need Explanation. EFI Intake Air Temperature

Hoping someone in the hive is EFI savy enough to add to my education.

Had the Molvo out for another round of trying to understand the Fitech. Inasmuch as it has always seemed to behave in a totally random manner from day to day, it was becoming apparent that on warm days it was much better to drive than on cold days. The Fitech really hated cold weather.

So yesterday was a HOT day, and a cursory drive around the block indicated that it would be happy to drive to work. ( Same drive around the block a week earlier in cool weather barely made it home)

So I log the drive to work data and have a look. What I noticed for the first time was that the Intake Air Temp shown on the log was 64*F. Weird, cause ambient temp was 86*F. It ws a HOT day. Hhmmm?

Is this normal for the IAT to be lower than ambient temp? I would have expected it to be higher since it was drawing hot underhood air.

Also what if any effect would be expected from wrong IAT data?

Pete

I would expect the IAT to be at least the same as the outside temperature or higher, depending on where it's picking up the air from.

As cooler air is denser and thus has more O2, it's likely running richer than it would be if the IAT reading was correct.

Would be interesting to see if the IAT is reading a constant value by any chance no matter what the actual temperature is, or if it's off and reading lower than expected.

Is this a case where the IAT is showing Celsius and it's actually reading ~147F?

Defintely not normal. IAT will pretty much always be hotter than ambient. Forced induction makes this worse. Incorrect IAT signal will skew fueling tremendously. In the same way a choke works, an IAT reading cold will add a ton of fuel. Half of the original EFI "cheats" were just resistors inline with the IAT so the car would fatten up the mixture under the guise of moar powa.

Bad sensor?  Bad iat data wonks everything up. Tbi gm vehicles it was always the first thing to check when they were not running right.  

Wow, trying to do any research on that product just results in lots of slick PR and very little actual information.

It truly is built for the Summit/Hot Rod guys who don't want to know anything about it and just want to buy something and plug it in and go.

Sorry, I got off on a tangent there.  Yes, the IAT is very important and if it is wired incorrectly or otherwise not working you'll have issues like you've noted.

See if you can remove it and test it (if you can determine what its specs are so you can put a known temp to it and measure the resistance) and possibly replace it or repair the wiring.

If anything intake air temperature will be MORE than the ambient due to heating in the engine bay or other components. Different manufacturers use different temperature curves and some are close to eachother (GM and VW/Saab/Bosch) and some are vastly different (Ford). Do you know the sensor the FITech is expecting, is it included? It is also possible that the 64F is a default value for open circuit/no sensor, so check that out as well. You should be able to put a potentiometer in the sensor leads and generate resistances that make the reading in the ECU change.

If the sensor is bad/wrong, it reading colder can make the system run richer than it should. 

A bad IAT reading would mess with fuel trim and possibly a little bit with timing (but the timing likely wouldn't be so far off that you would notice)

Colder air is denser and it bumps up the fuel delivery.  If it's reading colder than it actually is, you're running rich.  Verify that your sensor is reading correctly.  If it is a threaded sensor, make sure it has zero ohms (or darn close to it) from the housing to ground.

Pretty much impossible for IAT to be lower than ambient unless you have a really efficient liquid cooled intercooler. A lower than actual IAT will cause an overly rich condition. 

If you have some sort of venturi effect going on with your air intake, there could be a drop in temperature according to the ideal gas law (PV=nRT) https://en.wikipedia.org/wiki/Ideal_gas_law  However, that doesn't seem likely.

So do you have any old logs on a cold day to see the IAT?

Where are you measuring IAT?  Before or after the throttle body?

If it's before the throttle body then you have a sensor issue.  Could be bad wiring, could be a calibration problem.  IAT sensors are typically thermisters (a resistor whose resistance varies with temperature).  The precise mapping from resistance to temperature varies between thermister designs, and is also non-linear.  Your ECU needs to know how to read the sensor that you have installed.

If it's after the IAT (so inside the intake manifold) then you can theoretically see temperatures drop below ambient due to the pressure drop across the throttle body.  This can lead to them icing up in cold climates, which is why they usually have coolant running to them.

In a speed/density engine, IAT is used to trim the fuel.  The ECU wants to know air mass, and is inferring that from air pressure, but since temperature also affects pressure it needs to be compensated.

In a mass airflow engine the ECU is reading air mass directly, but a hot wire MAF sensor needs an IAT in order to calibrate those readings correctly.

In either case, an incorrect IAT reading will result in fueling being off.

Throw the Fitech in the dumpster.  Problem solved.

So with my '10 Forte, the stock intake IAT temps would show about 20-25* over ambient. The cold air intake I used dropped that to 15-20 and then I wrapped the intake with header wrap and a shield over the the filter from the engine bay and that was down to 5-10 over ambient. 

I solved the iat problems.....

alfadriver (Forum Supporter) said:

So do you have any old logs on a cold day to see the IAT?

This is a Fitech TBI. So the sensor itself is located who knows where as part of the TBI unit.

I have old logs taken on colder days, but since the logs don't show a date, I cant even go back and check with weather records. However one that I recall had a temperature of 20 and that being well below freezing, I would not have been out in the Molvo. Pretty sure that this thing has a 20 degree offset on the IAT sensor.

It would also explain why the car is psychotic in that it is trying to trim fuel to meet the target AFR  while the IAT keeps telling it to add more fuel.

Have to go see what I cn learn about replacing the sensor.

If it was gm, id say unplug the sensor and intentionally put in in limp mode to see if it turns consistent. 

Zero out your IAT correction table.(no correction)

Tune the car when the weather is normal for when you will be using the car.

Tune the cold start and water temp correction after the tune is done. 

When all that is done it will just be a matter of adjusting the IAT correction a couple percent.  It needs to be done after everything else or you are just chasing your tail.

Intake Air Temp is going to play a bigger part in timing control than fuel.  It's normal to start pulling timing above ~140-150deg.

In reply to Patientzero :

Not sure if any of that is applicable to a Fitech?

"IAT correction table" ¯\_(ツ)_/¯

I am guessing that correcting the table is a way of offsetting the  iac temp 

In reply to NOHOME :

No, the table is to correct the fuel correction % based on the air temp.  I'm willing to bet your water temp correction is the more likely problem.  The intake temp correction doesn't change enough to make the huge difference you're describing.  

I'm pretty sure you can change the Air temp correction from the handheld.

A wideband O2 would be useful to see what's really going on.

In reply to jwagner (Forum Supporter) :

The Fitech has a wideband. 

In reply to NOHOME :

A good check for all the sensors is what are they saying after the car has sat for 8+ hours.  They should all be the same.  And MAP should be whatever BP is.  Of course, this is before you start the car....