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Anyone ever taken an external thermometer and put it inside the air duct next to the IAT to see if there is a difference between the two readings? I am seeing up to 30° differences after a heat soak situation (scan tool reporting higher than my external thermometer). But then after driving for a while, the difference got down to between 10 or so degrees difference. No significant drivability problems. No check engine light. Just been noticing my fuel trims seem high at idle, when restarting a hot engine.
So I just did a test. In a second vehicle of the same brand but a slightly smaller engine, I inserted my thermometer probe into the intake near the IAT sensor. Started the vehicle and the two temperature readings were very close to the same, within a degree or two. Took the vehicle on a test drive at both low and high speeds and the two temperatures remained very similar to each other the whole time. Turned the vehicle off, waited for around 15 minutes, then restarted the vehicle. Now, the scan data was showing a temperature around 25° higher than my inserted thermometer. Took the car for a low speed drive around my neighborhood and during the drive, the scan data temperature and my inserted thermometer got pretty close, within about 5° of each other.
What is that telling me? I'm not sure. One possibility is that the first vehicle should have done the same as the second. The first vehicle had about a 10° higher reading on scan data compared to the inserted thermometer when the vehicle was cold started. When it was restarted again with a hot engine, the variation jumped up over 30° and stayed between 20 to 30° variant the whole drive.
I could understand small variations between one thermometer to another, but I am surprised that in both vehicles, at times, there was as much as nearly 30° difference between the reading of the IAT and the reading of my inserted thermometer, both in the intake airstream near each other.
Thanks. Yes, I did leave the inserted thermometer in the air duct, undisturbed, the whole time. I was hoping to find some silver bullet to identify why I'm having high fuel trims at idle, when the hot engine is restarted, with no vac leaks detectable by my smoke machine. The fuel trims are much lower on a cold start.
What got me to looking at my IAT was I noticed on a 79 degree day, that my IAT was showing 139 degrees during extended idling. I was surprised the IAT could be truly seeing a temperature that much higher than the outside temperature.
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