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TDI Fuel Economy Discussions about increasing the fuel economy of your TDI engine. Non TDI related postings will be moved or removed.

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Old August 25th, 2008, 22:39   #1
ndamico
 
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Default 2003 Jetta Automatic Poor Mileage

First off I know automatics get worse mileage. Here is the scenario. My friend and I both have 2003 Jetta TDI's. mine is a manual w/ 114k on it and his is an auto with 78k on it. both have same size tires. Mine is a GLS, his is a GL. Mine get great mileage (Upper 40's) and his is stuck in the mid to high 20's...

Observations/what we've done:

Mine is substantially faster (i know mine is a manual and his isn't)
We have removed the snow screens from both cars (they were both plugged)
we have reaplced the air filters and fuel filters with OEM replacements.
Both cars idle smooth, start the same, run quietly and otherwise appear just fine.
I have made a few datalogs with Vag-Com which i will attach. I just got the software a week ago and haven't had much time to use it.


However as i said his mileage somewhat sucks and mine is great and we are having a very hard time believe its just because his is an auto.

any help would be greatly apprecaited!

Here is my acutal vs desired timing.. scale on timing is 10*:



Here is the same thing from the auto with poor mileage...scale on timing is set to 20*:



it appears his may be off more at low RPM, but does it matter?


Here is the automatic's MAF data. it does appear under WOT the the maf reading for actual will start to fall. is this ok?




Here is a screenshot of his measuing blocks i took:






Lastly, after he left my house i realized how to get the timing plug-in to VAG-COM to work . Here is what it shows on my vehicle. It says "Timing within spec, slightly advanced". is that good?





Thank you again. I've found this site invaluable. I come from the Duramax diesel family so i'm new to TDI's but not to diesels.
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Last edited by ndamico; August 25th, 2008 at 23:46.
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Old August 26th, 2008, 06:47   #2
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Can you define (in numbers) what you mean by "his MPG sucks" ?

From long experiance, if the automatic is getting 37-40 MPG in mixed use, he's doing pretty good.
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Old August 26th, 2008, 06:54   #3
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He said mid-upper 20s.
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Old August 26th, 2008, 07:33   #4
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1) try running w/o the maf...see if things improve (just unplug the wire harness) if so then try cleaning the wire harness and maf connector with deoxit and wiggling it around a bit...see if that fixes it. if not...and running without helped, then replace the maf.

2) check to make sure no vac lines are unplugged

3) if no vac lines are unplugged then replace all vac lines (about an hour and $10)

4) if none of the above then check the turbo actuator (there are threads on how).

5) if none of the others then try the other things in the 101 section on troubleshooting.
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Old August 26th, 2008, 07:36   #5
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Intake plugged?
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Old August 26th, 2008, 11:00   #6
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We tried unplugging the MAF already (forgot to mention that) and the car runs worse for sure. from what i can tell that means the MAF isn't bad, correct?

i'll have him check vacuum lines.

we've monitored the map sensor and it definately makes boost when it should. the car isn't slow and seems to run great, minus the poor mileage.

intake plugged is possible, but i thought that would manifest itself in the MAF not being able to get to 800 on a WOT run?

i am getting a clean intake manifold for my car and once i swap it we're going to clean mine and move it over to his car.

any comments on the data i posted about timing, etc?
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Old August 26th, 2008, 11:51   #7
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Do you have a Scangauge?
That would make things alot easier.

Do you drive his car and get same bad mileage as him?
What if he drives your car (if he drives standard)?

Same size tires and the pressure is the same as yours?
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Old August 26th, 2008, 12:19   #8
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we both have scanguages..

we haven't switched drivers, but we have followed eachother on some trips to compare data while driving. we are going the same speeds and basically accellerating in a similar fashion.

we both have the same size tires. air pressure is within a few pounds between cars.

our scanguages seem to accurately reflect the mileage differences we are seeing.

he is trying to post in this thread but i think he didn't activate his account. he's emailed the admin so hopefully he'll get that resolved and will be able to comment as well.

Quote:
Originally Posted by tttthumper
Do you have a Scangauge?
That would make things alot easier.

Do you drive his car and get same bad mileage as him?
What if he drives your car (if he drives standard)?

Same size tires and the pressure is the same as yours?
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14 Passat SE 6-Speed
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Old August 26th, 2008, 18:26   #9
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regarding his scangauge, we swapped MAF's today and no change. if he disconnects his MAF his car has much less power, but crusing down the freeway at 65MPH nets him about 20 MORE MPG according to the scanguage.

what does that mean? also the load value on the scangauge is lower.
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03 Golf 1 Micron Fuel Filter Kit and Nozzles, , DC Stage 4 Clutch, 6-Speed, Peloquin
14 Passat SE 6-Speed
02 Chevy Duramax Nicktane 1 Micron Fuel Filter Kit 800+HP #2 only, TTS, II, J&H
ARP Head Stud TDICLUB Special - $150 Shipped
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Old August 26th, 2008, 21:31   #10
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Nick , thanks for helping me with my poor fuel economy problem,, so .....
I purchased a 2003 jetta 77k miles auto trans 1 owner last month
runs good , seems to have ok power,, pulls through entire rpm range
checked fuel consumption on long highway trip using pad and pen, 28mgp!
replaced fuel and air filter ,checked vaccume lines , unplugged (ran poor with unplugged) mas air sensor ., borrowed nicks mas and no change to mgp using scan guage 28 mpg at 60 mph with no a/c
now here's where it gets interesting !!!! I forgot to plug the mas sensor in today when leaving nicks house and guess what -- 45 mpg at 60 mph using scan guage! I did get p0102 (mas air error ) while drive the 20 miles home but 45 mpg !!!! so... i plugged in the sensor and did a test drive on the freeway at 60 mph , now back to 28/29 mpg for a 20 mile run,, so... unplugged the mas and back to 45 mpg !!
Here is a bit more info , lod at idle in park w/no a/c i get 24 lod
unplug the mas and i get at idle in park w/no a/c lod of 18
Everything seems better (lod /mpg ) with mas unplugged ,except much less power.
I dont know if this info helps, but thanks in advance as i am a new tdi owner
Thanks also to my new friend Nick who is also an new tdi owner who just happens to live in Sacramento and is one hell of a guy!
Sorry for the long reply ,but the belt and tensioner were also replaced (2 months ago ) at the dealer before i purchased the car
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Old August 27th, 2008, 05:53   #11
bayshorecs
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Talking out of my a$$ here, but just expanding on a thought...

- you swapped the MAF with a known working MAF. Same results, low MPG. This rules out the MAF itself.
- you unplug the MAF so the ECU no longer reads the measured values and defaults to control values, MPG increases. This means there is something air related.
- You cleaned the snowscreen and air filter
- You have not checked/cleaned the intake

Sounds like you have an air restriction to me. I would clean the intake and maybe the intercooler to be sure you are getting good air flow to the motor. There may be enough air flow to breath well enough with the MAF unplugged, but with it plugged in, the car is reading the true values and trying to compensate.

Could be COMPLETELY wrong, but the intake cleaning is what I would try next.
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Old August 27th, 2008, 06:13   #12
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Quote:
Originally Posted by bayshorecs
Talking out of my a$$ here, but just expanding on a thought...

- you swapped the MAF with a known working MAF. Same results, low MPG. This rules out the MAF itself.
- you unplug the MAF so the ECU no longer reads the measured values and defaults to control values, MPG increases. This means there is something air related.
- You cleaned the snowscreen and air filter
- You have not checked/cleaned the intake

Sounds like you have an air restriction to me. I would clean the intake and maybe the intercooler to be sure you are getting good air flow to the motor. There may be enough air flow to breath well enough with the MAF unplugged, but with it plugged in, the car is reading the true values and trying to compensate.

Could be COMPLETELY wrong, but the intake cleaning is what I would try next.

agreed, also check the intake where the egr connects, could be fully of carbon
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Old August 27th, 2008, 06:45   #13
riverman
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thanks for the thought ,, i feel as if i am talking out of my a$$ when it comes to a diesel... i can repair most problems with my wifes subaru but the jetta is a different story
I told Nick last night ," I have never worked so hard to repair a car that runs so great " usually when the subaru is having problems it acts up ,kicks,sputters and a diagnosis is very simple
My jetta runs great,shifts great and has a fair amount of power, but 28 mpg on the highway is what the gasser will do !
Next week i will clean the intake and get back to you guys with the results.
Mabey unplugging the MAF is a good way to check if your manifold it blocked !
Thanks to everyone
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Old August 27th, 2008, 07:37   #14
tttthumper
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Just my $0.02 worth.
Would the intake get that clogged up after only 74k?

Great troubleshooting, it's always great to know that your narrowing down the problem, and say...... Nope it's not that or that, that's working etc.
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RIP 08/05/2009 - 2003 TDI Jetta 4dr 5spd - Galactic Blue, Alligator Tune, Scangauge II, Ventectomy, Garmin Etrex Cx, PowerService, 18" Moda R6 w/ Triangle Talons, 1500W Zerostart, Defender Skid plate
Best Tank(s) 4.267LHK (55.44 mpg) - 1312kms (820 miles)
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Old August 27th, 2008, 07:59   #15
bayshorecs
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Depends on the driver.

Once a problem of clogging starts, you get more and more restriction causing the air flow lower and lower which will increase the clogging rate.

Basic rule of thumb on diesels before any other troubleshooting. You need:

Air
Fuel
Compression

Without knowing those 3 are good, you are "pi$$ing in the wind".
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