It took me over a year to solve this MPG puzzle

U

U4ick

Veteran Member
Joined
Nov 16, 2014
Location
texas
TDI
2003 jetta tdi
I have to confess that I'm at Witsend to understand what effect AAT has on fuel mileage.
Perhaps it is time for an explanation from the honorable and esteemed OP.
Lest we begin to think that he has dropped a stinkbomb and is now in the background laughing. :rolleyes:
 
BobnOH

BobnOH

not-a-mechanic
Joined
May 29, 2004
Location
central Ohio
TDI
New Beetle 2003 manual
Herm TDI said:
................snip............................
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 .
Click to expand...
With out knowing the nuts and bolts of the AAT input, this seems like a pretty good explanation, if anecdotal.
Having similar disappointing MPG, I'll be checking mine soon.
 
nicklockard

nicklockard

Torque Dorque
Joined
Aug 15, 2004
Location
Arizona
TDI
SOLD 2010 Touareg Tdi w/factory Tow PCKG
jettawreck said:
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.
Click to expand...
You could be 100% right, but isn't it plausible that the ECU checks AAT at startup to see if it needs to add extra fuel during a cold winter day to speed warm-up and get emissions under control AFAP? If I recall correctly, it also checks ambient pressure to see if it's at high altitude and adjusts fueling (QTY and timing) accordingly. What I wonder is after all these years, we still don't know the exact control methodology. Can some VW engineer take pity on us and just post the diagrams please? Surely the patents have expired by now anyways.
 
K

KrashDH

Top Post Dawg
Joined
Dec 22, 2013
Location
Washington
TDI
2002 Golf
nicklockard said:
You could be 100% right, but isn't it plausible that the ECU checks AAT at startup to see if it needs to add extra fuel during a cold winter day to speed warm-up and get emissions under control AFAP?
Click to expand...
What engine are you talking about?
There is no AAT temperature readings that anyone has confirmed by giving a block, or where the reading comes from. IAT is measured in the MAP, that's the only thing we know for sure.

We know the *sensor* under the cowl on the drivers side is directly correlated to the A/C compressor operation. There is no proof (yet) that it does anything else. Hence there are no ambient air blocks for an ALH. I wouldn't think the engine would give a hoot about AAT as far as fueling? It gets the IAT from the MAP. Sitting overnight, IAT=OAT therefore it could fuel accordingly. As IAT changes, the fueling would change based on this. Unless they are WAY off from each other, which kind of-possibly-might be the scenario the OP was describing, you'd have a faulty MAP.

We all keep going around and around with this. Unless the OP chimes in here on this one it's all snake oil. Tasty snake oil. That I'm drinking
 
nicklockard

nicklockard

Torque Dorque
Joined
Aug 15, 2004
Location
Arizona
TDI
SOLD 2010 Touareg Tdi w/factory Tow PCKG
Okay, fair enough. Herm's never been a troll; he's just busy I bet.

But, if *I* were building the control methodology from scratch, I definitely would NOT want to be 100% dependent on an IAT/BOOST combo sensor that regularly goes wonky. That'd be a very fragile system, meaning your car would be in open-looped mode a ridiculous amount of the time, and you'd probably rarely have it in emissions compliance for the life of the vehicle. Besides, being 100% dependent on a sensor that only gives ambient temps after overnight (or sufficiently long) cooldown is stupid. What about when the car is only parked for 2 hours, but a storm comes in and the ambient drops 40 degrees? So your engine control methodology is wildly fragile to pretty common scenarios, and only reliable in ONE special use case?

That engineer needs a punch in the nose for being stupid, if they did just depend on "IAT = AAT after "ong-ish enough." That's one lazy, or stupid, but definitely mostly non-compliant engineer.
 
jimbote

jimbote

Certified Volkswagen Nut
Joined
Jul 10, 2006
Location
spiral arm, milky way (aka central NC)
TDI
Tacoma 4x4 converted to TDI
AAT sensor has zero to do with how the ecm controls fueling... only the IAT which is integral with the map sensor has anything to do with how the engine operates... the ambient sensor is only connected to the hvac controls and has no link whatsoever to the engine controls. If you experienced increased FE after replacing the AAT it was unrelated.
 
W

wonneber

Top Post Dawg
Joined
Oct 12, 2011
Location
Monroe, NY, USA
TDI
2014 Jetta Sportwagon,2003 Jetta 261K Sold but not forgotten
The two wires for the temp sensor connects to the injector pump and the ECM.
 
A

afterthisnap

Veteran Member
Joined
Jul 24, 2012
Location
Mountain West
TDI
Jetta wagons, ALH/CJAA
At the very least this thread made me poke around the cowl. I found a literal mouse nest of random shredded fibers and detritus that wasn't there last year, so thanks!
 
Nero Morg

Nero Morg

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Joined
Oct 19, 2017
Location
OR
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2014 A6 TDI, 2001 Jetta TDI, 2014 Passat TDI
Well I'm sure a NMS has an ambient temp sensor ;) until Herm chimes back in it might be moot if the ALH has one.
 
B

benshaw

Veteran Member
Joined
Jul 8, 2004
Location
51
TDI
Jetta bew
im not gona do it but why not unclip it and check the cluster before and after reading.
 
K

KrashDH

Top Post Dawg
Joined
Dec 22, 2013
Location
Washington
TDI
2002 Golf
Hey Herm we know you're in here, saw you today...:D please come over to your thread and explain your magic!
 
Nero Morg

Nero Morg

Top Post Dawg
Joined
Oct 19, 2017
Location
OR
TDI
2014 A6 TDI, 2001 Jetta TDI, 2014 Passat TDI
Are we all boycotting Herm now? LOL
 
C

csstevej

Top Post Dawg
Joined
Aug 12, 2004
Location
north nj
TDI
2001 golf tdi 4 door auto now a manual, mine, 2000 golf 2 door M/T son's,daughters 98 NB non-TDI 2.0, 2003 TDI NB for next daughter, head repaired and on road,gluten for punishment got another tdi 2001NB,another yellow tdi NB
Someone drag him out of his barn......
 
Nero Morg

Nero Morg

Top Post Dawg
Joined
Oct 19, 2017
Location
OR
TDI
2014 A6 TDI, 2001 Jetta TDI, 2014 Passat TDI
Smh I'll bandwagon this. But only because I need that sweet juicy fuel eco ;)
 
AndyBees

AndyBees

Top Post Dawg
Joined
May 27, 2003
Location
Southeast Kentucky
TDI
Silver 2003 Jetta TDI, Silver 2000 Jetta TDI (sold), '84 Vanagon with '02 ALH engine
My experience with the OAT sensor. (2000 Jetta)

In late winter-early spring of 2006, I had surgery. I was laid up several weeks. When I got to drive, it was early June and hot out side. My AC would not come on. A shop could not figure-out the issue..... Long story short, a mouse had chewed thru one of the two wires to the OAT sensor located under/behind the cowl on the left side.

Anyway, I drove the vehicle at least two weeks without AC and never saw any difference in MPGs... When I repaired the wire to the OAT sensor and got AC, still basically zero difference in MPGs.

I agree with Jimbote, there is no communication between the OAT sensor and the ECU, thus, no DTC for it!
 
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