Originally Posted by BobC92

Well, I read several posts by a number of people in various import car and "tuner" forums who vigorously maintain that modifying the IAT is pure snake oil. In one forum it was even listed in a thread called Crap that Doesn't Work - right along with fuel line magnets and the "tornado" intake inserts. SO, I read up on it some more and really got my head around exactly how the IAT mod works. I also read a few articles on aftermarket "piggyback" performance chips, and replacement programmable ECUs. From this I determined that modifying the signal from the IAT is one of the tricks that both these devices employ to enhance performance. Based on my research, I decided to go ahead with giving it a try.

Reading the IAT diagnostic procedure in my Chilton's manual told me that the IAT's resistance at an ambient air temperature of 32 degrees should be around 6000 ohms, and at 68 degrees it should be around 2700 ohms. The wiring diagram showed me which pins on the combined MAF/IAT to measure to check it, and sure enough, the resistances were right about what it said they should be. My wife thought I was nuts when she saw me putting the airbox in the freezer to get it down to 32 degrees for the test! So, at this point I had all the info I needed to proceed.

Rather than pay some eBayer $5-$40 for a five-cent resistor in the 3k-5k range and a set of instructions I decided to do it a little differently and make mine adjustable. I bought a 5k linear potentiometer, 5 foot of 18 guage speaker wire, and a 5-pack of snap together male/female "bullet" connectors at Radio Shack - all for about 6 bucks total.

I cut the wire to the IAT (red & black wire - pin 6 on the 7 pin connector to the airbox MAF sensor) and installed a male snap connector on one end and a female snap connector on the other end. I figured that, if it didn't work, I could just plug the two ends of the wire back together to UN-do the mod quick & easy. Next I threaded the speaker wire through one of the rubber grommets in the fire wall and put a male snap connector on one wire and a female snap connector on the other wire and plugged them into the ones I put on the IAT wire. Next, inside the cab, I soldered the two ends of the speaker wire to the center and end terminals of the potentiometer. VIOLA' - an adjustable IAT!

With the potentiometer turned all the way down to zero it is like nothing has been done. The ECU reads the actual resistance of the IAT. This a good thing for those of us who live in areas where passing an emissions "sniffer" test is required to renew your tabs. With the potentiometer turned all the way up, the ECU reads the combined resistance of the IAT and the 5k potentiometer - roughly 7k-8k of resistance. This translates to the ECU getting a signal telling it that the air temperature is somewhere in the 10-15 degree range.

Since colder air is denser air, the ECU adjusts the pulse width to the injectors to hold them open a tiny bit longer - to push through a little more fuel and try to maintain the correct air/fuel mixture. But, since the engine temp sensor is telling the ECU that the engine is fully warmed up, it doesn't advance the igniton timing - like it would if it were running in open-loop (warm up) mode. This is very important because one of the problems I was experiencing was knocking and pinging when engine was pulling hard at low RPM with the throttle wide open (a.k.a. "lugged down"). When taking off from a dead stop - especially uphill. I was having to rev the engine up into the 2000 RPM range (that's just an estimate - since it doesn't have a tach) and then slip the clutch to avoid "lugging" the engine and having it ping and lurch etc.

I took it for a test drive and the difference/improvement was quite obvious. IT WORKS! Not what I'd call a dramatic improvement, but definitely noticeable. I went over to the parking lot at my son's school to do a little low-speed testing. The lot is set up with two parallel parking lanes about 100 yards long with a row of parking stops in between, and it is on a slight hill. I did about a dozen "laps" around the row of parking stops in 2nd gear to test it at various settings of the potentiometer. Everytime I came around the end of the row I let it slow all the way down to just under 10 mph, and then as I started the uphill run I'd floor it. With the potentiometer set to zero the car would lurch and ping very badly and barely manage to accelerate to 12 or 13 mph by the end of the row. With the potentiometer set to the middle setting (~ 2500 ohms) it was a little better, and with the potentiometer turned all the way up to 5000 ohms it was MUCH better. Barely pinged or lurched at all and it accelerated to 15 mph!

Next I drove it around the neighborhood doing multiple starts from dead stopped with various potentiometer settings. My "seat of the pants tachometer" tells me that if it takes 2000 RPMs to get a smooth startup with the dial set to zero, then it probably only takes 1500 RPMs to get a smooth startup with the dial set to 5000 ohms. That's a BIG improvement. I'll probably hook up a tach to test it and get some actual numbers, but there is no question, based on the old "butt dyno" that there is a definite improvement.

As my last test I decided to do a high-speed hill climb. We live on top of a hill that is about 600 ft high, and one route to the top is a 3/4 mile long, divided, 4-lane street, that is a 6% grade. I hit it in 3rd gear at 40 mph with my foot on the floor and the dial set to the 5000 ohm position. Initially I was accelerating up the hill, but as it got steeper it stabilized at around 50 mph as it got into the long steepest section in the middle (50 mph is the speed limit on that road BTW). After a couple of seconds I spun the dial to the zero ohm setting to see what would happen. The engine almost instantly started to ping and ever-so-slowly loose speed. After a few seconds I then spun the dial back to the 5000 ohm position and, just as quickly, it STOPPED pinging and started slowly regaining the speed it had lost! Heck, for my money, that would be worth the 6 bucks and 3 hours invested (including tracing the wiring, testing the IAT, and the trip to Radio Shack) right there!

So, now the only thing that remains to be seen is the mod's effect on gas mileage. The eBayers claim up to a 20 HP gain and equal or better mpg from this mod. Now, I know that the boost is less than that - 20 HP added to an engine that only has 112 HP to start with would be a MUCH more dramatic-feeling improvement. However, I'd be willing to believe that it is somewhere close to 10 HP, give or take a pony or two. If that means I don't have to floor it quite so often, then just maybe the mileage won't be negatively impacted - or at least not much. Of course, I do have the option of turning the mod off every time I get on the freeway if I like, so maybe I can just get in the habit of turning it on and off depending on how and where I'm driving. Or maybe setting the dial at the halfway point to get a balance between the max power boost and max fuel economy will be the answer. Who, knows? We'll just have to see how it goes.

I'm still looking for other suggestions, looking for an adjustable cam gear, and wondering about the manifold & throttle body swap though. So, if you're reading this little book I've written, and have some more suggestions for cheap mods, or info about the intake swap idea, then bring 'em on! I'm not satified just yet - but I wanted to share what I did and what the results were with anyone who might be interested.