See 1000q: limp mode fix for the solution. Although you could be in permanent limp mode, it should be fairly obvious because engine error codes will be set and you may see the check engine light/malfunction indicator light. Read both articles since each situation has overlapping causes/solutions, this article is more for permanent low power without the sudden loss of power.
Since any cause of low power is possible in your specific case, this article is divided into more common, less common, and rare causes. The numbers are in no particular order, it's just for organization. Of course, if you have any error codes tripped or have a clue to the cause of low power you should address those causes first, so get the car scanned for error codes before throwing time and parts at it. This can be done for free at many auto parts stores. Further details for each each section are given in links to other articles. If you need more help, ask your question in the myturbodiesel forums.
If you have a problem that isn't caused by something obvious, you need a Ross tech VCDS cable. This is a laptop computer diagnostic cable to talk to the car's computer. Without it you cannot do the more advanced tests.
Note about generations - some generations have similar engines:
Mk3= 1996-1997 3rd generation Passat TDI or 1996-1999 3rd gen Jetta TDI
Mk4= 1998-2006 New Beetle, 1999-2005 Jetta, 1999-2006 Golf, 2004-2005 Passat TDI
Mk5= 2005.5-2010 Jetta TDI, 2009 Sportwagen
Mk6= 2010-2013 Golf TDI, 2011+ Jetta, 2010+ Sportwagen, 2012+ Passat
Remember, an engine needs fuel, air, and compression to run. Low power is related to a lack of one of these or a sensor problem making the computer thinking there's a lack of these. Any sensor problem could also be caused by a bad ground or broken/chaffed wire so also check every section of the wiring of the suspect sensor for breaks.
1. Bad MAF sensor - very likely cause on the mk4 TDI. Not common
on the mk3 TDI (1996-1999 Jetta/Passat). Early mk4
MAFs failed often, see 1000q: MAF FAQ and 1000q:
mk4 MAF for more details and the part numbers. Error codes normally do
show up with a faulty MAF since the signal degrades instead of going out
completely. Through VCDS, checking
MAF actual vs. specified at idle, high rpm, and high load will quickly show a
bad MAF or other problem causing a low MAF reading. Start the engine and
click on "engine", then click on "measuring blocks" in the
You should see the screen below. Hit
"up" until you see "group 3". Below is a bad MAF - the
specified value and actual values are off. Bad MAFs don't go over 400-500
because the signal degrades. Also check for faulty
wiring or a bad plug.
Below is a good MAF - the specified value and actual value are close and both
can reach 800+.
2. Clogged intake manifold - carbon buildup chokes the intake manifold, starving the engine of air. Only ultra low sulfur diesel is sold in North America now so there should be much less buildup in the future. Always use good quality synthetic engine oil on your TDI. See 1000q:mk3 intake cleaning, 1000q: a4 intake clean, or 1000q: a5 intake removal for the detailed procedure.
3. Anti shudder valve shut or almost shut (does not apply to mk3 TDI, more for mk4 TDI) - there is a spring loaded valve right before the intake manifold. Newer TDI use an electronic valve and are not as susceptible to sticking. If there is excess carbon buildup, it could shut in a partially closed position. See 1000q: intake manifold cleaning for detailed pictures and operation.
4. Clogged snowscreen/air filter - a clogged air filter will starve the engine of air. A clogged snowscreen (large debris air pre-filter) shouldn't block off all air unless the aux-intake flap is also clogged. See 1000q: mk3 air filter, 1000q: mk4 air filter for details on changing them.
5. Clogged fuel filter - change interval is 20,000 miles but biodiesel
use (cleans out old buildup) or bad fuel could clog it early, resulting in fuel
starvation. Algae or bacterial growth in the fuel tank could also clog the
lines. See 1000q: mk3 change fuel
filter, 1000q:mk4 fuel filter, or 1000q:
mk5-mk6 fuel filter for details on how to change it. Here is a video showing more about clogged fuel filters and pickups (The video may not specific to your model)
6. Boost leak - a cracked hose or loose connector lets measured air out. No air or major leaks = poor engine running or stuttering. A visual inspection may not reveal all the possible or hard to see spots where leaks can form. See 1000q: boost leak check for a foolproof method of finding all leaks. Common places for a boost leak to occur is at hose ends (the hoses pop off due to a loose connector or worn locking tab), the corners, or places where it rubs against something else. mk3 Jetta TDI tended to wear out the rubber hose before the intake manifold because it's close to the hot exhaust piping. mk4 Passat TDI tend to wear out one of the lower intake hoses because it rubs against a ground cable.
2004 and newer engines use a quick connect tab-spring connection. If
it's only locked on 1 end, it can pop off halfway. For some reason, mk4
engines tended to wear the tabs at the intake manifold (probably due to a looser
fit) which makes the hose pop out. If it's loose you can use an aftermarket
doggie collar to lock it in place or replace the hose. The BEW engine
hose is VW# 1j0 145 838 t. Other engines also have quick connect couplers at other
7. Hose inside ECU (mk3 TDI only, does not apply to mk4 or newer TDI) - this hose leaks and normally sets a check engine light, see 1000q: ecu hose for the fix.
8a. Vacuum lines to/from turbo and n75 solenoid - these dry out over
time and crack or can rub through. It's possible they are clogged. The n75 solenoid controls the turbo
wastegate or VNT vanes with either vacuum or pressure. b4 Passat - on
firewall above coolant reservoir, a3 Jetta - on pass side near air box, a4
Jetta/Golf - on firewall above brake fluid reservoir. Below are vacuum
line diagrams for the mk4 ALH (below left) and BEW (labeled BEW) engines.
8b. Problem with the n75 solenoid, VNT actuator, VNT vanes, or vacuum
lines. Only mk4 and newer have VNT. You should have already checked the vacuum lines, the below test
will inspect the entire system. Start the engine and through VCDS, click
"engine"-->"measuring blocks"-->hit "up"
until you reach "group 11". Compare Specified vs. Actual
MAP. This compares what's actually happening and being observed from the
boost sensor (barring a faulty sensor/plug/wire) to boost the computer is requesting (what should be
happening). They should be relatively close. If they are far off
this normally results in limp mode but it could also be contributing to the
problem. If you have a mk3 you have a conventional turbo but you can still
use this test to check the n75 solenoid, the wastegate, and vac lines.
However, wastegates are much less susceptible to sticking vs. VNT vanes.
The videos below show how it works. The lever on the outside is welded to
a lever inside the turbo housing. This is how it moves the VNT vanes.
See the below videos to see how smoothly and free the lever should move.
It should not stick or bind at all. Vacuum is being applied to the can,
The lever moves a ring and the ring moves the vanes. These vanes change
the angle and speed of exhaust hitting the turbine wheel. If these vanes
are sticking it will cause low power or limp mode, depending how how badly
they're stuck and at what angle.
Here is a Borg Warner turbo - it works in a similar fashion but the actuator moves in the opposite direction.
Below is a log of a turbo which was recently cleaned. Some initial spikes are normal but boost should level out.
If you're quickly stomping on the acceleator/gas pedal, expect more
spikes. If you're smoothly applying the accelerator pedal there should be
less spikes. A faulty log could also indicated a faulty
actuator, vacuum lines, or n75
8c. If the test shows poor response or no response at all, it could be sticky VNT vanes/actuator (mk4 and newer TDI only) - see 1000q: turbocharging for a description of what a VNT turbo is. The vanes or actuator can stick or fail to function, see 1000q: VNT actuator diagnosis and repair to check the actuator. The lever should move freely as shown in the above link and videos.
8d. If the actuator is fine, also check the n75 solenoid and vac lines. The n75 solenoid controls vacuum or boost to the vacuum line going to the turbo wastegate/VNT actuator. To test, apply voltage to the solenoid or swap with a known good unit. If you have a mk4 TDI, you can swap it with the EGR solenoid to test. Also check the plug for corrosion and the wiring harness for chaffing. If those are good, disconnect the VNT actuator rod and move the vanes by hand. If the vanes are stuck then remove the turbo and clean the inside of the exhaust housing to free then stuck vanes, see 1000q: turbo VNT vane cleaning and 1000q: turbo removal ALH BEW engine
If you need a new n75 solenoid, click the below links:
N75 1996-1999 mk3 body, VW #028-906-283-F (pierburg type n75)
N75 1998-1999 mk4 body, VW #1H0-906-627-A, for Up To Vin # 9 MX 999000
N75 1998-2005 mk4 body, VW #1J0-906-627-A, ALH and BHW engine, for From Vin # 9M Y 000 001, D shaped connector
9. Faulty injection pump's fuel injection quantity adjuster - these are occasionally set wrong from the factory or after seal replacement. It's also possible the fuel pump's internal quantity adjuster is faulty. Applies to 1996-2003 TDI only or TDI that use a Bosch VE injection pump (not pumpe duse or common rail). See 1000q: fuel injection quantity testing and adjustment and hammer mod for more details. Injection quantity should be 3-5 at idle and up to 36-38 at full throttle.
10. Exhaust leak. It's possible for the exhaust manifold or EGR hoses to leak out gasses that should be powering the turbo. This should give a sooty stain around the leaking area. The EGR recirculates exhaust gases to the intake to lower NOx emissions. The EGR hoses are metal but it's possible for them to leak at the gaskets or crack. There's a video of a loose EGR hose in the boost leak article mentioned earlier.
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Fuel pickup in the fuel tank......
Are you sure it's not......