Diagnosing and Fixing Limp mode for A4 1.9TDI [low power troubleshooting]

Madmax78

New member
Joined
May 9, 2021
Location
Montreal
TDI
2000 Jetta Sedan
Just wanted to write Canadian Grizzly (if he’s still around) and thank him for posting his piece on diagnosing limp mode problems (trouble shooting) with the 1.9l ALH engine. I recently installed a new turbo and his step by step analysis of the vacuum system saved the day - specifically his reference to the N75 solenoid having « unregulated » vacuum - meaning straight from the brake booster line. On close inspection (tracing out his diagram of the system he posted) - I noticed the T connectors to the N75 and N18 were crossed! Problem solved. Runs like a top now. Many, many thanks.
 

Jenhockey

Member
Joined
Oct 18, 2021
Location
Kelowna
TDI
2004 VW Passat
Where can one find vacuum line to replace the lines in my car? It seems simple enough to do myself, hopefully fix my stupid limp mode problem, and also doesn't seem like it would take too long. I just don't know where to find the vacuum line, what it's called, size, etc. Any help?
The main vacuum lines are just regular 5mm (inside) tubing. You can find them almost anywhere I got mine from lordco
 

Jr mason

Veteran Member
Joined
Oct 20, 2013
Location
Ohio
TDI
01 Beetle, 2012 Jetta
Should not cause limp mode.

I'd take a hard look at all the turbo, intercooler, associated piping and connection clamps.

Do you have a way to monitor boost?
 

TornadoRed

Top Post Dawg
Joined
Aug 3, 2003
Location
West Des Moines (formerly St Paul)
TDI
2003 Jetta TDI wagon, silver; 2003 Jetta TDI wagon, indigo blue; 2003 Golf GL 5-spd, red (PARTED); 2003 Golf GLS 5-spd, indigo blue (SOLD); 2003 Jetta TDI wagon, Candy White (SOLD)
Hi everyone, just want to know do cracked/leaking egr pipes cause noise when shifting (owl noise) and limp mode?
First off, some TDIs make that sound and some don't. And most people seem to like that sound and wonder how to replicate it in a TDI that doesn't make it. It really has nothing to do with limp mode.

Secondly, many things can cause limp mode, but typically either the turbo actuator is sticky or there are one or more leaks in the vacuum hoses. Remember, limp mode is generated by the engine computer to protect the turbo.
 

lauski_don

Member
Joined
Mar 29, 2023
Location
Banja Luka
TDI
1.9 TDI
Should not cause limp mode.

I'd take a hard look at all the turbo, intercooler, associated piping and connection clamps.

Do you have a way to monitor boost?
Thank you for reply. My mechanic tested the boost, and he says it's fine. I also have elm327, if that can help.

I started driving a lot in city, and sometimes when I speed up to 2.3k rpm, I got limp mode. But I just shut on and off and it went fine.
But it started to get more frequent. So I went to my mechanic. He replaced the pipe and said it's fine.
After a few days I needed to drive ~100km and started gassing it, then it happened again and my ic hoose popped of at 120kmh, which is below car battery. I managed to get it back and drove back, barely!
Went to mechanic again, and he just put screw right through that pipe and said that turbo is a bit dirty but boosts well and it shouldn't pop off hosses. Then I tested it with driving for 30mins with 3-5k rpm (when it heated up of course). And no hosses popped off but I still have that noise when I let off gas pedal, like a whoosh, and still get limp mode when trying to go over 100km/h. I can make that sound constantly without changing gears, just it's more louder when I change gears.
I went to second car mechanic and I noticed when he was in neutral revving engine from egr cooler side there was a blunt of smoke when he hits 3-4k rpm.
Really don't know what to do now? Change car mechanic or try to wiggle it on my own?
My theory is that some of the piping is cracked or leaking under pressure and causing that sound (and smoke), but I'm not sure.
And I'm sick of changing mechanics.
 

lauski_don

Member
Joined
Mar 29, 2023
Location
Banja Luka
TDI
1.9 TDI
First off, some TDIs make that sound and some don't. And most people seem to like that sound and wonder how to replicate it in a TDI that doesn't make it. It really has nothing to do with limp mode.

Secondly, many things can cause limp mode, but typically either the turbo actuator is sticky or there are one or more leaks in the vacuum hoses. Remember, limp mode is generated by the engine computer to protect the turbo.
@TornadoRed
Thank you for reply. Do you think is that noise dangerous in any way? Can I drive without thinking about it?
I have tested the turbo actuator before I went to my car mechanic. Also I changed my tandem pump recently.
Vacuum hose from solenoid box to turbo actuator is changed also. And my mechanic says that turbo is dirty but it give good boost (and he said it gives maybe a bit more boost than it should).
 

TornadoRed

Top Post Dawg
Joined
Aug 3, 2003
Location
West Des Moines (formerly St Paul)
TDI
2003 Jetta TDI wagon, silver; 2003 Jetta TDI wagon, indigo blue; 2003 Golf GL 5-spd, red (PARTED); 2003 Golf GLS 5-spd, indigo blue (SOLD); 2003 Jetta TDI wagon, Candy White (SOLD)
@TornadoRed
Thank you for reply. Do you think is that noise dangerous in any way? Can I drive without thinking about it?
I have tested the turbo actuator before I went to my car mechanic. Also I changed my tandem pump recently.
Vacuum hose from solenoid box to turbo actuator is changed also. And my mechanic says that turbo is dirty but it give good boost (and he said it gives maybe a bit more boost than it should).
I just remembered the name of the sound you described; it's called a "choo." It's more common with the VE engines, but if you have a tandem pump then you must have a PD engine. I've not owned one of those and don't know enough to make any other suggestions.
 

koen B

New member
Joined
Jan 17, 2024
Location
Kawartha lakes
TDI
2003 Jetta
I'm going to add a little bit to this... btw... great writeup

Limp mode is best described as a complete loss of turbo power, when you key off and key back on your power should be back,
If this is not your symptom, you should check the MAF 101 page... which just says unplug your maf, and if it's better or the same, then it's bad. BTW... if you are running a K&N air filter... unplug the MAF first.

I think you should scan the car first...

There are three common codes for limp mode.
Boost Deviation Intermittant
Boost Deviation Negative
Boost Deviation Positive

Boost Deviation Intermittant means you most likely have a vacuum leak somewhere. Check all lines, actuators, and plastic fittings.

Boost Deviation Negative means you have a clogged intake, actuator that's leaking, bad vacuum lines, or a sticky turbo.

Boost Deviation Positive means you have a N75 that's bad/malfunctioning, actuator that's leaking, bad vacuum lines, or a sticky turbo.

In addition... the A3 and B4 models (1998 and older, jetta, passat) have a Vacuum line in the ECU... but basically the same procedure... They run on boost, not vacuum, so a tire pump may be of use to see if the actuator is working on the turbo.
Could Egr delete fix this problem? I have my cat cut out and straight piped. I recently noticed the power loss all the sudden and it’s quite anoyying while merging onto a highway. What should I do? Scan it?
 

dilkie

Well-known member
Joined
Aug 24, 2008
Location
Ottawa
TDI
'01 Golf, '06 Golf, 90 Jetta
Could Egr delete fix this problem? I have my cat cut out and straight piped. I recently noticed the power loss all the sudden and it’s quite anoyying while merging onto a highway. What should I do? Scan it?
Yes, that fixed mine because the cause (after months of investigation) turned out to be a slow/gunked-up EGR valve that was causing boost and airflow to not match expected values. I just plugged the EGR hose (removable since, in those days, still had to take the car in for emissions testing every 2 years).. and ignored the CEL.
 

koen B

New member
Joined
Jan 17, 2024
Location
Kawartha lakes
TDI
2003 Jetta
Yes, that fixed mine because the cause (after months of investigation) turned out to be a slow/gunked-up EGR valve that was causing boost and airflow to not match expected values. I just plugged the EGR hose (removable since, in those days, still had to take the car in for emissions testing every 2 years).. and ignored the CEL.
Thanks! I have a delete kit coming soon. I can imagine my EGR is quite clogged up with 520,xxx kms on it. I figured I would install the delete kit before tearing into it more being I wanted to Egr delete it anyway. Hope it fixes my problem
 

gavo2023

Member
Joined
Jan 28, 2024
Location
Canada
TDI
BEW
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 l


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.

If the power is the same as it should be properly working or same as in it's still limping?
 
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