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

l_c

Veteran Member
Joined
Mar 4, 2003
Location
San Jose, CA USA
TDI
Wrecked and gone: VW Jetta wagon 2002 silver TDI
Occasional overboost came on ... looking for more N75 info

I've been trying to diagnose my limp mode (of the 2002 TDI), without too much pain.

On a trip from N to S California, after stopping for fuel, I started the engine and it raced for about 1/3 second (which it had never done before). After that, it was limp mode except for a couple of times when I started it up, after the engine was allowed to get nice and cold (then it would come back to normal) ... but after running it again, limp mode would come back reliably.

So for a couple of weeks now, limp mode all of the time. (Hill climbing really sucks!) Once in a while it still does that overboost after cranking up, but not on any of the other start-ups. I have checked for ECM codes using VCDS, didn't see anything.

Last weekend I reached in there as best I could and worked the VNT actuator linkage, dribbled some household oil on it (for the heck of it) ... put a vacuum on it (Mity-Vac style), and it seems to actuate all right (up to 20 inches Hg). The hose and diaphragm checked out working all right;

I've removed my N75 valve and it seems to never close off vacuum between the top two ports. Mine draws 770 mA at 12V. I believe I saw about 16 Ohms open-circuit (at its terminals). It makes a small thunk sound when I apply voltage, but definitely not any loud click.

My question is, should I see the valve CLOSED between the top two vacuum ports, when the valve is de-energized (no voltage applied), OR when 12V is applied?? Not sure whether it's NC or NO (i.e. normally closed or open). Also, I've capped off the side (vent) port during testing, is that correct?? Thanks for anyone's help/advice! Larry

P.S. I still have the original N75 in front of me, for reference (in case anyone has the same vehicle type / part), here are the numbers that are printed on it.
It's made by Pierburg (see the side where the bracket is), PA66-GF 35 (I think ... seems like a laser mark)
On the front (face) side -- 1J0 906 627 A (that's the VW/Audi part number)
35/01 (might be a datecode)
7.22903.25 01T243

I'm ordering a new one (made by Hella?) from autohausaz.com
 

e*clipse

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Joined
May 9, 2007
Location
Chico, CA
TDI
Toyota TDI swap
I am having similar issues with mine.

Do you have VAG-COM? Is there a DTC 00575?

The error is an intermittant difference between MAP spec and MAP actual. It will put the engine into limp mode.

This was happening on mine, and due to a stickey actuator. (among other things)

Since you have a mityvac, check the point that it starts moving. This should be around 3-4 in HG. Mine took a bit of working with before it was freed to this spec. When first starting the engine the actuator should pull the VNT lever toward the stop. The VNT should be at its most open adjustment when the engine is off.

Regarding the N75 valve, the continuously adjusts the actuator by varying the duty cycle of the n75 valve. Larger duty cycle = more open.

Good luck! :)
 

rdkern

Top Post Dawg
Joined
May 21, 2004
Location
Humboldt Co CA
TDI
Passat 1997 silver (sold after 11 years), Jetta 2000 atlantic blue
My understanding, which is not by any means perfect, is that the VNT closes when the engine is turned off due to the N75 shutting. I believe this is due to the power cutoff, but it could be due to the vacuum issue.

Put some power on it and see what happens. If nothing happens, replace that sucker. I suspect it should not connect the vacuum to the VNT when power is not applied - since it's my understanding that this is a "failsafe" mode.
 

l_c

Veteran Member
Joined
Mar 4, 2003
Location
San Jose, CA USA
TDI
Wrecked and gone: VW Jetta wagon 2002 silver TDI
e*clipse, no DTC was found on mine ...
The only fault code I got was when I had the harness to the N75 disconnected, the ECM thought that it was shorted to ground (that threw a MIL).

After I had the valve out and had been pulsing 12V on it, then put it back into the car, and cleared that code (mentioned above ... supposed N75 shorted to ground), I actually had boost for one driving trip. After it was warm and I started the engine again, brief overboost surge and then back to limp mode. I'm pretty sure that I simply have a failing N75.
(PWM isn't gonna cause the right amount of travel, if the actuator has impaired range;)

Keith_J had written in thread # 238710 ("N75 has no power to it") that you can read the duty cycle of N75, in VCDS. I looked earlier today (when I hooked it up) ... saw some really neat stuff in ECM Measuring Blocks, but I'm not sure where to find the Duty Cycle of this one.

UPDATE ... I see that in http://www.ross-tech.com/vag-com/cars/tdi.html they do explain how to plot boost from Measuring Block 011. Would like to try that!
Also, am seeing that Measuring Blocks 114 thru 119 have all sorts of Boost Pressure Control information. 118 and 119 show the Duty Cycle??

I saw written in this thread (1st post, by Canadian_Grizzly) that you can go into Basic Settings mode, Engine controller group 011, to force it to cycle (10 seconds on or off); Haven't yet tried that.

Will post back here, if/when I get this resolved. Thanks; Larry
 

e*clipse

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May 9, 2007
Location
Chico, CA
TDI
Toyota TDI swap
I was just going to say: Look at measuring block 11.

My demo version does not support the higher measuring blocks.

It does sound like a faulty n75 valve.

Just to double check - you do have a good vacuum going to it?

Good luck!
 

l_c

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Joined
Mar 4, 2003
Location
San Jose, CA USA
TDI
Wrecked and gone: VW Jetta wagon 2002 silver TDI
e*clipse, thanks a lot, I'll try and measure the live vacuum when I get some time (maybe I can tee in my hand-pump / gauge, and then start the engine) Larry.
 

e*clipse

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Joined
May 9, 2007
Location
Chico, CA
TDI
Toyota TDI swap
How a VNT turbo works

maybe I can tee in my hand-pump / gauge, and then start the engine
You shouldn't need to do that. The car will start and run with the vacuum off. It just won't run as well.

To understand the VNT, here's a video:

http://www.youtube.com/watch?v=Gc2awh0O0Bc

Note that when a vacuum line is connected to the top connector (the one on the far right in this video) the actuator will pull on the VNT mechanism. As the actuator pulls (more vacuum) the vanes "close" thus speeding up the turbine.

This is shown in this video, with the exhaust gasses animated:

http://www.youtube.com/watch?v=KgVrpiFfp8U&feature=related

As the motor speeds up (more exhaust gasses) the ECU adjusts the N75 duty cycle to release the actuator and open the vanes. :cool:

If your actuator was stuck in the released position (as mine was) you basically won't get boost until later in the RPM range.

Good luck!:)
 

JASONP

Top Post Dawg
Joined
Feb 24, 2008
Location
Guelph
TDI
2015 Golf TDI Comfortline 6spd
e*clipse said:
You shouldn't need to do that. The car will start and run with the vacuum off. It just won't run as well.

To understand the VNT, here's a video:

http://www.youtube.com/watch?v=Gc2awh0O0Bc

Note that when a vacuum line is connected to the top connector (the one on the far right in this video) the actuator will pull on the VNT mechanism. As the actuator pulls (more vacuum) the vanes "close" thus speeding up the turbine.

This is shown in this video, with the exhaust gasses animated:

http://www.youtube.com/watch?v=KgVrpiFfp8U&feature=related

As the motor speeds up (more exhaust gasses) the ECU adjusts the N75 duty cycle to release the actuator and open the vanes. :cool:

If your actuator was stuck in the released position (as mine was) you basically won't get boost until later in the RPM range.

Good luck!:)
Thats insane
 

windigo

Active member
Joined
Feb 6, 2006
Location
near Indy
TDI
none but looking
Great news for people with the Expensive N75 and EGR valves...
I have a 1999 TDI bug and they quoted me over $200 each...
I just installe the ones for a 2001yr TDI and they were less that $100 each...
Works great... All you have to do is to replace the electrical plug...
My friendly VW dealer even gave me the plugs so all I had to do was move the wires... Windigo is back and running well...
 

l_c

Veteran Member
Joined
Mar 4, 2003
Location
San Jose, CA USA
TDI
Wrecked and gone: VW Jetta wagon 2002 silver TDI
not as easy as I'd hoped

I guess I haven't totally understood how the VNT works ... here's what I think, and it might be wrong -- when the peripheral vanes are "open", the turbo is dumping a lot of the boost, i.e. venting pressure; when the vanes "close", it's sending more boost pressure toward the engine intake.

In the first video clip offered (above), that would mean when my actuator (linkage) is "up", it's dumping boost pressure, i.e. no boost; the linkage moving "down" (toward the belly pan) will actuate increased boost pressure. Just to straighten my addled thinking ... does the actuator hit the Stop as the linkage is pulled Down all the way toward the belly pan?

I might have mixed this up (again); I think when my engine is off, the linkage is "up", right? Spring pressure forces it up? Applied vacuum makes it pull "down"? I usually (now about 2/3 of the time) get that brief overboost, when starting the engine ... more often when warm-started (cold start usually no initial overboost). If my problem was, poorly regulated vacuum (meaning, N75 doesn't cut off the vacuum properly, as I suspected) then I think I got this right (i.e. when I start the engine, it pulls downward too much on the linkage).

I need to do a couple of things (in any order) ... first, from post #1 in this thread (Anut's instructions)
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).
That sounds easy, but ... (silly question) where is the Rigid Brake Booster Vacuum Line ???

I checked for fault codes (in 01 / Engine controller) this morning ... nothing. Though I'm getting N109 malfunction codes, which I didn't expect (I removed my cutoff plunger a couple of months ago ... but the solenoid is still connected).

Also, I have not yet turned on Basic Settings while getting Measuring Blocks 011 (but I did drive around this morning, while glancing at the 011 display in VCDS) ... will try that probably on the weekend (when there's daylight w/o too much rain). I guess I'll use my little angled mirror to look for the linkage, when I hit Basic Settings.

Thanks for your help, gents. (Sorry I wrote so much this time) Larry
 

e*clipse

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Joined
May 9, 2007
Location
Chico, CA
TDI
Toyota TDI swap
I might have mixed this up (again); I think when my engine is off, the linkage is "up", right? Spring pressure forces it up? Applied vacuum makes it pull "down"? I usually (now about 2/3 of the time) get that brief overboost, when starting the engine ... more often when warm-started (cold start usually no initial overboost). If my problem was, poorly regulated vacuum (meaning, N75 doesn't cut off the vacuum properly, as I suspected) then I think I got this right (i.e. when I start the engine, it pulls downward too much on the linkage).
My setup is completely different, with the turbo and motor rotated, so "up" and "down", "front" and "back" get confusing. How about "toward the stop screw" and away from the stop screw"? :cool:

The stop screw is a minimum open setting that should ONLY be adjusted if you really know what you're doing and can ensure that it won't completely close. As you can imagine, there can be all sorts of dynamic problems caused by an incorrect set screw adjustment.

When the actuator is pulled toward the stop screw, the vanes close. When there is no vacuum (car off) the vanes open. When you start the car, if there is vacuum and if the computer doesn't have fault codes (the N75 duty cycle is correct) and if the N75 valve is working, it will move to the high boost position.

The cool thing about this system for VNT control is that it is fail safe. In other words, if you loose vacuum pressure or have other problems, the turbo defaults (by spring force) to the open, low boost setting. Of course a sticky actuator could stick in the wrong place.

Regarding the vacuum lines, start looking for the vacuum pump, near the oil filter.
 

l_c

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Joined
Mar 4, 2003
Location
San Jose, CA USA
TDI
Wrecked and gone: VW Jetta wagon 2002 silver TDI
I'm puzzled ... about 2/3 of the time, when I start up, it stays in "high boost" i.e. overboost for a fraction of a second. Other times when I crank up, it sounds normal. But in either case I'm stuck in limp mode.

Yesterday I manually worked the actuator (reaching down), left work and surprisingly had boost. Later when I stopped to fill the tank with #2, it went back to limp mode. So I've been thinking for weeks now, "linkage might be sticking when hot" (except that it doesn't come back to normal in the morning).

Since I replaced N75 and didn't get improvement, I have a new question -- is there any chance that a plugged-up EGR valve could push me into limp mode (without throwing a fault code in the ECM) ?? My latest EGR valve has been drooling black oily discharge, for the past 1 to 2 years. I have a brand new EGR valve that's waiting to be installed.

Thx; Larry
 

l_c

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Joined
Mar 4, 2003
Location
San Jose, CA USA
TDI
Wrecked and gone: VW Jetta wagon 2002 silver TDI

l_c

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Mar 4, 2003
Location
San Jose, CA USA
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Wrecked and gone: VW Jetta wagon 2002 silver TDI
Limp mode went away

Unfortunately I didn't have much time to fiddle more with my car (diagnosing vacuum, actuator, start-up characteristics, N75 etc.)
but last weekend I did go in there, re-checked hand vacuum on the turbo actuator, replaced the EGR. (Also replaced the fuel cutoff solenoid ... I'd seen a Malfunction code on this, not sure why ... but I figured it shouldn't affect anything, since I didn't have a plunger in it.)

My previous EGR (it was my 2nd ... now I'm on my 3rd @ 150k miles) didn't look too shabby; when I apply vacuum on it (both main actuator, and switchover) it seems to move correctly, and not leak vacuum. Yeah, it's dirty ... and maybe the response is not quick enough like the ECM would want. To explain why I replaced it, it's because it'd been dribbling messy black oily sludge out of the hole (what is that hole for, inspection or vent??) for many many months and I was tired of the mess.

Anyhow, that work I did on Saturday didn't seem to do a darned thing, vis-a-vis my limp mode. On startup, sometimes I'd get a brief overboost, and other times I didn't notice any attempted boost. But in either case, always limp mode. I also did not bring a computer and scan for fault codes, on Saturday.

On Sunday morning, I started the car, and wow, joy -- back to normal ... boost ... so I've now had 4 days of consistent proper running. I'm kind of enjoying driving this car once again. What I'm wondering is, replacing my EGR was enough to make my limp mode go away? Larry.
 

vwjettadsl

Vendor , w/Business number
Joined
Feb 24, 2005
Location
Missouri
TDI
2013 Touareg TDI 182k, 2012 Jetta TDI 175k, 2012 JSW TDI 200K, 2012 Golf TDI 110K, 2003 Jetta 200K
Why isn't this thread a sticky? I think if it was made a sticky it would cut down on all the newbie posts about low power.
 

need4speed

Veteran Member
Joined
May 4, 2004
Another vote for STICKY.

But this problem is actually VERY complex. Limp Mode is a SYMPTOM - that can be triggered by at least four different causes (probably others). Most common seems to be underboost, followed by overboost, and intermittent, and "boost pressure xxxxxx". And these, in turn, are SYMPTOMS, each with it's own set of possible underlying causes. I think that a lot of the advice offered on this site is good - but I also think that a lot of folks are steered down the wrong path inadvertently (troubleshooting underboost-related issues; when their problem is overboost).

I can tell this subject deserves a troubleshooting flowchart. . . but I won't attempt one until I actually solve mine.
 

claudiu_c

Member
Joined
Apr 18, 2009
Location
Cluj-Napoca, Romania
TDI
A4 B5 1.9 tdi 200hp
hello. i´m new to this site, but come also with a limp mode problem. i have a audi A4 B5 1.9 tdi, PD, 116hp, with biger turbo, vnt17 (so i think), FMIC, straight 3inch exhaust with sportcat, and unichip pigyback software. initialy the car had 180hp, but now, after 3 months, in the 3-rd gear flatout it gets in limp mode. the problem is the turbo makes more than 2.5bar overboost, read on the vagcom (max 2.5bar), and on a boostmeter (more than 2.5bar). i have changed the maf, map, and turbine controling senzor, but has the same simptoms.
have any ideeas what i could do, so the limp mode doesen´t apear anymore?
 

e*clipse

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May 9, 2007
Location
Chico, CA
TDI
Toyota TDI swap
On a modded car like yours, the ECU needs to know what's happening. The information from the sensors is most likely being altered by the piggyback software. The ECU doesn't have direct control, and can only react when things get out of control by forcing limp mode.

Over/underboost is one of the primary reasons for the ECU to force limp mode; it's a safety mechanism to protect the engine.

If your are seeing 2.5bar overboost, it's a good thing your ECU is going into limp-mode. :p

I would look into the VAG logs of the N75 duty cycle, and the MAP pressure. At this point the problem could be control or even a stuck actuator. I would also look into a proper tuning; not a piggyback system.

Good luck! :)
 

claudiu_c

Member
Joined
Apr 18, 2009
Location
Cluj-Napoca, Romania
TDI
A4 B5 1.9 tdi 200hp
ok. on the original ecu of the car, was wired up another control system, a unichip (this i think is called a pigyback chiptuning), and it was tuned on a dyno, the only one i know to be in romania, so it should be well tuned. my ecu had prior of this a chiptuning, but after the problems apeard, i wrote back the original soft.
i will also change all the vacuum tubing next week.
 

CoolAirVw

Vendor
Joined
Nov 9, 2005
Location
Kansas City Missouri
TDI
Jetta
Here's some threads documenting limp mode.

I wanted to post the first ones because there's alot of graphing that pinpointed the problem. But then as I kept going there was alot of these threads so I thought I'd list them.

These two are need4speed's that he fixed by adjusting the VNT actuator.

http://forums.tdiclub.com/showthread.php?t=244619

http://forums.tdiclub.com/showthread.php?t=245445


Following 2 are spleecho's. That he fixed by adjusting the rod lenth. I'm not sure I saw that as a fix in the original post in this thread, also the rod siezes in the lever.

http://forums.tdiclub.com/showthread.php?t=176604

http://forums.tdiclub.com/showthread.php?t=245203


In the following thread Dieselphreak gets a nut stuck in his actuator and also had a vacuum leak. Then later in the thread AusgezichnetTdi has his acutator rusted.

http://forums.tdiclub.com/showthread.php?t=241626

In this thread Dura08max adjusted the actuator to fix it.

http://forums.tdiclub.com/showthread.php?t=244588

This guy thought he had limp mode, and titled the thread that way, but turned out the MAF needed cleaned.

http://forums.tdiclub.com/showthread.php?t=244757

Here's another one with no fix posted yet. Probably just a vacuum hose since it started after intake cleaning.

http://forums.tdiclub.com/showthread.php?p=2536204#post2536204

FYI, I found all these threads in 3 pages of current threads!

Months later EDIT: More info...

Following thread shows where Dogdots thought a MAF would help but it didn't, then found exhaust leaking at the egr tube at the cooler due to gasket and bolts missing. NOTE: PRE-TURBO EXHAUST LEAKS CAUSE LACK OF BOOSTAND LIMP. Later in post 11 greengeeker found rocks jamming up his vnt actuator.

http://forums.tdiclub.com/showthread.php?t=259707
 

TDI-Newbie

Veteran Member
Joined
Aug 6, 2008
Location
Toronto-Bolton , Ontario
TDI
VW Golf 2003
Ok... they only time my golf 1.9 tdi goes to limp mode is when i accelerate past 120 -130 klm per hour... It sounds to me like a hose leak but... any other thought would be appreciated. Its ok on steep hills.. I'm also going to clean the intake in a few weeks... I hope that will help as well.
 

PicknCrew

Member
Joined
Mar 28, 2006
Location
North Franklin, CT
TDI
2011 Golf TDI MT
Replacing parts and cleaning things still didn't resolve my limp mode problems. After much time and expense I resolved my intermittent limp mode symptoms with a google search and spending $50 on a boost controller (check valve). Inherent design flaw in these TDI engines in my opinion.
 

whitedog

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Joined
Jul 12, 2004
Location
Bend, Oregon
TDI
2004 Jetta that I fill by myself
PicknCrew said:
Replacing parts and cleaning things still didn't resolve my limp mode problems. After much time and expense I resolved my intermittent limp mode symptoms with a google search and spending $50 on a boost controller (check valve). Inherent design flaw in these TDI engines in my opinion.
You took your car to the dealer and they replaced a few things, then you decided to take over. We don't know all of what was done to the car so it's difficult to tell if you checked all that is needed.

There are thousands of these cars running fine without the boost controller, so I'm thinking "no" on the inherent design flaw.
 

richmt

New member
Joined
May 1, 2009
Location
montana
TDI
2000 jetta
hi all,
I also had the code p 1577 and after replacing the air filter, n75 and all vacuum line, cleaning the snow screen I still had the code usually tripped going up hill in 4th or 5th gear. I finally purchased a mityvac and checked the actuator and travel as outlined in the TSB for actuators and found that at 17hg the vnt mover was all the way against the stop. After removing and adjusting the travel to the correct specs. I have not had anymore mil lights and the car runs great 75mph up steep hills with a.c. on. Thanks Canadian Grizzly for you post helped alot
rich 2000 jetta tdi 147,000 miles
 

roachxp

Member
Joined
Jan 13, 2004
Location
Milford, MA
TDI
2001 TDI GOLF GLS EURO-MODLE
Quick question is the N75 the #4 or # 6 in the first post. I was going to replace mine because of the fault code and notice 2 that looked the same. I did #6 hopefully thats the right one.
 
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