In 4 years of driving a 2005 FEH with a ScanGauge attached....

IAT is ALWAYS 10 degrees +/- 2 degrees warmer than outside ambient under all kinds of weather conditions. Maybe closer to 12 when it's 100 outside, and closer to 8 degrees when it is zero outside, but the long-term average, under average conditions is indeed, 10 degrees.

When stationary, parked or otherwise at idle, the engine is A) not drawing in much air mass, and B) the compartment under the hood where the air is drawn in gets warmer... and the IAT is all over the place.

It is not uncommon for me to see IAT of 70 degrees on a 10 degree day if I'm standing still with a hot engine.

When traveling at ~25 MPH+ the results are extreemely consistent.

 

Modern cars are just that... MODERN, and conventional wisdom, and conventional thinking needs to be thrown out in some cases.

The FEH, and I'm sure other cars, use lots of modeling, predictions, and assumptions to operate. One assumption by itself can be extreemely inaccurate. Many factors taken together as a whole can derive great simulations of actual conditions without actual measurements.

Does the car really care what the OAT is? I think not.
The temperature of the air entering the engine matters, and that is all that is measured.

 

I was using a AutoXray 6000 scanner but have a SG-II also. I just looked at the manual and the IAT is one of the hardwired guages. I'll have to see how it compares.

Even then if you read what the manual states about IAT, it states exactly what I saw on my scanner or a few degrees above ambient while driving and much higher at idle and not what you stated! Can you explain why your reading differ from mine and what the manual states?

If it did use IAT to calculate OAT, it would have to 10F+ all the time, not part of the time.

 

Disregard what John claims, no where in the manual does the PCM calculate OAT from the IAT. The IAT sensor does what the manual states and that's the bottom line. If Ford had a reason to calculate OAT with the IAT and other sensors (RPM, speed, and time etc.), I'm sure they could. The fact that Ford and other manufactures locate the IAT sensor with the MAF is because OAT is not what's needed to adjust fuel and ignition timing. The fact that the engine compartment temperature and engine temperature effect IAT, the OAT can never compare the same or within so many degrees all the time.

GaryG

 

I'm not concerned if the IAT is used to calculate OAT or not, it's his statement about +10F above OAT all the time. Neither me or Linear Logic seem to agree with that!

I'm wondering if he is using multiple SGs also? I think that can create problems or false readings.

 

Wouldn't the air entering the engine be the same temperature as the OAT?

 

The HVAC, climate control system, DEFINITELY needs to know OAT. The A/C compressor MUST be shut down, disabled, if the OAT begins to decline, or declines, to very close to freezing. There is also the issue of the US patent that Ford has that involves reducing the level of regenerative braking that is used if the OAT begins to decline to freezing.

 

Only when driving it comes close it but that's it. At idle it pulls in the warmer air of the engine compartment.

 

Nobody disputes that! The debate right now is about the incorrect comparison values of OAT to IAT stated by gpsman1 and how he arrived at them.

 

I punched up the IAT gauge on my SG-II. My OAT was 36F most all the time but showing 35F once in a while.

Driving at 50MPH the SG-II showed 36F also for several miles then went down to 33F! How can it show below OAT??

I stopped at a drive thru car wash where it went up to 60F, half mile later, stopped at a party store, in/out about 5 minutes. At 50MPH it showed 44F.

I passed my turn to go home and drove another 3-4 miles. The SG-II slowly settled down to 39F.

There is something wrong here! I can understand slight differences between it and my scanner but how can it show below OAT??? Readings appear erratic too!

I now have serious doubts about anything it shows!!