Canadian_Grizzly
Veteran Member
Diagnosing and Fixing Limp mode for A4 1.9TDI
By Canadian Grizzly
Read the entire post prior to jumping to conclusions.
This is a general overview on how to diagnose and fix a A4 1.9TDI that goes into “limp” mode due to an overboost of the turbo. While I’m by no means an expert I’ve gleaned this information through information on this forum and my own experiences.
Limp mode happens when you are accelerating, often up a hill, or passing and the car suddenly seems to lose all power. Power is immediately restored when you cycle the ignition. What has happened is the computer has sensed that the turbo has created too much boost and the computer shuts it off and goes into safe mode.
There are a number of different reasons why the car may overboost… some being very simple to verify and fix and others more complex. The majority of overboosts are created by:
Bad Maf (Oxygen Sensor)
Leak in the vacuum system
Faulty N75
Bad actuator
Dirty gummed up turbo causing variable vanes to jam or stick
Some members have experienced overboost due to coked up intakes as well. This is a maintenance issue. See below for links.
If it only occurs occasionally try to find a long uphill slope on the highway and try to accelerate and reproduce limp mode. This will help later to see if the problem is resolved when you try again with similar circumstances.
Bad MAF
One of the easiest to check...but you will get a CEL (Check Engine Light). All you have to do is disconnect the MAF and give it a test drive. If power is same or improved, replace. Read this thread for one example... http://forums.tdiclub.com/showthread.php?t=215165
Vacuum Leaks
Most problems with the turbos are simple leaks in the vacuum lines. It is hard to see on the cloth covered stock hoses and if your car is past warranty it is best to simply replace all the lines. Here is a link of what you need… http://forums.tdiclub.com/showpost.php?p=1566427&postcount=30 Note to Canadians... mcmaster won't ship to us so I recommend sourcing the lines (and any other necessary parts) from Brian at Roseland http://www3.ns.sympatico.ca/roseland/VWPartsA4.htm
Be very systematic. Remove one hose, cut new one to length and replace that one hose and so on. That way you won’t screw up. When removing lines carefully work them off with a screw driver or carefully slice them with a razor and then slide off. Be especially careful if its cold. When you encounter a check valve ensure that it is also working correctly.
Also check the large vacuum line on brake booster…if you have experienced any hard brake pedal situations that will likely be the culprit. Ensure that the hose to the bottom of the airbox is plugged in as well, this one is commonly overlooked. If you haven’t replaced the hoses and think you have ruled out a leak… start checking again as this is the single most common reason for most overboost situations.
The top line on the N75, labeled 'vac' is where unregulated vacuum is drawn. Regulated vacuum is pulled through the larger 5mm ID line labeled 'out` The 5mm ID line opposite the other lines is the vent which vents the regulated line to atmospheric pressure.
Even if the N75 is good, it it doesn't have solid vacuum getting to it (top hose 'vac' ) it will not send vacuum properly out the 'out' port just below the vac port. It is also important that the vent port is clear. It's on the same side of the valve as the connector.
2 test options posted by Anut:
1. disconnect the 'vac' hose. Connect a mighty vac to the hose. Start the car. You should get >25 inHg vacuum.
2. connect a vacuum hose directly from the rigid brake booster vacuum line to the 'vac' port on the N75. You'll get an EGR code, but your turbo oughta work great.
If 1 doesn't occur, of if 2 fixes the problem; find the vacuum leak. Vacuum is drawn from the rigid line. The line splits twice such that you have 4 unregulated vac lines. One to the N18 (EGR), one to the N75 (turbo), one to the vacuum reservoir and one to the N239 (anti-shudder valve).
Faulty N75 solenoid
The N75 is the solenoid that adjusts the vanes on your turbo. On your car it will be on the firewall just to the left of the airbox. It has a grey top and some vacuum lines attached to it. If it is not working then you would be getting an overboost situation.
You can also check that the N75 is working and moving the actuator on the turbo. This is a little more difficult. Remove engine cover… use a mirror and a flashlight and watch or feel the actuator as another person puts the ignition key on. It should cycle and move properly. As in diagnosing many things on these cars a vagcom comes in very handy as you can ask the computer to cycle the N75 while you watch the actuator. With the engine running, in VAG-COM go to Group 011. Down in the lower left of that VAG-COM page is "basic settings." Click this on. You'll see the N75 go "off" and "on." Then, put a light down against the lower firewall (just in front of where the passenger's feet would be) and position a mirror so you can see the back of the turbo. As the N75 cycles, you'll see the VNT actuator rod move up and down, about 3/4" each way. If this test is ok, you can assume that the N75 is working ok.
If it doesn’t seem to work then swap the EGR solenoid, N18 (same looking part just to the left of the N75 and with black top not grey) with the N75 and try again.
Faulty Actuator
If this doesn’t solve then you either have a faulty actuator or stuck vanes in the turbo. If you can get your hands on a vacuum pump attach it to the actuator and see if it holds vacuum. If not then purchase a new actuator. Dealers normally don’t carry the actuator as a separate part as they want to sell you an entire turbo. These can be purchased by a number of vendors in the states and through Roseland in Canada.
Test the adjustment of the VNT actuator. It should start to move at 3-5 inHg, and be on the stop (fully open) at 18 inHg. Less vacuum necessary, and it will spike. More, and it takes too long to make boost, and drivability is poor.
Here is the TSB from VW on swapping and testing the actuator. http://pics.tdiclub.com/data/500/Actuator_TSB.pdf
Jason has a couple videos of a actuator in action. http://forums.tdiclub.com/showthread.php?t=330149
Sticking turbo vanes
Now failing all of the above it might be the hardest job which is cleaning the actual turbo. Note I said clean not replace. Here is the link thanks to Drivbiwire on how to accomplish this task… http://forums.tdiclub.com/showthread.php?t=140910
PLEASE NOTE: If you take it to a dealer they will diagnose a blown turbo and replace what is probably a perfectly good dirty turbo. If you are daunted by the task find a TDI Trusted Guru, look at the sticky in TDI 101 or go to a local GTG (get together) general discussed and planned the regional forum areas.
Other power sucking maintenance that should be performed.
Snowscreen cleaning… http://forums.tdiclub.com/showthread.php?p=1344400#post1344400 For Beetle http://forums.tdiclub.com/showthread.php?p=332954
Intake Manifold cleaning (just do it…really) http://pics2.tdiclub.com/gwillie/VW/Sballintake/INTAKEMAN101b.htm
Thanks to Fred
Without Fred and this forum none of this information sharing would be possible. If this thread was helpful and you saved mega bucks fixing or diagnosing the problem yourself, please consider a donation (see paypal button on home page) to ensure that this site remains available in the future.
By Canadian Grizzly
Read the entire post prior to jumping to conclusions.
This is a general overview on how to diagnose and fix a A4 1.9TDI that goes into “limp” mode due to an overboost of the turbo. While I’m by no means an expert I’ve gleaned this information through information on this forum and my own experiences.
Limp mode happens when you are accelerating, often up a hill, or passing and the car suddenly seems to lose all power. Power is immediately restored when you cycle the ignition. What has happened is the computer has sensed that the turbo has created too much boost and the computer shuts it off and goes into safe mode.
There are a number of different reasons why the car may overboost… some being very simple to verify and fix and others more complex. The majority of overboosts are created by:
Bad Maf (Oxygen Sensor)
Leak in the vacuum system
Faulty N75
Bad actuator
Dirty gummed up turbo causing variable vanes to jam or stick
Some members have experienced overboost due to coked up intakes as well. This is a maintenance issue. See below for links.
If it only occurs occasionally try to find a long uphill slope on the highway and try to accelerate and reproduce limp mode. This will help later to see if the problem is resolved when you try again with similar circumstances.
Bad MAF
One of the easiest to check...but you will get a CEL (Check Engine Light). All you have to do is disconnect the MAF and give it a test drive. If power is same or improved, replace. Read this thread for one example... http://forums.tdiclub.com/showthread.php?t=215165
Vacuum Leaks
Most problems with the turbos are simple leaks in the vacuum lines. It is hard to see on the cloth covered stock hoses and if your car is past warranty it is best to simply replace all the lines. Here is a link of what you need… http://forums.tdiclub.com/showpost.php?p=1566427&postcount=30 Note to Canadians... mcmaster won't ship to us so I recommend sourcing the lines (and any other necessary parts) from Brian at Roseland http://www3.ns.sympatico.ca/roseland/VWPartsA4.htm
Be very systematic. Remove one hose, cut new one to length and replace that one hose and so on. That way you won’t screw up. When removing lines carefully work them off with a screw driver or carefully slice them with a razor and then slide off. Be especially careful if its cold. When you encounter a check valve ensure that it is also working correctly.
Also check the large vacuum line on brake booster…if you have experienced any hard brake pedal situations that will likely be the culprit. Ensure that the hose to the bottom of the airbox is plugged in as well, this one is commonly overlooked. If you haven’t replaced the hoses and think you have ruled out a leak… start checking again as this is the single most common reason for most overboost situations.
The top line on the N75, labeled 'vac' is where unregulated vacuum is drawn. Regulated vacuum is pulled through the larger 5mm ID line labeled 'out` The 5mm ID line opposite the other lines is the vent which vents the regulated line to atmospheric pressure.
Even if the N75 is good, it it doesn't have solid vacuum getting to it (top hose 'vac' ) it will not send vacuum properly out the 'out' port just below the vac port. It is also important that the vent port is clear. It's on the same side of the valve as the connector.
2 test options posted by Anut:
1. disconnect the 'vac' hose. Connect a mighty vac to the hose. Start the car. You should get >25 inHg vacuum.
2. connect a vacuum hose directly from the rigid brake booster vacuum line to the 'vac' port on the N75. You'll get an EGR code, but your turbo oughta work great.
If 1 doesn't occur, of if 2 fixes the problem; find the vacuum leak. Vacuum is drawn from the rigid line. The line splits twice such that you have 4 unregulated vac lines. One to the N18 (EGR), one to the N75 (turbo), one to the vacuum reservoir and one to the N239 (anti-shudder valve).

Faulty N75 solenoid
The N75 is the solenoid that adjusts the vanes on your turbo. On your car it will be on the firewall just to the left of the airbox. It has a grey top and some vacuum lines attached to it. If it is not working then you would be getting an overboost situation.
You can also check that the N75 is working and moving the actuator on the turbo. This is a little more difficult. Remove engine cover… use a mirror and a flashlight and watch or feel the actuator as another person puts the ignition key on. It should cycle and move properly. As in diagnosing many things on these cars a vagcom comes in very handy as you can ask the computer to cycle the N75 while you watch the actuator. With the engine running, in VAG-COM go to Group 011. Down in the lower left of that VAG-COM page is "basic settings." Click this on. You'll see the N75 go "off" and "on." Then, put a light down against the lower firewall (just in front of where the passenger's feet would be) and position a mirror so you can see the back of the turbo. As the N75 cycles, you'll see the VNT actuator rod move up and down, about 3/4" each way. If this test is ok, you can assume that the N75 is working ok.
If it doesn’t seem to work then swap the EGR solenoid, N18 (same looking part just to the left of the N75 and with black top not grey) with the N75 and try again.
Faulty Actuator
If this doesn’t solve then you either have a faulty actuator or stuck vanes in the turbo. If you can get your hands on a vacuum pump attach it to the actuator and see if it holds vacuum. If not then purchase a new actuator. Dealers normally don’t carry the actuator as a separate part as they want to sell you an entire turbo. These can be purchased by a number of vendors in the states and through Roseland in Canada.
Test the adjustment of the VNT actuator. It should start to move at 3-5 inHg, and be on the stop (fully open) at 18 inHg. Less vacuum necessary, and it will spike. More, and it takes too long to make boost, and drivability is poor.
Here is the TSB from VW on swapping and testing the actuator. http://pics.tdiclub.com/data/500/Actuator_TSB.pdf
Jason has a couple videos of a actuator in action. http://forums.tdiclub.com/showthread.php?t=330149
Sticking turbo vanes
Now failing all of the above it might be the hardest job which is cleaning the actual turbo. Note I said clean not replace. Here is the link thanks to Drivbiwire on how to accomplish this task… http://forums.tdiclub.com/showthread.php?t=140910
PLEASE NOTE: If you take it to a dealer they will diagnose a blown turbo and replace what is probably a perfectly good dirty turbo. If you are daunted by the task find a TDI Trusted Guru, look at the sticky in TDI 101 or go to a local GTG (get together) general discussed and planned the regional forum areas.
Other power sucking maintenance that should be performed.
Snowscreen cleaning… http://forums.tdiclub.com/showthread.php?p=1344400#post1344400 For Beetle http://forums.tdiclub.com/showthread.php?p=332954
Intake Manifold cleaning (just do it…really) http://pics2.tdiclub.com/gwillie/VW/Sballintake/INTAKEMAN101b.htm
Thanks to Fred
Without Fred and this forum none of this information sharing would be possible. If this thread was helpful and you saved mega bucks fixing or diagnosing the problem yourself, please consider a donation (see paypal button on home page) to ensure that this site remains available in the future.