Limp mode: What is it, what are the causes, and how to fix it for the Audi and VW TDI diesel turbo

1000q: mk3 jetta/passat "how to" index list
1000q: mk4 "how to" index and checklists
1000q: mk4 "how to" index and checklist for pumpe duse

1000q: mk5 Jetta TDI "how to" index

Introduction

Limp mode is when your VW or Audi TDI suddenly loses power while driving and is restored after you turn the car off and back on.  This article shows how to fix limp mode.

It feels as if the car is being held back when you request more power or as if the parking brake is on.   You might not notice it on a level road with light engine load because you don't need much power to drive slowly on a level road.  You're more likely to trigger/notice it on highway on ramps or going up hills because this requests more power/turbo boost.  The check engine light (CEL) or malfunction indicator light (MIL) on the instrument cluster doesn't always go off.  If you have always have low power without the sudden cut out feeling or the engine just won't rev high then read 1000q: constant low power on TDI.  Many causes and symptoms overlap so read both articles - this article focuses more on power suddenly cutting out instead of constant low power.

What isn't limp mode

Limp mode is not when the engine starts fine but suddenly shuts off after a second.  If this happens look for the immobilizer "yellow car" symbol on the instrument panel, see 1000q: immo FAQ.  If this is the case the anti theft immobilizer is shutting off the engine.  If the car shuts completely off well after 2 seconds it's not an immobilizer problem.  Check the air intake piping, fuel filter, fuel pickup in the fuel tank, and fuel shutoff solenoid at the injection pump (1996-2003 cars only, single black wire leading to solenoid).

It's not when you turn the key and the engine doesn't even try to start or turn over.  You have a low battery, ignition key, or starter problem.  See 1000q: starter check and diagnosis to see the fix if you have a mk3 or mk4 car.  (Or see 1000q: b5 Passat starter, 1000q: mk5 Jetta starter FAQ, if you have one of those cars.)

If the engine shuts off suddenly during normal driving and you don't see the glow plug light go on during failed restarts then replace the black relay 109 with the newer gray relay 109 (1996-2003 cars only, it's under the steering wheel in the relay panel).  It sometimes cools and lets you restart.  The problem with black relay 109 was bad soldering.  This relay controls power to the ECU and prevents the glow plug light from coming on at all during failed restarts.  

If the engine stays at a constant 1200 RPM and won't go higher your accelerator pedal or wiring could be faulty - this should trip an error code.  This could produce constant low power instead of limp mode but the error code and other symptoms should tip you off.  The pedal is drive by wire, meaning there's no cable attaching it to a throttle, it's just electrical wires and sensors.  In the mk3 TDI the pedal sensor is a potentiometer.  In the mk4 TDI there are 2 pedal sensors (g79 and g185 which are probably potentiometers), if one fails you should get an error code, if both fail the ECU will limit engine RPM to 1200.   This should be obvious when reading the accelerator pedal value through VCDS.  Turn the car on, plug in VCDS, and it will tell you how far the accelerator pedal is being pressed.  

If the car ran fine before but is suddenly bucking or jerking at a certain RPM range you probably have a loose/cracked intake piping.  It's subjected to heat and pressure on turbo cars and can crack, releasing the intake air.  A clip may have also released, letting a hose slip off.  Look around for loose hoses and then do a boost leak check.  See 1000q: boost leak testing for more details.  A major boost leak may result in hard starting but smooth but smoky idling.

If the engine RPM goes up but the car's speed stays the same or doesn't increase with RPM, then you have a bad/slipping clutch or transmission.  If the engine RPM stays as sluggish as the car's speed the transmission/clutch is probably fine.  See 1000q: clutch FAQ for more details on clutches used on the TDI and the dual mass clutch.  If you have a DSG transmission and the PNDRL "shifter" gear selector lights are all on or blinking, you have a low battery (also check voltage regulator or alternator).  Some DSG are starting to show defective transmission temp sensors which cause false neutrals. 

It's never the glow plugs or glow plug harness, these don't have any effect on power.

If the car's top speed is 20 mph, there are other major problems.  Limp mode should still let you slowly accelerate to 40-50 mph.

Why is there a limp mode?

The car's computer is constantly monitoring sensors such as air flow, EGR flow, fuel flow, etc., and is expecting certain values.  If a sensor sends a signal that is "out of bounds" through malfunction or failure, the car's computer will fall back to a safety map or cut fueling and turbo boost to prevent damage.  This is because overboosting or overfueling could cause engine damage.  A diesel is built more sturdy than a gasoline engine and boost spikes are normal in a diesel.  Even still, any engine has limits so the computer pulls back engine power to prevent these limits from being exceeded.

The effect is that you lose the power from the turbo or don't get fuel, making it feel as if you lost the turbo or lost all power right when you need it.  Also see: 1000q: boost and vacuum leak testing , 1000q: turbocharging FAQ, and 1000q: MAF FAQ for some more background information.  Again, if you have constant low power or can't rev the engine high, also read 1000q: constant low power on TDI.

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 sedan and wagon
Mk6= 2010 Golf, Jetta wagon, and 2011 Jetta TDI

Procedure - Troubleshooting steps for the VW or Audi TDI engine

With any problem on a modern car, first scan the car's computer for error codes and see what you have.  If you don't have a code scanner, many auto parts stores will scan it for free.  To access sensor readings or for problems that aren't caused by something obvious, you need a Ross tech VCDS cable.  This is a VW Audi specific laptop computer diagnostic cable to talk to the car computer.  It is much more advanced than what the generic error code scanners can do and without it you can't do the more advanced tests.  Below are some of the more common codes that show up from limp mode.

Then inspect all the vacuum lines for worn ends or split corners.  Also inspect boost hoses for loose couplers or do a boost leak test.  The reason boost leak test is also listed above is because sometimes it causes limp mode and sometimes it causes stumbling and bucking.

Since any cause of limp power is possible in your exact 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 what's causing limp mode you should address those causes first.  If there is an article showing more details, it will be linked from this page, otherwise ask your question in the myturbodiesel forums.  Touareg troubleshooting is not included here but the basic principles are the same and I think that many of the codes will be caused by the same problems.


More common causes of limp mode

1.  Boost Deviation Intermittent, Boost Deviation Negative, Boost Deviation Positive, intake manifold pressure xxxxx, charge pressure control deviation

These codes show a problem with the system that senses turbo boost and controls the turbo.  Most of the time, a sudden cut of power is related to sensor or turbo problems.  These codes generally show an underboost or overboost situation which sets you into limp mode.  The N75 solenoid is the solenoid that controls pressure to the turbo wastegate (mk3 TDI) or vacuum to the VNT actuator (mk4, mk5, mk6 TDI).  Here's how it works: once the intake manifold pressure sensor shows boost pressure reaching the desired value, the ECU activates the N75 solenoid which fluctuates vac/pressure to the wastegate/VNT actuator to stabilize the boost pressure at the desired value.  If the vacuum lines are broken or the solenoid is bad, it won't control pressure which causes a problem which the ECU sees.  You can try removing the N75 and flushing it out with electronic cleaner.  Once dry, try a squirt of WD40 inside to help the solenoid move freely.

Check the wiring harness grounds for corrosion or breaks.  Bad grounds can cause mysterious electrical problems.  Also check the wiring/connector for breaks or corrosion coming to and from the N75 valve.   If that doesn't work, N75 replacement may be needed, see below for the part numbers.  The N75 in the 1996-97 passat is on the firewall next to the coolant reservoir.  The 1996-1999 Jetta's N75 is in the engine bay on the passenger side fender by the fuel filter.  The 1998-2006 mk4 generation N75 is on the firewall behind the accordion-like hose coming from the airbox.

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

If you have an mk4, swap the EGR solenoid with the n75 solenoid as a temporary test for the N75.  It's possible for the EGR solenoid to cause limp mode but this is not as common as the other problems, cleaning it out may be the solution.  Again, check the vacuum lines first.

1a.  Boost Deviation Intermittent means you most likely have a vacuum leak somewhere.  Check all vacuum lines, turbo actuator, and plastic fittings.  There are vacuum lines that connect the turbo, car computer, sensors, etc., and tend to get frayed at the ends or rub through.  

1b. Boost Deviation Negative means you have a clogged intake, turbo actuator that's leaking, bad vacuum lines, or a sticky turbo actuator lever.  If you have a VNT turbo, it may be because of stuck VNT vanes which requires removal and cleaning of the vanes.  See the below section for related causes.

1c.  Boost Deviation Positive means you have a N75 that's bad/malfunctioning, actuator that's leaking, bad vacuum lines, or a sticky turbo VNT or wastegate lever.  If you have a VNT turbo, it may require removal and cleaning of the vanes.  See the below section for related causes.

For mk3 cars only:  The A3 and B4 models (1996- 1999 jetta/passat TDI) have a vacuum line inside of the ECU.  If you get error 65535 - Internal Control Module Memory Error and some boost codes, it's most likely this hose.  The plastic inside may be fragile so see 1000q: ecu hose for a "how to", pictures, and a diagram of the vacuum line routing.  Later cars do not have this internal line.  These cars do not have a VNT and the wastegate is fairly reliable.  To test the wastegate actuator, apply pressure to the vac line going to the wastegate can.  The wastegate arm should move.  If it doesn't, it indicates a problem with the actuator or vacuum line.

For mk4 and mk5 cars only:  The turbo actuator or turbo vanes can get stuck.  See 1000q: VNT actuator check and removal and 1000q: turbo cleaning for more details and the below section.  Below are vacuum line diagrams for the mk4 ALH (below left) and BEW (labeled BEW) engines.  (1Z and AHU engine vacuum diagram was in the ecu hose article linked above).  There should be stickers above the radiator support that have vacuum diagrams there too.

2.  Bad/sticking VNT actuator or turbo vanes

If you have an mk4 or newer TDI, your car uses a VNT turbo.  This section does not apply to the mk3 TDI.  See 1000q: turbocharging FAQ to see exactly what a VNT turbo is.  The actuator or adjustable vanes inside the turbo can stick or go bad from soot and carbon clogging the VNT vanes.  On a regular basis like highway on ramps, you should rev the engine high and hard to raise exhaust gas temperatures (EGT).  Raising EGT will burn up and blow out carbon and soot build up in the exhaust system and turbo.  Only do this once the engine is fully warmed up and let the car cool down with a period of normal driving afterwards.  More detailed procedures and pictures here: 1000q: VNT actuator check and removal and diagnosis TDI.  This link gives instructions on how exactly to check it.  Again, the mk3 TDI uses a wastegate, use pressure to check it instead of vacuum.

If it's sticking, try to free it up by lubricating it, manually moving it, or by giving it a hard run.  This may burn up or loosen any sticking bits inside.  Give it a blast with PB Blaster or another penetrating lubricant.  PB Blaster eats rust more than WD-40.  This might only provide a temporary solution.  If you read the above link on detailed diagnosis and need a new one, click the links here to compare current pricing, shipping, tax, etc.: kermatdi VNT actuator, idparts VNT actuator.  Dealers don't normally stock this part or sell it because it's more profitable to sell an entire turbo and selling an entire turbo also fixes/rules out any internal turbo problem and fixes the problem in 1 visit.

If you're lucky the VNT actuator or vacuum lines are leaking or the N75 valve is bad.  If not then the turbo vanes or lever are stuck and the turbo should be removed for disassembly and cleaning.  See 1000q: turbo removal ALH/BEW engine.

The videos below show how the VNT actuator and vane work.  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, not pressure.  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.
 

If you have a pumpe duse Jetta, Golf, or New Beetle (2004-2006), your car has a smart VNT actuator that has a sensor to check its position, pictured right.  The wires for the sensor tend to break or rub through.  2004-2005 VW Passat TDI and earlier cars don't have this sensor.

3.  Check for intake piping that is loose, not clamped correctly, or split hoses.  See 1000q: boost leak check for more details.   A fault in intake manifold pressure could indicate this or worn vacuum lines.  Often, intake hoses will split due to age or rubbing/proximity to exhaust piping.  Mk3 Jetta tend to have the rubber elbow hose before the intake manifold split on the far side where you can't see it.  If you worked on the car recently did you attach and tighten all piping?  Did someone stuff paper towels into the intake piping and forget them?

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 snap hose couplers.  These quick connect tab-spring connections can pop off if it's not tight or wear down the locking tabs.  If it's only locked on 1 end, it can pop off halfway and let boost out.  For some reason, mk4 engines tend to wear the tabs at the intake manifold (probably due to a looser fit) which made 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 (from worldimpex).

4.  If the fuel filter has not been changed in the last 20,000 miles, it's possible that it's clogged and is restricting fuel.  A bad batch of biodiesel or diesel fuel could clog the fuel filter.  If you have a high mileage car, using a high percentage of biodiesel can loosen built up deposits that travel through the fuel lines and clog the fuel filter.  If your fuel injectors have been sitting dry outside of the car, diesel fuel can go bad and gum up the tiny components inside the fuel injector.

5.  The intake could be so clogged that it is causing limp mode.  Remove the intake piping before the intake manifold and look with a flashlight.  If you've never seen a clogged intake, you will be surprised at the amount of carbon buildup in the intake manifold.  Up to 1/5-1/6" of buildup on the inside is normal.  Please refer to 1000q: mk3 intake cleaning and removal, 1000q: mk4 ALH intake cleaning, or 1000q: mk5 intake and EGR cleaning.  Mk3 and mk4 cars are more prone to carbon buildup due to design and EGR metering.  Now that all fuel sold in North America (not Mexico) is ultra low sulfur diesel (USLD) only, clogging problems should be much less.

6.  It could be the MAF air flow sensor contributing to the problem although this normally isn't the sole cause.  See 1000q: MAF FAQ or 1000q: MAF testing, replacement and troubleshooting for more details on mk4 and newer cars).  And see 1000q: diagnosing constant low power - MAF problems could be causing both constant low power and triggering limp mode.  This problem is common on mk4 and newer cars but rare on mk3 TDI.  A bad MAF does not normally trip an error code.


Less common causes of limp mode

1.  If it's near freezing, is the fuel gelled?  Since diesel fuel is an oil, it waxes over and gels if it's cold enough.  Biodiesel gels at a higher temperature.  Gel temperatures vary according to additives put in the fuel by the retailer and if the car was preheated.  It's possible the fuel is also bad but this shouldn't cause limp mode, just poor running.  Some powerservice diesel fuel additive for low temperatures should lower the gel point and help un-gel the fuel.

2.  It's also possible algae or bacterial growth in the fuel tank has clogged the fuel pickup.  It will show up as black sludge on the fuel pickup in the fuel tank.  If there's clogging and growth, clean the fuel line all the way to the fuel filter and put some diesel fuel biocide in the fuel tank.  After the next tank, change the fuel filter.  Lift the rear seat and remove the access panel cover (3x phillips screws).  Unplug the fuel feed, return, and fuel gauge sensor and unscrew the retaining ring.  The fuel pickup will pull straight out.  Remove it and clean it since there could also be a blockage in the pickup's tubes.

3.  It probably isn't a bad turbo but it's possible.  Sticking VNT vanes or a bad actuator are more likely causes on an otherwise good turbo.  Sticking vanes are caused by soot buildup and corrosion inside the turbo (not applicable to mk3 TDI).  Repeated heavy and high rpm use should burn up the soot deposits and help prevent it from happening again.  If it's really bad and the above method doesn't help after repeated tries, you have to remove the turbo and manually clean out the soot.  This is not for people without at least a basic level of mechanic experience.  In some cases a worn turbo with excessive play will grind the turbo wheels against the housing.  This will cause a delay (followed by a spike) or just permanent low boost.  Many times a turbo will just break and pour oil if it get to this amount of wear.


Rare causes of limp mode

1.  Bad ECU - the car's computer.  Although possible, it's rarely the car's computer.  The mk3 TDI use a vacuum line inside the ECU and this is often misdiagnosed as a bad ECU at a cost of a $1300 replacement.  See 1000q: ecu hose for the fix.  This hose was not in later cars.  Although it's a rare failure, mk4 TDI ECU have had a few reports of confirmed failed ECU.  More likely is a bad sensor or faulty wiring somewhere between the sensor and ECU.  Also check for bad grounds.  The only way to 100% test it is to swap with a known good ECU.  For newer cars with an immobilizer you must have access to the SKC to swap the ECU.  

2.  Bad injection pump or bad turbo.  It's possible a replaceable component has failed.  Do more tests before getting a new/rebuilt injection pump or turbo.

3Coolant migration - this is a rare but very bad problem where a faulty coolant reservoir tank leaks coolant through the wiring harness through capillary action.  This will ruin the electrical parts of the car.  This never happens on mk3 TDI but is possible on mk4 and early mk5 TDI.  Late mk5 TDI had a new coolant tank design and should not see this problem.

4.  Was the timing belt just done?  Did they use the correct TDI specific tools?  Diesels are sensitive to cam and injection pump timing.  2004-2006 TDI do not have adjustable injection pump timing but some had problems with soft camshafts.  Remove the valve cover and look for excess camshaft wear.  For mk3, see 1000q: mk3 TDI timing belt part 1 , for mk4 ALH engine see 1000q: mk4 ALH engine timing belt 1.  For the mk5 BEW engine see 1000q: a5 VW Jetta TDI timing belt part1.  For the mk4 BHW TDI engine in the Passat, see 1000q: BHW engine timing belt.

5.  Other sensor or electronic problem contributing to the problem - In measuring group 7, you can see fuel temp, intake air temp, and coolant temp.  Below are normal readings on a warmed up ALH engine.  Let the engine cool overnight and check these values again while the car is off and fully cooled - they should all read ambient temperature and be about the same.

6.  Going back to the N75/vac line/VNT actuator problems which are more common, it's rare to have a vacuum system failure since it should produce other noticeable problems.  If the vacuum pump at the end of the camshaft/tandem pump on pumpe duse engines fails, you won't have vacuum pressure and the actuator system won't work.  If you don't have vacuum at all then the brake pedal will feel very hard since the brake booster also works on vacuum.  It's possible the pump is bad or the vacuum line between the pump and other components are bad but you already checked the lines in "more common causes".  There are some lines going from the vacuum pump to the solenoids  so just check those.

7.  I believe that it should result in constant low power instead of limp mode, but if your diesel particulate filter (DPF) is clogged, it could result in sudden power loss.  The DPF is equipped only on 2009 or newer TDI (or 2006-2008 Touareg V10 TDI).  There are warning lights and messages on the instrument cluster if this is the case.  Earlier cars had no DPF.  See 1000q: VW and Audi DPF filter and Adblue FAQ for more details on this system.

Do you know of more causes of limp mode?  Please add your knowledge by posting your case in the myturbodiesel.com forums, thank you!