It took me over a year to solve this MPG puzzle

Herm TDI · Jul 16, 2020

Car Info:
2002 Golf GL TDI w/ 5-Sp manual
Perf. Modifications:
Stage 3 Tune w/ EGR delete
11 MM Inj Pump W/ PP 520 nozzles
Stealth race pipe
The Problem:
About two years ago I noticed a steady drop in my MPG. I have no active DTC's
I went through the usual checks.
The static & dynamic timing were "spot-on"
Wheel alignment were well within tolerances .
Tire pressure has been unchanged since I've own this car (32 PSI on all 4 corners).
Air cleaner replaced (my preferred brand Mann w/ snow guard layer)
Fuel filter replaced (my preferred brand Mann)
Checked and checked again no dragging parking brake.
Replaced the MAF (still no change in the low MPG)
Replace the Eng Temp sensor (OE)
This went on ...I continued to ponder this matter. My MPG in normal summer driving condition was in the high 30's and some times into the low 40's (MPG).
I started to look into the parameters that the ECU uses to compute the fueling maps... Throttle position sensor, Fuel temp sensor, MAF , MAP and the ambient air temp sensor.
I was rooting through VCDS to check these #'s output to the ECU.
Then I found an odd reading...the Ambient Air Temp (AAT)sensor. This sensor is one of the vital fueling inputs .
I was reading a constant -2.2 Celsius (28F) in late June !!!
The Aaaahhhaaa moment !
Here I was driving around in June and the ECU is fueling the engine for February weather !

I replaced the AAT sensor and my MPG went up to 46 MPG over the next fuel fill of the tank .

NOTE:
I never once had a DTC for a faulty AAT sensor .

Nero Morg · Jul 16, 2020

Well shoot, I know what I'm checking tonight. My scangauge has been reading crappy fuel eco the last few weeks in 80f-ish weather.

KrashDH · Jul 16, 2020

Interesting, I too have an '02 Golf and my fuel mileage has never been where I think it should be for this car. You talking about the sensor down behind the bumper?
Also which/where is that parameter in VCDS? I might go hook up...

Johhny04 · Jul 16, 2020

Which measuring block in VCDS can i find the reading for AAT?

Nero Morg · Jul 16, 2020

KrashDH said:
Interesting, I too have an '02 Golf and my fuel mileage has never been where I think it should be for this car. You talking about the sensor down behind the bumper?
Also which/where is that parameter in VCDS? I might go hook up...

Not sure of parameter, but I think the sensor is the one in the cowl panel, drivers side.

KrashDH · Jul 16, 2020

Nero Morg said:
Not sure of parameter, but I think the sensor is the one in the cowl panel, drivers side.

I think there's more than one though right? The one under the cowl is directly tied into the A/C system and it's operation is solely for the A/C?

Herm TDI · Jul 16, 2020

KrashDH said:
Interesting, I too have an '02 Golf and my fuel mileage has never been where I think it should be for this car. You talking about the sensor down behind the bumper?
Also which/where is that parameter in VCDS? I might go hook up...

The AAT on my Golf is located near the cabin air filter.

super1 · Jul 16, 2020

Herm TDI said:
The AAT on my Golf is located near the cabin air filter.

Next to the Cabin Filter ? Need to remove the cover to access the filter or up by the cowling?

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gforce1108 · Jul 16, 2020

Wow - I figured IAT would affect MPGs, but not the AAT. Good to know!

KrashDH · Jul 16, 2020

gforce1108 said:
Wow - I figured IAT would affect MPGs, but not the AAT. Good to know!

That's what I'm confused about.
I haven't seen a block in VCDS that directs to AAT? There's IAT in group 7...
That sensor, isn't it just a switch that allows/doesn't allow the compressor to kick on at a certain temp? Ie at a certain resistance value at the temperature disables the ability for the compressor to run?

I guess I'm not well versed in that sensor and didn't realize it pulled double duty?

I think I'm left with more questions than answers haha!

tgray · Jul 16, 2020

Good info to know. This makes me wonder what would happen if we make the sensor send the opposite signal like the air outside was real hot instead of cold. Would that make the MPGs go up any from stock? I know the fuel temp sensor behaves differently on this matter.

Vince Waldon · Jul 16, 2020

Yeah, humbly and with all due respect still confused, as I thought the Ambient Air Temp (by the cabin filter) was only used for A/C - related decisions.

Always figured the car's only really concerned about the temp of the air it's actually ingesting (since that relates to air density/O2 content etc) which is IAT as built into the MAF. EDIT: MAP.

Or so I thought...incorrectly I guess.

Dh4276 · Jul 17, 2020

What measuring block did you find the reading under, I’ve been getting same kind of mileages since I bought my golf. Was actually worse economy on stock tune!

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KrashDH · Jul 17, 2020

Vince Waldon said:
Yeah, humbly and with all due respect still confused, as I thought the Ambient Air Temp (by the cabin filter) was only used for A/C - related decisions.

Always figured the car's only really concerned about the temp of the air it's actually ingesting (since that relates to air density/O2 content etc) which is IAT as built into the MAF.

Or so I thought...incorrectly I guess.

IAT is built into the MAP which is what block 7 reads.

Vince Waldon · Jul 17, 2020

True dat... good catch... corrected.

nicklockard · Jul 17, 2020

I wonder if that's why my 2010 Touareg Tdi gets awful mpg (lucky to get 19 mpg driving 85% freeway at 70 mph). According to owners here, on fuelly.com and fueleconomy.gov, my combined average should be 24 + 1.5 mpg.

I purchased a VCDS enthusiast dongle to check AAT.

eddieleephd · Jul 17, 2020

After reading this post I looked at readings and saw coolant temp in engine to be 20-21C and IAT to be 25C then I read looking for AAT reading and in instruments I found coolant temp sensor that reads 17C. No climate readings could be accessed. So it makes me wonder which sensor is showing in each spot for coolant.
I thought the dash read the same sensor as the engine, however, I guess not.

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super1 · Jul 17, 2020

KrashDH said:
IAT is built into the MAP which is what block 7 reads.

So it’s mounted on top of my IC ?

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eddieleephd · Jul 17, 2020

super1 said:
So it’s mounted on top of my IC ?

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Yes, the MAP sensor is located on the intercooler. The easiest way to get to it is to remove the bumper cover.
Sounds troublesome until you've fought with getting it other ways. Then the screws in the wheel well, and four or five in the front, pull the how lever off and the bumper cover slides off.
Then you can see the intercooler clear as day.

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KrashDH · Jul 17, 2020

This thread still needs answers.
IAT is read from the MAP and falls under block 007.
Coolant temps are different all together.
Ambient Air temp is not an option in VCDS for the MKIV platform. Except likely with models that have a sensor. Gas models have it down behind the bumper on the driver side. But if you have an AAT sensor you're also going to have a temperature reading on the cluster for the outside temp.

It doesn't make sense to me that the sensor under the cowling is related to fuel at all. It's an open/close switch (to my knowledge) that controls the A/C compressor.

One could argue that if their IAT differed wildly after the vehicle had sat overnight with the local outside temperature (just google what the temp is at that time) then the MAP sensor would be bad as it integrates the temp as 1 unit. But IAT temps should vary during engine operation from ambient temp. I feel like IAT differing wildly from ambient would be a sign, but you would have to let the engine reach ambient before you checked the reading.

Hopefully the OP has something up their sleeve that they could let us in on. Wondering why that sensor would change anything with how the car fuels. Also where in VCDS for clarification because nothing I can find points to this.

super1 · Jul 17, 2020

eddieleephd said:
Yes, the MAP sensor is located on the intercooler. The easiest way to get to it is to remove the bumper cover.
Sounds troublesome until you've fought with getting it other ways. Then the screws in the wheel well, and four or five in the front, pull the how lever off and the bumper cover slides off.
Then you can see the intercooler clear as day.

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Yes I just change the headlights on my G/F Golf super easy
Thanks

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eddieleephd · Jul 17, 2020

KrashDH said:
This thread still needs answers.

IAT is read from the MAP and falls under block 007.

Coolant temps are different all together.

Ambient Air temp is not an option in VCDS for the MKIV platform. Except likely with models that have a sensor. Gas models have it down behind the bumper on the driver side. But if you have an AAT sensor you're also going to have a temperature reading on the cluster for the outside temp.

It doesn't make sense to me that the sensor under the cowling is related to fuel at all. It's an open/close switch (to my knowledge) that controls the A/C compressor.

One could argue that if their IAT differed wildly after the vehicle had sat overnight with the local outside temperature (just google what the temp is at that time) then the MAP sensor would be bad as it integrates the temp as 1 unit. But IAT temps should vary during engine operation from ambient temp. I feel like IAT differing wildly from ambient would be a sign, but you would have to let the engine reach ambient before you checked the reading.

Hopefully the OP has something up their sleeve that they could let us in on. Wondering why that sensor would change anything with how the car fuels. Also where in VCDS for clarification because nothing I can find points to this.

I hear you, I think it's similar to the altitude factor in this situation. The density of O2 in ambient air is also varied on altitude. The ECU looks at MAP readings when the ignition is first switched on and bases all readings on that reading after the vehicle is started. It then varies fueling for air density on those readings.
This works pretty good and ultimately effects gassers negatively more than diesels. The greatest negative effect is going from sea level to high altitude and it will over fuel. Simple solution is shut it down and restart so the ECU senses the correct air density and fuels correctly.
Therefore, if the AAT is looked at initially and parameters are set for cold weather, then it makes sense to have an effect. The only time the sensor is looked at is at key on, then afterwards it's ignored. That's how I see it working anyhow.

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Dh4276 · Jul 18, 2020

KrashDH said:
This thread still needs answers.
IAT is read from the MAP and falls under block 007.
Coolant temps are different all together.
Ambient Air temp is not an option in VCDS for the MKIV platform. Except likely with models that have a sensor. Gas models have it down behind the bumper on the driver side. But if you have an AAT sensor you're also going to have a temperature reading on the cluster for the outside temp.

It doesn't make sense to me that the sensor under the cowling is related to fuel at all. It's an open/close switch (to my knowledge) that controls the A/C compressor.

One could argue that if their IAT differed wildly after the vehicle had sat overnight with the local outside temperature (just google what the temp is at that time) then the MAP sensor would be bad as it integrates the temp as 1 unit. But IAT temps should vary during engine operation from ambient temp. I feel like IAT differing wildly from ambient would be a sign, but you would have to let the engine reach ambient before you checked the reading.

Hopefully the OP has something up their sleeve that they could let us in on. Wondering why that sensor would change anything with how the car fuels. Also where in VCDS for clarification because nothing I can find points to this.

I have a MK4 golf BEW and found the ambient air temp reading under adv measuring blocks!

I went in myself after reading this article but I found it and the intake temp sensors to be reading the same! So that is not where I’m loosing my economy!

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clyde · Jul 18, 2020

Herm TDI said:
The AAT on my Golf is located near the cabin air filter.

Just checked my 2003 Jetta wagon; it has a sensor at the far left of the cowl area, right beneath the hood hinge on driver side. Didn't check part # but it looks identical to the AAT sensor used on Mk2 and Mk3 cars with MFA. That MFA includes an ambient air temp readout.

This German-built Jetta has no sensor near the cabin air filter.

STDOUBT · Jul 20, 2020

Dh4276 said:
I have a MK4 golf BEW and found the ambient air temp reading under adv measuring blocks!
I went in myself after reading this article but I found it and the intake temp sensors to be reading the same! So that is not where I’m loosing my economy!
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No, wouldn't it stand to reason that the AAT and IAT would have to be different at operating temps? If yours are both reading the same... that seems like an error.
So, at what outside temp did you measure and at what point in the engine's heat cycle?
What was your reading on each?

KrashDH · Jul 20, 2020

STDOUBT said:
No, wouldn't it stand to reason that the AAT and IAT would have to be different at operating temps? If yours are both reading the same... that seems like an error.
So, at what outside temp did you measure and at what point in the engine's heat cycle?
What was your reading on each?

This is correct, they should be different at operating temp/under load. IAT would be warmer...

People keep finding AAT in VCDS but no one is specifying which block. I get it's the advanced measuring blocks.
Can anyone CONFIRM an ALH takes an ambient air reading and if so, it's from that sensor under the cowl?

As far as I know my golf has one one the driver side under the cowl, which is opposite of the cabin filter.

@Dh4276, do you have a temp reading on your cluster?

Nero Morg · Jul 20, 2020

I couldn't find an ambient temp in the engine controller, but I did find one under instruments. But that might be because I put in a FIS w/sensor...

Dh4276 · Jul 20, 2020

KrashDH said:
This is correct, they should be different at operating temp/under load. IAT would be warmer...

People keep finding AAT in VCDS but no one is specifying which block. I get it's the advanced measuring blocks.
Can anyone CONFIRM an ALH takes an ambient air reading and if so, it's from that sensor under the cowl?

As far as I know my golf has one one the driver side under the cowl, which is opposite of the cabin filter.

@Dh4276, do you have a temp reading on your cluster?

No, I do not have a temp reading on the cluster.

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Dh4276 · Jul 20, 2020

STDOUBT said:
No, wouldn't it stand to reason that the AAT and IAT would have to be different at operating temps? If yours are both reading the same... that seems like an error.
So, at what outside temp did you measure and at what point in the engine's heat cycle?
What was your reading on each?

It would depend on where the IAT is reading from! I checked a new beetle today that has a BEW as well. It was reading 34c on both right after start up. We happened to be in the system checking other stuff and looked at while in there. The temp outside was around 92F

On my Golf, I checked it after a 45 min drive and was reading 30c on a 92f day!

Where is the IAT located in the system? If it’s after the turbo, then I would agree something is off!

After doing some quick searching, looks like the MAP sensor also functions as the IAT!

Guess it’s time to check the MAP sensor!

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jettawreck · Jul 21, 2020

I've never seen (or looked) an AAT in VCDS but long ago I installed a CheckTemp display and sensor in the old 2001 Jetta. Typically the ambient temp and IAT (monitor that field with ScanGauge) are about 10-15*F different at steady speed cruise with MAP in the 15-17 range. Get some prolonged boost and, of course, the spread increases quite a bit.
The 2003 under similar boost levels always seems to run a bit higher IAT's (engine has been in two different chassis) and those temps spike quicker under higher boost. Currently the 2003 is using the intercooler and sensor from the 1.8T gasser donor chassis. Perhaps the sensor in the 2001 reads a bit low but it's not enough to care about.
I really have my doubts the AAT would affect much since the ECU should really only care what the coolant, fuel and intake air temps are, but would seem there would have to be other fields (boost, injection timing, etc) out of sorts in relation to a wonky AAT sensor to impact the fuel mileage to any degree. But, I'm always ready to learn the results.

It took me over a year to solve this MPG puzzle

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