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Old June 3rd, 2007, 15:45   #1
Canadian_Grizzly
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Arrow Diagnosing and Fixing Limp mode for A4 1.9TDI [low power troubleshooting]

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.p...7&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...00#post1344400 For Beetle http://forums.tdiclub.com/showthread.php?p=332954
Intake Manifold cleaning (just do it…really) http://pics2.tdiclub.com/gwillie/VW/...AKEMAN101b.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.
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Having trouble with limp mode... Troubleshooting Low Power

Having trouble searching? Read this thread on advance search in forum http://forums.tdiclub.com/showthread.php?t=133579

Last edited by Canadian_Grizzly; September 18th, 2011 at 08:19.
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Old June 3rd, 2007, 18:56   #2
alphaseinor
 
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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.

Last edited by alphaseinor; January 5th, 2008 at 17:42.
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Old June 3rd, 2007, 19:30   #3
skrewdriver
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Can a clogged exhaust also cause this problem?

I have a '98 Beetle TDI w/ 169K on her. My car throws the 'Boost Deviation Intermittent' code, and performs just like the limp mode description given above (it's fine till a certain RPM/engine load, then absoltely no turbo. After restarting it's fine again for a while). I've been wrestling with this, replacing and cleaning things for a month and a half now. I replaced all the vacuum lines on the turbo path (I haven't done the ones on the EGR path). I replaced the N75. I pulled off the turbo and cleaned it good (all vanes were free moving when it went back together). I replaced the turbo actuator while the turbo was off. I'm soooo frustrated, I'm on the verge of taking this thing to the stealership to get fixed.

The current state of the problem is this:
The car whistles when you accelerate. I got under it just a bit ago, and it really sounds like it's coming from inside the catalytic converter. The catalytic converter looks like it is the original one that came on the car. It's pretty rusted and has a big dent on it.

So, could a clogged CC be my prob?
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Old June 3rd, 2007, 19:38   #4
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Yes, a clogged cat could cause it...
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Old June 3rd, 2007, 19:48   #5
ymz
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Good work, Grizzly and Alpha...

Just a bit of caution for new-bie mechanics: when removing the vacuum hoses from the N75 or other devices, be careful not to break off the nipple to which the hose is attached... many of them are made of plastic, and after a couple of years, the hoses are pretty well stuck onto them...

Some useful tips for removal: a hook/pick set with different angles (or a dental pick set) will be helpful in breaking the bond between the hose and nipple... just be careful not to puncture the hose unless you're replacing it. An easier but more destructive approach is simply to cut the hose just beyond the nipple and then slice the remaining piece so that you can peel it off... there may be sufficient length in the hose to simply lose the final 1/2" inch or you may need to replace the entire length, so it's best to think ahead and have some spare tubing...

I'm not sure that the dealerships, with all their manuals and special tools, are any better equipped to diagnose and repair these boost problems - except for their penchant to simply replace everything in sight... (turbo included...)

Yuri.
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Old June 3rd, 2007, 20:00   #6
tnteggroll
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Just finished spending 16 days waiting for the dealership to "fix" the turbo limp-mode problem in my 2001 Jetta TDI. Same basic problem, would go into limp mode randomly, typically at 60-70+km/hr, or going uphill/passing. 16 days later, rebuilt turbo, car running beautifully.

For four days. Craps out again this morning, only went into limp mode once on the way home, which is significantly better than before, but still, #$@%. I have a 1300km trip (each way) on Wednesday, kind of buggered of what to do, take the trip with a potentially persistant problem? Race around and try to get it fixed in 2 days, while working 5am-1pm each day? Why would it be running better with the turbo rebuilt, if it was something other than the turbo (ie: N75 solenoid, vacuum lines, etc)? That doesnt really seem right. Unless with the "better condition" turbo, it takes a little more of a problem than before to send it to limp mode. I dont know...

All I know is I'm getting super frustrated with my car, and debating just getting rid of it and finding something with less problems (and with only 90k, there's been a lot more problems than there should be). And that's really, really depressing, I love my Jetta.

Last edited by tnteggroll; June 3rd, 2007 at 20:03.
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Old June 3rd, 2007, 20:14   #7
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well... you have a guide above to start troubleshooting... that's what it's there for...

By the way... doesn't sound like your dealer is very good at diagnosing your issue if you are having them again... I think they treated the symptom, not the disease.
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Old June 3rd, 2007, 20:51   #8
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I think this should be stickied and but in the how to section great work guys
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Old June 4th, 2007, 14:58   #9
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Quote:
Originally Posted by tnteggroll
Just finished spending 16 days waiting for the dealership to "fix" the turbo limp-mode problem in my 2001 Jetta TDI. Same basic problem, would go into limp mode randomly, typically at 60-70+km/hr, or going uphill/passing. 16 days later, rebuilt turbo, car running beautifully.

For four days. Craps out again this morning, only went into limp mode once on the way home, which is significantly better than before, but still, #$@%. I have a 1300km trip (each way) on Wednesday, kind of buggered of what to do, take the trip with a potentially persistant problem? Race around and try to get it fixed in 2 days, while working 5am-1pm each day? Why would it be running better with the turbo rebuilt, if it was something other than the turbo (ie: N75 solenoid, vacuum lines, etc)? That doesnt really seem right. Unless with the "better condition" turbo, it takes a little more of a problem than before to send it to limp mode. I dont know...

All I know is I'm getting super frustrated with my car, and debating just getting rid of it and finding something with less problems (and with only 90k, there's been a lot more problems than there should be). And that's really, really depressing, I love my Jetta.
Okay, even weirder... It cut out the once yesterday, CEL stayed on all day, but the turbo didnt go into limp mode again. Today, when I was on my way to take it to the shop, the CEL went off, and still no limp mode. So what the hell? Theres not much they can tell me if the CEL isnt on, and the turbo is working fine right? Its a 45min drive to the shop, so didnt think it was worth it to head out there.
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Old June 4th, 2007, 15:06   #10
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If the CEL isn't on, a code is still stored... I would have someone with a VAG-COM diagnose it... where are you?
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Old June 4th, 2007, 15:10   #11
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Vancouver, was taking it to JS Performance in Abbotsford.
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Old June 4th, 2007, 15:19   #12
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I would hope they would be able to help... and I'm positive they will have a VAG-COM...
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Old June 4th, 2007, 15:40   #13
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Nice write up CG. and Alpha.
Does this diagnosis apply to the PD's as well?
In particular, the N75 which I think I have or the 109 relay
which I see mentioned all the time.
I try to follow along with my Bentley manual but I couldn't find these
components.
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Old June 5th, 2007, 07:54   #14
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Quote:
Originally Posted by JB05
Nice write up CG. and Alpha.
Does this diagnosis apply to the PD's as well?
In particular, the N75 which I think I have or the 109 relay
which I see mentioned all the time.
I try to follow along with my Bentley manual but I couldn't find these
components.
I believe it does. 109 relays were a weak spot in older cars...haven't heard much about them in the PD's.
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Having trouble with limp mode... Troubleshooting Low Power

Having trouble searching? Read this thread on advance search in forum http://forums.tdiclub.com/showthread.php?t=133579
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Old June 5th, 2007, 09:42   #15
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The initial relay 109's were much less reliable than the newer ones. I believe the upgrades and better designed one came in in 02 or thereabouts. Hopefully the PD's will not have any problems with these items.
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