Limp mode: What is it, what are the causes, and how to fix it
for the VW diesel turbo TDI
back to 1000q: general turbodiesel FAQ and general "how
to" index
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
Introduction
Limp mode is when the car suddenly loses power while driving. When you request more power, you notice a lack of power as if the car is being held back. You may not notice it if you are on a level road with light engine load because this may not be enough to trip you into limp mode. The key symptoms of limp mode are turning off the car restores power and being triggered through mid-hard driving such as going uphill or highway driving. It is not when you start the car and it shuts off right away. If the engine shuts off after 2 seconds and you see the immobilizer car on the instrument panel, see 1000q: immo FAQ .
It can be caused a few ways but the direct reason it happens is because the car's computer senses a problem and puts the car into limp mode to prevent damage. The effect is that you lose the power from the turbo, making it feel as if you lost power right when you need it. See: 1000q: boost and vacuum leak testing , 1000q: turbocharging FAQ, and 1000q: MAF FAQ.
Troubleshooting steps for the VW TDI engine
As with anything else, first scan for codes and see what you have. If you don't have a code scanner, many auto parts stores will do it for free. Here are some common codes that show up from limp mode.
Boost Deviation Intermittent,
Boost Deviation Negative,
Boost Deviation Positive, intake manifold pressure xxxxx
Boost Deviation Intermittent means you most likely have a vacuum leak somewhere.
Check all lines, actuators, and plastic fittings. There are vacuum lines
that connect the turbo, car computer, sensors, etc., and they tend to get frayed
at the ends or rub through. Even a tiny hole will throw you into limp
mode.
Boost Deviation Negative means you have a clogged intake, 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.
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.
The A3 and B4 models (pre 1999 jetta/passat TDI) have a vacuum line inside of the
ECU. The plastic inside may be fragile
so see 1000q:
ecu hose for a how to and pictures and a diagram of the vacuum line routing.
Also 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.
How do these cause limp mode?
Basically, the car's computer is monitoring air flow, EGR flow, fuel flow, etc., and is expecting certain values. If a sensor fails or sends a signal that is "out of bounds", the car's computer will fall back to a safety map to prevent damage.
Again, first scan the car for codes. 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.
If the fuel filter has not been changed in the last 20,000 miles, it's possible that it is clogged and is restricting fuel. It's possible that a bad batch of biodiesel or diesel fuel has also clogged your fuel filter. If you have a high mileage car, using a high percentage of biodiesel can loosen built up deposits that end up clogging the fuel filter.
The N75 solenoid is the solenoid that controls the turbo wastegate or VNT vanes if you have a VNT turbo. It could be causing an underboost or overboost situation which sets you into limp mode. The way it normally works is by controlling air pressure/vacuum to the ECU. Once boost pressure gets to a certain point, the ECU activates the N75 solenoid and the vacuum/pressure in the vacuum lines actuates wastegate/VNT actuator. This is why vacuum lines should be checked in addition to solenoid/electrical faults.
During limp mode, the ECU sees a spike in the sensors or some other problem due to slow response or malfunctioning of this valve. You can try removing it and flushing it out with electronic cleaner. Once dry, try a squirt of WD40 inside to help the solenoid move freely. 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
Note that if you have an mk4 or mk5 body style, your car uses a VNT turbo. Have a helper turn the ignition on and off, the VNT actuator lever on the turbo should move. It is analogous to the wastegate arm on earlier cars. You can also use a vag-com cable to cycle the N75 using their software. With the engine running, go to group 011. Go to "basic settings". Click this on and you should see the n75 cycle the VNT arm. These cars also have a problem with soot and carbon clogging the VNT vanes. On a regular basis, 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 out of the turbo VNT vanes and help prevent them from getting stuck. Only do this once the engine is fully warmed up and let the car cool down with a period of normal driving afterwards. This only applies to mk4+ cars (1998+ new beeltle, 1999+ jetta, golf, passat), as earlier TDI use a conventional turbo.
If you find that the VNT actuator is faulty and can't be fixed with the above "hard run method", you can buy one at either vendor, click the links to compare pricing, shipping, tax, etc. kermatdi VNT actuator, tdiparts VNT actuator. If the lever arm is sticking, lubricate it or replace. This often leads to the entire turbo being replaced by the dealer because it's more profitable than replacing just the actuator. Here is the TSB and "how to" article : 1000q: replacing turbo actuator.
Another test is to swap the EGR solenoid with the n75 and try it again as a temporary test for the N75 solenoid. Note that it is possible for the EGR solenoid to cause a limp mode problem but this is not as common as the other problems, cleaning it out may be the solution. This could also indicate a faulty wastegate/VNT actuator but this is more rare than an N75 or vacuum line problem. It is possible to buy just the actuator if you look around but again, this would be an unusual problem.
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 1/4" 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.
What it probably isn't
It probably isn't the car's computer. Although possible, it is 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. And although it is rare, the mk4 ALH engine (4th gen TDI in the US) has had some reports of confirmed failed ECU.
It probably isn't the transmission or clutch. If the RPM goes up but the speed stays the same or doesn't increase with RPM, then it could be a bad/slipping clutch or transmission. If the RPM is as slow as the 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.
It probably isn't a bad turbo. Although soot buildup can clog the turbo and cause the VNT vanes to stick (only applies to mk4+ cars), 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 exhaust housing and manually clean out the soot. This is not for people without at least a basic level of mechanic experience.
It's never the glow plugs or glow plug harness, these don't have any effect on power.
It could be the MAF/MAS air flow sensor contributing to the problem. See 1000q: MAF FAQ or 1000q: mk4 MAF replacement and troubleshooting for more details.
Do you know something that should be added to this article? Post your comments in the myturbodiesel.com forums