03 Jetta tdi 34mpg AT BEST

josh higbee

Member
Joined
Feb 6, 2022
Location
Phoenix, AZ
TDI
03 Jetta 1.9L TDI 5spd
03 Jetta tdi 5 spd picked up a few months ago bone stock with shifting at 3k rpm’s max mixed hwy and city gets 31-34. Best mpg has been 34 here are a list of things that have been replaced since I’ve had it

all vacuum lines, n75, maf, air filter, fuel filter, fuel lines ( in the engine bay), cat delete, cleaned intake and egr, turbo actuator (verified correct positioning).

I have zero maintenance records on the car so not sure what has all been done to it. Brakes don’t seem to drag and yes I have yet to get the timing belt done. Only mod to the car is a tune from kerma. Any insight would be appreciated.
 

GlowBugTDI

Veteran Member
Joined
Jul 20, 2018
Location
Cambridge, MN
TDI
2001 Beetle GLS TDI (BIODSL). Glow Bug TDI (sold)
Do you know what tune?

Also, have you checked to see if your brakes were dragging by actually spinning the tire in the air?
My bug gets 38hwy, with a lil city and hard driving here and there.
 

dhangejr

Veteran Member
Joined
Mar 1, 2017
Location
PNW is my home
TDI
mk4 Jetta
First guess…Clogged intake manifold. Pop off the ASV and have a look

Bad Injector? VCDS Chanel 13 could try some diesel purge, but check the intake first. .


dragging breaks PIA to diagnose
Stick veins also PIA die to e clip both are less likely I guess.
Leaking fuel ? Also annoying to find.
 

josh higbee

Member
Joined
Feb 6, 2022
Location
Phoenix, AZ
TDI
03 Jetta 1.9L TDI 5spd
Do you know what tune?

Also, have you checked to see if your brakes were dragging by actually spinning the tire in the air?
My bug gets 38hwy, with a lil city and hard driving here and there.
Tune is just a basic one from kerma. Downloaded my file, sent it to them, and they returned a new file for just a basic tune. Tires were off the ground when looking for dragging.
 

josh higbee

Member
Joined
Feb 6, 2022
Location
Phoenix, AZ
TDI
03 Jetta 1.9L TDI 5spd
First guess…Clogged intake manifold. Pop off the ASV and have a look

Bad Injector? VCDS Chanel 13 could try some diesel purge, but check the intake first. .


dragging breaks PIA to diagnose
Stick veins also PIA die to e clip both are less likely I guess.
Leaking fuel ? Also annoying to find.
I’m thinking it may be an injector issue. Intake has been removed and thoroughly clean. Does have 210k miles so I’m thinking it may be an injector issue. Also has a strong smell of diesel from behind the car even before the cat was removed. Have run some cleaner additive for the injectors and haven’t noticed much
 

Zak99b5

Veteran Member
Joined
Apr 30, 2021
Location
Albany NY
TDI
2003 Jetta TDI
Was the injector cleaner added to the fuel tank or run straight (recirculated) to the IP?
 

Zak99b5

Veteran Member
Joined
Apr 30, 2021
Location
Albany NY
TDI
2003 Jetta TDI
I'd try running a can of straight Diesel Purge through the IP. FIlter in the supply line and all.

In the fuel tank it gets pretty diluted and won't clean as well.
 

turbobrick240

Top Post Dawg
Joined
Nov 18, 2014
Location
maine
TDI
2011 vw golf tdi(gone to greener pastures), 2001 ford f250 powerstroke
My guess would be a poor spray pattern from gunked up injectors. If the brakes are dragging or a wheel bearing is bad you can diagnose by going for a brisk drive without stops and coasting to a stop, then feeling the hubs for excess heat.
 

Bobo4255

Active member
Joined
Nov 25, 2009
Location
Charlotte, NC
TDI
2003 Golf GLS TDI Manual
And change the coolant temp sensor and/or check the connector wiring. I had a bit of damaged wiring going to this sensor (behind the vacuum pump facing firewall), and repairing the wiring was all I needed. But if this sensor is bad, you could be over fueling constantly. Also agree with others, injectors have to get done, too, given age/mileage.
 

hooville

Veteran Member
Joined
Jan 13, 2003
Location
Marietta, GA
TDI
2000 Golf GLS, 2016 GTI, 2015 TDI Sportwagen S
A bad thermostat will also result in low MPG. It's summer in Phoenix, so the temp gauge should get up to the 190° mark and stick there in around 10 miles or less I would expect. If not the thermostat may not be closing completely or opening too far at a given temp. I installed a non OEM thermostat in the Golf and it started failing in under 6K miles: taking 15-20 mins to come to temp, and then not getting to temp at all... MPG dropped to 35-37 and is now back to 43+.
Not a hard job, but plan on replacing 2-3 liters of coolant, plastic housing and o-ring as well. Since it's best to replace the water pump when doing a timing belt that would be the ideal time to replace the thermostat since it looks like that's on the docket soon!
 
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