda, because I'm almost sure that honda will use different ratings than saturn. When I get my hands on a multimeter tomorrow I hope then I will measure the resistance across my sensor at like 70 degrees, and then mehbe try to boil it and measure the resistance at like 200 degrees. I think that if flatland only got that much increase that the resistance wasn't changing it to 240 degrees, but mehbe like 120 degrees or something. I dunno though, that resistance needs to be figured out.

[Matt Timion]

My only concerns with this that I think are worth mentioning here is that I think flatland should've figured out the resistance crap for honda, because I'm almost sure that honda will use different ratings than saturn. When I get my hands on a multimeter tomorrow I hope then I will measure the resistance across my sensor at like 70 degrees, and then mehbe try to boil it and measure the resistance at like 200 degrees. I think that if flatland only got that much increase that the resistance wasn't changing it to 240 degrees, but mehbe like 120 degrees or something. I dunno though, that resistance needs to be figured out.

I'm fairly certain this information would be in the service manuals. Perhaps one of us can track one down and try to get the needed info.

[kickflipjr]

I have a saturn. So I will probally do this when i have the time.

[SVOboy]

I looked up the IAT part of the 88-91 manual and the closest it said was measure it and if it was from 1k-4k ohms it was still good. This seems way high, especially considering flatland got an improvement with 110 ohms. I guess it'll just take testing.

[Flatland2D]

I'm guessing the 1k to 4k thing is for checking when cold. That should cover temperatures from about 10 degrees F to 100 degrees F according to a chart I have (although I think it's for Toyota's). I'd imagine that the resistences used are pretty standard for these sensors.

I might go outside in a little bit and measure the sensor when cold, then get it up to operating temp and see what it gives.

[Matt Timion]

I'm guessing the 1k to 4k thing is for checking when cold. That should cover temperatures from about 10 degrees F to 100 degrees F according to a chart I have (although I think it's for Toyota's). I'd imagine that the resistences used are pretty standard for these sensors.

I might go outside in a little bit and measure the sensor when cold, then get it up to operating temp and see what it gives.

I'm guessing you could actually hook up the multimeter to your IAT and then drive around for a while and see what it says. Put the multimeter in your car and run the wires through the firewall to see what the resistance is.

But then again, finding a suitable shop manual might be better.

[SVOboy]

I put together my 110ohms and stuck them in the IAT, and will begin testing tomorrow, hopefully I'll get some good results, and throw up a picture of the stuff.

[SVOboy]

http://www.filefarmer.com/cruisur/DSC02565.jpg

This is what my set up looked like, smelled like hell, undid it, smell went away. Obviously it is no good for my damn engine, but as larry has done it on his, and flatland on his, it isn't bad for everyone's.

[Scorn]

The idea behind this mod is to trick the ECU into thinking that the air coming in is the perfect temperature so that it gives a better air/fuel ratio. Considering all cars are different, and all AIT senseors are different, the size resitor that you use on your car should be for your car only. The way to test and see what kind of resistor to use is to check the reading coming from the sensor with an ohm meter on a day when the temperature is nice and the engine seems to be running better because of it. Whatever reading you get is the size resistor to use, and you only need one.

[SVOboy]

Yep, that's true, but a lot of cars within the same make share resistors, for example, all obd0 and obd1 hondas use the same stuff, and the reason I used two resistors was because I ran them in parallel to halve the resistance of a single resistor, which gave me the proper resistance in the end.

[SVOboy]

Yep, so I got a different kind of tape, threw the resistors back in, no stink!!! So this test is back on, hopefully I'll find some good stuff out in the next few fill ups.

[Explorer]

I've been doing a little looking around on the net at some different websites that explain the theory behind using the resistors. I'm not sure if you guys are only trying this on the passanger cars or not, But in my findings I found that on my particular SUV (99' Ford Explorer XLT 4.0L) that the resistors that most people are using, uses a resistance value of 3,300 ohms to override the sensor and signal the computer that it is receiving air at an optimal temperature of 58 degrees. this info was found on an EBAY ad claiming to performace increases of up to 20HP. One question i have is first wouldn't this end up hurting your overall gas mileage conciderig that your letting the injectors stay open longer spraying longer? and second question is exactly where do these resistors plug into? Do they plug into the harness going to your MAF? I'm kind of new to the whole boosting Hp using resistor's only reason i'm pursuing this method of performance tuning is i'm not to ready to drop $800+ on a programmer for my truck yet. Any additional info would be appreciated. And thanks for having me in the forum.

[zpiloto]

I tried this IAT mod with no increase in FE. Tried it with IAT temp reading 240 and also 160 (according to the scan gauge) with no increase. I think the ECU changed the timing thus netting no result.

[SVOboy]

We're doing the resistors for fuel economy. As stated for some people it works and for some people it does not, depending on how the car works. Resistors are a ****ty way to get power, and will kill gas mileage if you try.

[Matt Timion]

I tried this IAT mod with no increase in FE. Tried it with IAT temp reading 240 and also 160 (according to the scan gauge) with no increase. I think the ECU changed the timing thus netting no result.

It looks to me like Casper is obd2, which most likely has variable timing. I believe this modification only works on obd1 and pre-obd cars.

[diamondlarry]

It seems to work on my OBD2 '97 Saturn. My air temp is reading 247 degrees F. As for timing, it seems to run as high as 39 degree btdc which is about where it was before the mod.

[davidjh72]

IAT mod rules! I've been averaging 34 MPG per tank previously. 41.1 MPG tank today with the IAT mod done midway through this tank of gas!

I unplugged the IAT sensor and put two 1/4-watt "470-ohm" resistors paralleled into the IAT connector. Scangauge reports IAT 198F. Car drives the same as before, though I keep RPM's below 3500 just to be safe. Ignition timing is the same so there doesn't appear to be any compensation of the timing by the ECU because of the high IAT temp reading. With the engine running leaner now (less consumption), I'm not going to put the engine under and undue strain. Shifting at 2500 RPM. Driving for enconomy!

I found that a 4700-ohm resistor sets IAT to 55F. Perfect for the 47F temp today. IAT is 5 to 10 degrees warmer than ambient temp at-speed anyway. Saves unnecessary disconnect/reconnect of the IAT sensor in A-B-A testing. One resistor for "normal", the other for "super MPG". I need to put in a switch so I can select low/high resistor. IAT 55F, MPG dropped to the high 30's. Set the IAT back to 198F with the 230 resistance and MPG jump to the high 40's, even 52 MPG.

I'll check the spark plugs after a few days to check for signs of "severe lean-burn"

Yesterday, Today, and Current Trip reporting with ScangaugeII...
http://s139.photobucket.com/albums/q299/davidjh72/car%20stuff/?action=view&current=031107-47F-9SW-N131-198F-IAT-52MPG.flv

[davidjh72]

3-15-07
So much for improving MPG with the IAT resistor mod. ScanGauge under-reported gas usage at 7.0 gallons, a whopping 1.465 gallon error. While SG showed 44.3 MPG, I actually got only 36.50. So I've put the IAT sensor back in place.

3-19-07
64 mile commute to work each way. 36 miles country roads, stopping at regularly spaced stop signs every 4 to 5 miles. 26 miles @60 MPH freeway. 65 MPH max. With IAT un-modded, I get the usual MPG and closer reporting of the ScanGaugeII to the actual gallons used. SG reported 9.6 gallons used (35.5 MPG). FE about the same as the last tank, IAT-modded. Proves that at least with my Saturn, tricking the ECU into thinking the engine is breathing hotter air that it actually is, only affects my SG to make me think I'm getting way better FE, until I gas up. SG honest with an un-modded car. Trick it out, and SG tricks the owner!

[repete86]

When I tried it in my Accord, it murdered my FE. FE dropped to almost 30 mpg at 55 on the highway, which is about what I used to be getting at 80 mph with the AC on!

[diamondlarry]

3-15-07
So much for improving MPG with the IAT resistor mod. ScanGauge under-reported gas usage at 7.0 gallons, a whopping 1.465 gallon error. While SG showed 44.3 MPG, I actually got only 36.50. So I've put the IAT sensor back in place.

3-19-07
64 mile commute to work each way. 36 miles country roads, stopping at regularly spaced stop signs every 4 to 5 miles. 26 miles @60 MPH freeway. 65 MPH max. With IAT un-modded, I get the usual MPG and closer reporting of the ScanGaugeII to the actual gallons used. SG reported 9.6 gallons used (35.5 MPG). FE about the same as the last tank, IAT-modded. Proves that at least with my Saturn, tricking the ECU into thinking the engine is breathing hotter air that it actually is, only affects my SG to make me think I'm getting way better FE, until I gas up. SG honest with an un-modded car. Trick it out, and SG tricks the owner!

I think that once the Team Challenge is over that I may return my IAT back to stock to test this on my car. I'm getting great mileage now and I would hate to jeapordize that.

[rockouthippie]

This isn't gonna work for most cars. If you have a O2 sensor, the computer probably will override your new mixture settings. This sounds a lot like an oxygen sensor mod I did on a Eagle Talon. It would go like raped batsh*t, but running that rich screwed up the cat converter. It got 12mpg city, but I didn't care.

That's the whole point of this system. To keep the cat converter from getting unburned gas through it. Clogs em up tighter than a drum. 2 cat converters in 3 years taught me the error of my ways.

I think if I was gonna try this, I'd make sure it had a switch for overriding it most of the time. This would give you control over your fuel mixture, if the O2 sensor doesn't override your trickery.

So, if I figure this right, you could either rich the engine out or lean it out with this trick. Being too lean will burn valves. Being too rich will clog the cat converter up. Neither is going to pass DEQ.

If you guys want to really beef up your fuel injected stock rods, you should use a bigger throttle body. It's pretty much "bolt on" 25 hp. I got an aftermarket one for my Lincoln for $200.

Kevin
retrovision.tv

[Bill in Houston]

I think I know what you are saying as far as the O2 sensor and closed loop mixture control. However, I think that the thought is that some ECUs will allow a leaner condition at some IATs than at others. For example, my ECU might say it's okay to run lean when the IAT is 80 deg, but not when it's 30 deg. So on a 30 deg day, the mod gets me to a lean-burn condition that I would not have otherwise been able to achieve.

[Jim Dunlop]

Why not just remove the IAT from the airbox/intake and move it closer to the engine?

[Lug_Nut]

The VW TDI diesels apparently respond to increased resistance (colder apparent temperatures) by advancing the initial injection. These are direct injection engines with the fuel injected into the combustion chamber at or very near to TDC, not duty cycle dependent injectors spritzing into the intake tract before the intake valve. This difference (injection timing vs. duty cycle) may be why the colder apparent reading in the TDI is desired for improved mpg, rather than warmer temps for gassers. This TDI version was developed from owners of converted TDI running on heated veg oil discovering a reduction in fuel economy related to retarded injection timing as the waste fat fuel heated up.
I'm still not entirely sold on the idea, although for the admission price of a pack of five resistors, it's a cheap experiment. I'll post if I think it's worthwhile after a suitable trial phase.
Details on the TDI "FAT" Fuel and Air Temperature mod, also known as "KFC" Kerma's Fuel Cooler can be found by using the search feature at www.TDIClub.com

[Snax]

This page (http://www.streetrays.com/catalog/product_info.php?products_id=2970) illustrates the output behavior of a wideband sensor, and that would be the place to start if you want to start hacking signals on an OBDII car. A simple adjustable voltage divider would allow tuning how the compter responds.

[cheapybob]

this is the same stuff I did a year ago. best economy i can get is with actual intake air temp about 160 (preheated intake) and resistance set for 242 degrees IAT. I had to use a switch because although these settings improved fuel economy by about 15% on the road, they make it hard to start when cold.

[lca13]

I been thinking about doing this... pretty easy to try... I have a 5K pot to use and would just run it into the cab and using an ohmmeter, set it initally to what it should be based on air temp, and then adjust "small amounts" after that.

But the O2 sensor has the final verdict... all the inputs are for the ECU to guess correctly then verify after the fact, so I am under no impression that I can simply move the AF curve left or right much with this simple mod.

Another idea.... how about an oscillator that applies a voltage or ground to the o2 circuit peridically, thus overriding the actual output... the frequency could then be increased to send increasing "rich" signals to the ECU... has anyone tried this?

[parasonic]

There is no need for an oscillator. The value coming from an O2 sensor is DC.

[lca13]

>>There is no need for an oscillator.
>>The value coming from an O2 sensor is DC.

No,, not was I was thinking.... a motor or oscillator that could close a circuit with a certain adjustable frequency to ground the output of the o2 periodically... something that would allow an increase in the fequency of the "rich" reading feedback to the ecu... something you could adjust from no change to a little to more... etc.

Wonder if something like that would work... just thinking about it.

[Snax]

There are piggyback fuel computers that can do just that. I believe most of them offer rpm and pressure dependent compensations.

[88HF]

I think the potentiometer idea has merit. If you had a pot that only controlled the range of acceptable values the IAT would send.

[kitcar]

I think the potentiometer idea has merit. If you had a pot that only controlled the range of acceptable values the IAT would send.

Taking the IAT out of the circuit completely and replacing it with a POT would work great! Even if the O2 sensors sneak the mixture back up, you'll still have an ignition timing advantage over the stock sensor.

I've been reading this tread with a lot of interest. I did the IAT deal on my '99 Dakota; it's an old trick for Dodge trucks and Jeeps for a few free horsepower and torque. The problem with most of these things is that the colder the air, the higher the resistance so when you add a POT or resistor to them they think it's colder, so they advance the ignition timing, thus you need to clip the wires and make a manual IAT or even just have a series of switches with different values. Turn them all off and you're at 0k and the vehicle thinks it's a million degrees.

I had a couple of scares when I had mine hooked up. In cold weather with the adjuster turned all the way down to 4k or so, the thing would stall from lack of timing and fuel - so care has to be taken when using them. First time it happened I thought I threw a rod.

This is the Dodge/Jeep/Chrysler baseline for the IAT; most should be similar.
Air temp........resistance
32 deg. 29,000 to 35,000 K
50 deg. 17,990 to 21,810 K
68 deg. 11,370 to 13,610 K
77 deg. 9,120 to 10,880 K
86 deg. 7,370 to 8,750 K
104deg. 4,900 to 5,750 K

[tjts1]

I'm reviving this old discussion after I put together my own IAT sensor pot and wiring harness.
http://farm3.static.flickr.com/2051/2475724126_f9eb3ee387.jpg
I haven't installed it yet but I'll update here as it progresses.
According to my BMW repair manual, the AIT sensor in this car outputs 2.5k ohm at 68f and 830 ohm at 122f. I'm using a 5k ohm pot.
http://farm4.static.flickr.com/3104/2451663089_638bf16fe5_o.jpg
The repair manual goes on to say that the ECU uses input from the IAT sensor to retard spark timing as air temperature increases. So in theory at least, colder IAT signal should increase both power and fuel economy... slightly. The beauty of using a pot instead of resistor is that I can check my instant fuel consumption at both extremes of the temperature range and everywhere in between simply by turning a knob.
I mapped out the pot for the 2 known data points. BMW claims that at operating temperature, the intake air temperature should be between 122-158f. Because of the high intake temperature, high compression ratio and lack of knock sensor, BMW requires minimum of 91 octane at all times. My goal of this project is to be able to run the car on 87 octane without compromising fuel economy or performance. The first step was to bodge together my own cold air intake through the inner high beam headlight. Its a sealed forward facing intake that takes advantage of the oncoming air for better throttle response and bit more power. According to some, this kind of intake can also improve fuel economy but I'm not quite sold on that front.
http://farm3.static.flickr.com/2322/2469911536_74e4d3300e.jpg?v=0
With just the intake, I already have it running on 87 octane without a hitch. Now we'll what happens when I take manual control of the AIT signal.

To be continued.
Justin