IAT and ECT - How much do they matter?

Wanted to get some opinions and thoughts from the community on this one. We're always told to watch IAT and ECT during engine performance diagnostics, as these are part of the 'critical inputs' that the engine computer uses for fuel management. But, how much weight do they actually carry? :huh: How much will they skew things like fuel trims if they're being reported incorrectly?

As always, I used the Scion xA as a guinea pig. Starting with the IAT, I installed a variable resistor on the signal wire and started turning the dial:



How much does it matter? Well, at idle, not much! The trims moved a few percentage points back and forth, but overall the car didn't notice much of a difference.

Now for the ECT. This one produced some noticeable changes:



I knew the ECT was important to idle speed, but MAN the engine was really unhappy with the resistor installed. :lol: There's a big dip in short term trims at frame 650, but I believe that's due to the flare in engine speed from spinning the resistor dial too fast. Making small, incremental changes in temperature varied the idle, but never changed fuel trims all that much. Calculated Load didn't notice anything was happening. :blink:

Next step is to rig the resistor in and go for a test drive. Wondering if the IAT will matter more at higher airflow rates...

Looking up the air density tables at just below freezing a cubic meter of air weighs 1.3 kg at 60c (140f) it weighs 1 kg.
The fueling map will probably only cover a credible range of temperatures anything below say -10c (14f) or over 60c(140f) won't be covered.

IAT usually has a major effect on ignition advance under load on most engines when I had a Honda it always seemed to have about 20% more power on a frosty morning.

Andy.MacFadyen wrote: IAT usually has a major effect on ignition advance under load on most engines when I had a Honda it always seemed to have about 20% more power on a frosty morning.

See, that's the kind of thing I'd like to witness on a test drive! Could I theoretically create a low power condition by changing the IAT and taking all the ignition advance away? Gotta be careful not to make it knock too much, either. :lol:

Pretty good idea Tyler, thanks for sharing your experiments with the community at large!

I have seen "Power Adders" wired into the IAT of cars... To what advantage, i do not know. I am interested to see it's effect under load, especially considering Andy's observation!

That Scion is a MAF car right?

Noah wrote: Pretty good idea Tyler, thanks for sharing your experiments with the community at large!

I have seen "Power Adders" wired into the IAT of cars... To what advantage, i do not know. I am interested to see it's effect under load, especially considering Andy's observation!

That Scion is a MAF car right?

Those resistors probably worked on engines without an oxygen sensor, these days I suspect the pcm will just ignore a sensor it sees a dodgy reading from.
Moisture in the air on cold damp morning also contributes to any power boost, the Honda concerned was actually Rover 623 really a rebadged version of the Honda Accord with H23A3 DOHC engine with a claimed 158bhp it never felt anything like that except when it was cold and damp. My current daily driver is also a MAP engine and it also has a bit extra power in the cold and damp but much less so than the Honda.

Noah wrote: That Scion is a MAF car right?

Yes sir! I actually just added the variable resistor in parallel to the IAT last time (since it's integral to the MAF), but I may take the IAT pins out of the MAF for the test drive. I've got a set of supah long test leads, so I can put the resistor in the cabin with me while driving.

Andy wrote: Those resistors probably worked on engines without an oxygen sensor, these days I suspect the pcm will just ignore a sensor it sees a dodgy reading from.

Probably right on here. I'm also interested to see how varying the IAT will change Calculated Load, as I'm told it's a factor in ignition advance.

Update! Did some driving testing with a variable resistor installed in place of the IAT. For your consideration:

Cruising at highway speeds.



Varying the IAT around 100 degrees, and the car didn't really seem to notice at all. :silly: MAYBE got the ignition advance to vary a bit?

Then I decided to do some WOT runs. Because of how the Scion data lists are configured in the Modis, I couldn't get IAT and the O2's on the same screen. The first run had the IAT set at 32 degrees, while the second was set to 150.





Perhaps a few more degrees of ignition advance when set cold? This would go along with what Andy said. The acceleration might have been a bit better, but it was tough to notice. No lack of fuel or change in Load.

Welp, there's a reason I've never changed an IAT for an engine performance problem, and all this testing spells it out. :lol: Maybe a larger displacement engine would notice more? Since I did this experiment, I've stopped looking at IAT during performance diagnostics - save the data stream bandwidth.

Lol thanks for doing the work for us. I've always said those little power adder boxes you wire in to the IAT are a joke !

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Ben wrote: Lol thanks for doing the work for us. I've always said those little power adder boxes you wire in to the IAT are a joke !

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No problem! Definitely NOT good for 5 HP at the wheels. :lol:

Maybe I run this experiment on my truck?

Awesome work guys!

ScannerDanner wrote: Maybe I run this experiment on my truck?

Sure! I'm happy to compare results if you end up doing this test.

Playing with the ECT now, seeing if I can create a no start. :lol:

Cool stuff Tyler!
It's always fun to glimpse into the word of the mad scientist!

What do you have for a variable resistor?

Noah wrote: What do you have for a variable resistor?

Thanks, sir! It's a part of a test set the shop bought off the Matco truck. VERY similar to the AESWave uTest kit. I have it strung into the cabin via some test drive leads.



I did some more playing around with the IAT during cranking, but I'll save you all some time: It didn't make any difference. :lol: I got cranking injector pulse width to very a few mS, but never enough to create a perceptible problem.

After that, I switched the resistor over to the ECT and started monkeying around. I went for a cruise on the highway, and saw this:



Swinging from overheating to below freezing only got me a few points of short term fuel trim change, but little else. Again, more evidence that the thermistors are just along for the ride when driving.

The fun came in when I started playing with injector pulse width during cranking. Just to see how much change I could get, I disconnected the injectors (no driver monitors on this car, so no codes ) and did some cranking:



That's what I wanted to see! I had always heard that ECT was a big player during cranking, but this confirms it. The next step: If the ECT is off far enough, can I get it to not start? I hooked the injectors back up, turned the resistor to HOT, and cranked it:



Frame 240 is the cranking attempt, when the engine was ACTUALLY at around room temperature. At frame 290, I dialed the temp down to something more reasonable, and got the engine to start.

I wanted to do this kind of testing in light of some recent threads involving cranking pulse width. It's a subject I've never seen covered in the aftermarket. I rarely look at it at all during diagnostics, but maybe I should be? :huh:

Thread Bump!
Would you be able to check the resistance of that variable resistor from stop to stop for me please? (Who am I kidding, you probably have a graphed resistance waveform!)
I tried doing this with a fuel tank gauge sender plugged into my ECT on my truck while I was waiting for someone to drop off a car.
I couldn't get the Coolant Temp PID below 300f. So of course the gauge in the cluster was buried hot.
Just not enough resistance...

They are nearly all negative temperture coefficient thermistors and work in the range 4,000 ohms (cold) falling to 300 ohms when hot.

The fuel tank sender's only sweep from a couple ohms to around 200ohms.
If 300 is the average lower end of the resistance scale on a NTC thermistor, that explains why I couldn't get the temp needle off H.
I'm wondering what kind of range the variable resistor in Tyler's kit has.

Noah wrote: The fuel tank sender's only sweep from a couple ohms to around 200ohms.
If 300 is the average lower end of the resistance scale on a NTC thermistor, that explains why I couldn't get the temp needle off H.
I'm wondering what kind of range the variable resistor in Tyler's kit has.

Sorry sir, I'll take a measurement for you! Don't have it graphed, sorry. :lol:

Off the top of my head, it's from 0 to 10K ohms.