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Today I expirimented with fooling my IAT sensor. I unplugged the wire from the sensor and tried putting various resistors in its place. My SGII reports an intake air temperature of 197F after I used a 220 ohm resistor. As a result the ECU pulled some timing and backed off on the fuel. It went from a idle/cruise of 0.4/0.5 gph to 0.3/0.4 gph. My around-town cruising mileage at night went from high 30's to low 40's.

I've had better success with this than with fooling my O2 sensor. A 150mv offset caused a few check engine lights, some hesitation and made the car anemic. So I pulled it soon after.

I'm going to keep an eye on the IAT mod to see if the ECU eventually wises up and cranks up the fuel. So far it hasn't. I'm pretty sure there are some nasty NOX compounds coming from the exhaust, but emissions aren't my primary concern for now. Maybe I'll increase the EGR rate to compensate later on.

DiamondLarry has has success with the resistor trick. It seems that Saturns like this mod. I tried it on the Del Sol via ECU reprogramming. Instead of fooling the sensor, I told the ECU to treat 50F temps the same way it would treat 212F temps. Less power and FE was no better.

DiamondLarry has has success with the resistor trick. It seems that Saturns like this mod.

That's true. I actually used 2-220 ohm resistors in tandem(110), any lower than that and I get a CEL. The SG reports a temp of 247 degrees. No noticeable change in timing. It's usual timing at a steady 55 mph cruise is around 35-38 degrees with an occasional high of 39. I too have noticed more gain from this than fooling the ECU about the O2 sensor.

If I was doing this, I'd use a resistor that gave the highest
temp reading you'd consider reasonable, and then put that in
series with a pot so I could fiddle with it on the fly.
.
I wonder how much mitigating influence even full EGR might have
on your car turning into a total NOXmonster, though..
.
_H*

I just finished a day of delivering pizza with the resistor mod. Also, it was the first day that temperatures dropped to the point where not running the AC would be practical. The engine stayed between 170-195 degrees the whole day so there was only one cold start. Deliveries ranged from urban to suburban and I ended up with 35.3 MPG. NOX or not, the resistor mod is here to stay.

what is the benifit of intake air temperature? is there an optimum range for fuel economy or does it differ for every enngine?

Supposedly warmer air is better because it is less dense. That way you can open the throttle further (reducing pumping losses) and still burn the same amount of fuel. Also when the air is less dense, the ECU injects less fuel.

Simply fooling the sensor is another thing. I just fooled my ECU into thinking that my intake temperature was 197F so it cut a bit of timing to prevent knock, and reduced the fuel injected. It seems to be working with no sacrifice in driveability. I probably will not be able to pass an emissions test though since NOX compounds are formed by running such a lean mixture. What a lot of people (especially hippes) don't realize is that fuel economy and emissions are sometimes at odds with each other. By defeating emissions controls, you can almost always squeeze a good amount of extra fuel economy from a motor.

I was surprized to find out that in Allegheny county PA, they only measure HC in the emmissions test, not NOx.

What makes you think you're running leaner? The O2 sensor will just cause it to run richer anyway, it simply takes a little longer for it to apply the long term fuel trim correction.

"As a result the ECU pulled some timing and backed off on the fuel. It went from a idle/cruise of 0.4/0.5 gph to 0.3/0.4 gph."

It's been days now and I'm still at 0.3gph at idle. I don't think it would take this long to correct. If it does correct somehow, I'll attack the problem from the O2 sensor angle. It's not too difficult to do.

BTW: Honda's lean-burn engines are illegal in CA and are possibly Honda's dirtiest engines due to NOX emissions (although very efficient). I'd still love to own one though.

I used to run an EFIE(O2 mod) but, since I started using the IAT mod(nearly 1& 1/2 years ago), I've found that my mileage is better without the EFIE. The problem is that I didn't think to test for this possibility until last year sometime.:(

BTW: Honda's lean-burn engines are illegal in CA and are possibly Honda's dirtiest engines due to NOX emissions (although very efficient). I'd still love to own one though.

I know that new cars can't be sold in CA with lean burn, but I find it hard to believe that if you move to CA from elsewhere that you aren't allowed to license your car. Are you sure that they are "illegal"?

It has to have something like 12,000 miles on it before you can register a car banned by CARB in CA. No new diesels for the last few years because of this. Of course they make an exception for large diesel pickups... Because those don't pollute nearly as much. :rolleyes:

7500 miles to be exact

BTW: Honda's lean-burn engines are illegal in CA and are possibly Honda's dirtiest engines due to NOX emissions (although very efficient). I'd still love to own one though.
I know for the 96-00 Civic, the HX was the lean burn model and had two changes from the standard Civic to keep the emissions in line. First, it had a close-coupled catalyst, built right on the end of the exhaust manifold, versus under the car on the standard Civic. Second, it was the only Civic with EGR (exhaust gas recirculation) which both together actually gave the Civic HX acceptable low emissions. They are also perfectly legal in California.

Hmm. The VX has both a close-coupled catalyst and an EGR, but Honda had to make a CA version in addition to the 49 state version.

What about the 00-05 HX? Were they offered in CA?

I used to run an EFIE(O2 mod) but, since I started using the IAT mod(nearly 1& 1/2 years ago), I've found that my mileage is better without the EFIE. The problem is that I didn't think to test for this possibility until last year sometime.

Has your car wised up to the mod yet? Mine hasn't. I don't understand why the car hasn't picked up on the consistently lean readings from the O2 sensor yet. Fuel consumption is definitely down, even at idle as reported by my SGII and I notice a decrease in power if I floor it. I just hope it doesn't get used to the resistor and runs super-rich once I pull it. What is the ECU doing with the O2 sensor readings if it hasn't picked up on running lean yet? It's been several days now, and I'm sure, longer for you.

I understand that much, but the readings from the O2 sensor should be equally erroneous after running consistently lean.

If my SGII reports that my fuel consumption at idle is 0.3 gph, is this information false? Before the mod, it ran at 0.4gph, so I assume that either it was running rich before the mod or it's running lean now. In either case, it makes me wonder what the ECU was doing with the O2 sensor readings. If the ECU is basing its fuel injection on the IAT, exactly what is it doing with the O2 readings if it hasn't adjusted the fuel rate yet? I'd assume that after a few minutes (or even days) of watching the O2 voltage being consistently high, it would crank up the injector duty cycles and return the idle fuel rate to 0.4gph but it has not done so yet.

It may be that it's just pulling timing and not burning as much fuel. I think it can do that w/o running lean.

Does the normal advance in timing result in more efficient operation than pulling timing and using less fuel at idle? Which seems to be what the IAT resistor mod does. I think idle is when the engine is most inefficient because of pumping losses, so reducing fuel consumption here may be the best mod for mpg.

Is it really that rare? Considering engine efficiency has done nothing but increase over the last century, and these increases in efficiency have been used to make more power, but keep efficiency the same, is it really that strange that shade tree changes in operation can increase efficiency. A model T pulled 25-30mpg, and over a ~century, we haven't improved on this in our fleet average? Manufacturers maximize power, conform to emissions regulations, and keep efficiency constant. I know this is rantish, but it's what has happened. If manufacturers balanced power, emissions, and FE we'd have gasoline vehicles that had ~60hp and got ~60mpg during the combined cycle that could do exactly what the vehicles we have do now. Instead they focus on power and we have a ~20+mpg fleet average.

AZBrandon, thats what he said. FE stays the same (or gets less), but cars are more powerful and cleaner.

edit- way too OT, pm'd a resp. to the wrong person too! :o

Car manufacturers have to work within certain bounds when building a vehicle. We can break out of these constraints. You can unlock plenty of mpg points at the cost of emissions, smoothness or power.

That's true. I actually used 2-220 ohm resistors in tandem(110), any lower than that and I get a CEL. The SG reports a temp of 247 degrees. No noticeable change in timing. It's usual timing at a steady 55 mph cruise is around 35-38 degrees with an occasional high of 39. I too have noticed more gain from this than fooling the ECU about the O2 sensor.

I have a guy on my Saturn forum that is using a single 150 ohm resistor to replace the IAT sensor and is using a hot air intake where he pulls the air from the cat area through an insullated tube. Do you think that this would cause our 1.9 DOHC to burn too lean? I have also been thinking about swithing to an MSD ignition system to help burn the fuel more efficiently? Your input?

I don't own a saturn, but from what I've read and my knowledge on how the iat works, the hot air intake wont let him run lean, the resistor may however.

I know I get a nice bump in my mileage by just pulling air off the O2 sensor hole in the manifold (I also have a partial radiator block). ~10% increase in mileage. On a hot day my IAT is about 130 degrees. And it keeps my engine temp down at about 190, which with just the radiator block would be around 210. On the 2000 Altima, the WAI definitely pulls timing.

I did the IAT mod today and when I started it, it ran a little rough until the engine warmed up a bit. Do you guys have that problem?

I also moved the air filter to roughly 4" above the header and 2" infront of the block by removing my WAI and putting the factory duct in (more flexible). The factory airbox was hacked up by the previous owner (he/she made a ghetto WAI) so I cannot reuse it and ducted the hot air into it. So right now I just hope that my filter doen't get too hot! After I find a good placement, then I will have a custom HAI made.

I would think that tricking the computer into thinking the air is cooler would advance spark timing and help fuel economy. Fooling it higher may make it think there is less Mass Air(hot air less density), but once in closed loop the o2 sensor would negate those effects and maintain 14.67AFR. This is just my opinion though.

I use a 3.3K ohm resistor and my AIT went to 70 degrees on my Scan Gauge 2. Colder air is denser or richer. The car thinks the mix is richer and tells it to use less fuel to balance out the mixture. My air to fuel ratio stayed at 14.7 before and after and my GPH idle the same, but at 55MPH from 34.25MPG to 36.25MPG, 5.9% better MPGs after already having a cold air intake. I noticed no changes in power and no engine problems, so its an awesome trick for $1.