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Old 01-23-2008, 11:42 PM   #1 (permalink)
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IAT sensor madness

I set my Scanguage II to give a readout of the intake air temperature. The given temps are all over the place.

Today at a steady 60 mph, the temps were ranging wildly from 58 to 89 with the outside temp being 53. I had pretty wild changes when I had a cold air intake on it as well. When I start it, the temp starts out around 90 wether the engine is hot or cold.

Is so much flux normal or should I be replacing the sensor? I know that depending on make, cold air or warm air may affect performance and/or FE.

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Old 01-26-2008, 09:42 AM   #2 (permalink)
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I'm at a loss to answer that one. I don't suppose you have a shop manual for the car - that's where I'd head.
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Old 02-01-2008, 12:46 AM   #3 (permalink)
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sounds like a bad one to me. you can test if you have the stuff. but you may want to go to a u-pick junk yard and get a used one and see. used they are cheap. after you read up on it. and it is a big deal.
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Old 02-01-2008, 01:04 PM   #4 (permalink)
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Green Machine -

Maybe a loose connection? Maybe you should subject the IAT sensor to controlled conditions, like dipping it in a cup of hot or icy water where you have a thermometer to know what the temperature is. Just make sure you don't submerge the wire-connecting part of the IAT sensor in the water too.


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Old 02-01-2008, 06:59 PM   #5 (permalink)
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attach a high impedance volt meter between the output signal at the IAT and a good ground and watch it in real time as you drive. it should not change fast, if it does also check the input volts to the IAT, should be constant 5vdc, if it changes problem is the input or your ground, if constant input, then problem is the IAT.

i have never used a scan gage, but think there would be a delay between it and the engine.

is that as clear as mud?
many times the prob. is self inflicted, corrosion, oil, bent pins, broken wires, bad ground somewhere, replacement sensor out of spec., high amp. radio messes with everything else electronic, someone else 'worked' on it etc...

Last edited by diesel_john; 02-01-2008 at 07:07 PM..
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Old 08-29-2008, 02:47 AM   #6 (permalink)
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I've had some strange readings as well.

I started out with figures in the area of outside temps. Upon cold startup, the outside temp. After going for a while, it's higher by 5 or so, due to the heat soak in the engine compartment.

Once it's gotten hot, the figure can vary by up to 10 over the course of a minute or two. I think that low flow (TPS) lead to higher temps. The incoming air is absorbing less heat from the intake path. Vehicle speed also has an effect==higher speed is more turbulent air in the engine compartment (has negative effect on IAT) and more air passing through intake tract, also negative. However a car going faster can cause increased underhood temps--so positive influence. Also, cruising at high speed can involve small TPSs so another positive influence.

Start after a heat soak and the IAT is way high like 120 or even 170.

For a few weeks I was getting IATs consistently 120 over ambient/actual. Then it went away and seems accurate again.

I will admit that I popped the sensor out a few times to do TB cleaning, etc, and messed with the wiring harness and cleaned the sensor itself with some MAF air sensor cleaner. I gave up after that, and after like a week of doing nothing, the readings all of a sudden were in the expected range (outside temp and up a few tens of degrees)
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Old 08-29-2008, 07:54 AM   #7 (permalink)
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Maybe it matters which way the wind is blowing? Might make a difference in how well the cooler outside air reaches your intake.

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Old 08-29-2008, 12:35 PM   #8 (permalink)
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Originally Posted by wagonman76 View Post
Maybe it matters which way the wind is blowing? Might make a difference in how well the cooler outside air reaches your intake.
You know that coincides with some very marginal anecdotal evidence of my own.

I am a fan of "spirited driving" on highway cloverleafs. Only in remote areas in low traffic conditions. I actually use the cruise control and see just how fast I can safely take some of the tighter ones, so I guess even my quest for speed is tempered by economy. Plus, it boils it down to the simplest type of driving--just steer.

So obviously my attention is on the road ahead above everything else. However, sometimes, I get the impression that the IAT temperatures change depending on the way I am turning. For example, cruising at 50 in a straight line, it remains constant. However let's say I take a ramp and am turning 270 degrees in the course of less than a 1/2 mile at the same speed and the IAT is different.

Also I think the value changes depending on whether I am turning left or right.

Again, I don't know which way it changes, nor do I plan to find out, but I seem to detect some kind of change in the value during or immediately after the turns.
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Old 10-04-2008, 02:57 PM   #9 (permalink)
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Might just depend on where your IAT sensor is located. Maybe the car has a thermostatic valve to pick up some warmer air off the exhaust manifold? If so, and the sensor is downstream of that, you could be seeing the valve cycling.
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Old 10-04-2008, 08:28 PM   #10 (permalink)
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I'm guessing: first, additional throttle is required to maintain speed in a corner, and secondly -- perhaps a change in wind-speed and direction.

On the off-ramp, you're changing direction and increasing throughput of air past the sensor as the frictional loss requires more throttle to maintain speed. I get the same condition because very little throttle is used until the tight off-ramp corner and merge acceleration. The throttle opens wider and more air is allowed to flow past the sensor as the frictional loss from the tires biting into the corner requires more power. This equals more airflow and many times, lower IATs.

It's entirely normal -- more air generally means lower IATs (or at least a change).


EDIT: If the ambient temp is hotter than IAT norms, the opposite might be true.

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Last edited by RH77; 10-04-2008 at 08:30 PM.. Reason: Hot Weather
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