The sensor is just a standard two wire NTC as the resistance goes down with temperature increase.white exec wrote:- to keep it clean, maybe? I presume it's just a NTC or PTC resistor. Be interesting to compare the resistances of all three (when you have the new one), and see if they're the same at the same temp. Wonder if a replacement needs calibrating, or whether the ECU somehow does that itself (not sure what it would check it against).
They don't really need calibrating from one sensor to another - these kind of sensors are better than 1 degree without requiring calibration and 1 degree is fine for this application - the experimental measurement errors from factors like the heat of the air filter box influencing the reading would be an order of magnitude worse than the tolerance difference between two sensors I suspect.
I don't think I have the original factory sensor with the broken tabs, I think I binned it, but I'm pretty sure they'd all read almost the same under static conditions, the difference is going to be the reading when the air filter housing is hot and the incoming air flow is cold - the open design of the one I just ordered should in theory read much lower (and closer to the real air temperature) in these circumstances while the one fully within the plastic sleeve will read a lot closer to the temperature of the housing.
That's not necessarily the answer - because what you really want is the air temperature just before it goes into the cylinders. That's what matters for calculating the charge of air inducted into the cylinders. Some of the temperature rise recorded by the sensor is actual heating of the air - I think a temperature rise of 20-30 degrees above ambient is normal when the engine bay is really hot due the air flowing through hot ducting, a hot air filter box, a hot intake manifold etc... putting the sensor right at the front before it has even gone through the air filter box would record a temperature that is a lot lower than how warm the air is by the time it has reached the valves in the engine.Wonder what would happen if the sensor were moved right "down front", so it was actually measuring the atmospheric air intake temperature.
But a temperature rise of 53 degrees above ambient is just not believable to me, especially when I've checked this reading on my old V6 on the Lexia many times and never saw the air intake temperature reading go above about 40 degrees even when the engine bay was stinking hot. So I think this current sensor is reading up to 25 degrees too high when the filter box is hot due to it not being cooled by the air flow properly.
The thermal lag of a sensor that is basically insulated by the plastic sleeve will be quite bad - the original sensor probably performs a lot quicker, and I'm hoping the one I've ordered will perform even better again as the sensor bulb is completely out in the open in the air flow with little direct conductivity back to the housing.Agree with your point about suddenly changing air flows, and a sensor with thermal lag. Doesn't make much sense, having the engine respond to conditions that no longer exist!
Modern MAF sensors which work on the heated wire principle work by heating a thin wire filament and then measuring the temperature drop of the filament caused by the air rushing past, which also requires an accurate measurement of the air temperature before it gets to the heated wire filament. By knowing the incoming air temperature and measuring the temperature drop of the heated filament you can calculate the air flow, the greater the air flow the more the heated wire cools. These type of sensors are extremely susceptible to carbon and dirt build up on the filament, which acts as an insulating medium, when they get dirty like this they don't see nearly the same amount of temperature drop which translates to the MAF sensor thinking there is less air flow than there really is - you then have a dirty MAF.
This air temperature sensor likewise will not drop in temperature as much (from the "ambient" temperature of up to 65 degrees inside the filter housing) when exposed to cold air flow if it is insulated instead of being exposed. The more it can be exposed to the air flow and the less thermal conductivity there is from the sensor back to the housing (through the base of the sensor) the more accurate its real world reading will be.
The symptoms of the insulated sensor would be that when the engine bay starts off cold the air temperature sensor will give an accurate indication of incoming air temperature and things will be fine, however as the engine bay gradually heats up and the air filter box gets hot the reading will start to increase well above the true air intake temperature reading, this will cause the engine to run progressively leaner and leaner (in open loop mode) due to the false high temperature reading.
So in theory replacing the sensor should make little difference in a cold engine bay but the engine performance should not deteriorate as the engine bay warms up.
Thanks James 
