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

I'm having a limp mode issue also. There is a whistle sound at lower rpm's that was never there before. The whistle disappears when it is wound out and it takes off like normal. Also, when the engine is cold it will barely move and then kicks down into 1st gear and builds enough boost to start going somewhere. Here is what I've done so far:

When it first happened a few months ago I thought the turbo went out so I change to a new VNT15 & actuator
All new silicone vacuum lines
New Bosch MAF
New N75


I am getting a code: P1550 Charge Pressure Control Deviation

At this point I'm not sure what to do. I don't see a crack or leak anywhere in the piping. I eliminated my EGR and cooler a couple years ago so my intake is good. I did read somewhere else on this forum that a clogged cat can cause limp mode. I think I may try that next. But would I get the whistling sound with a clogged cat? Is there something else I'm missing that I should try?

Whistling sound is never good...

1) possible pre-popped turbo
2) inlet pipe has come off
3) something else (gasket?) has failed

Honestly I don't think I've seen a clogged cat... that's WAY down on the probable causes list.

either way, take the inlet pipe off and check the turbo for play, or damage.

Good afternoon I am new to the forum as well as TDI's I have an 03 jetta that sounds like it has limp mode, but I have no CEL. will an obd scanner tell me what the problem could be.

TenBears said:

Good afternoon I am new to the forum as well as TDI's I have an 03 jetta that sounds like it has limp mode, but I have no CEL. will an obd scanner tell me what the problem could be.

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Welcome to TDIClub TenBears,
If you have no CEL a OSBII scanner may tell you what was set previously, however, will not tell you anything else.

TenBears
I am curious why you think you are having a limp mode issue.

For more in depth understanding and trouble shooting try looking at these links:
http://www.myturbodiesel.com/wiki/l...roubleshooting-vw-seat-skoda-audi-tdi-engine/
http://www.myturbodiesel.com/wiki/low-power-constant-tdi-engine-repair-and-solution/

Hey guys. Is this troubleshooting guide in order of likeliness or how easy it is to fix?
I mean, the guide mentions the MAF first, then the vacuum and so on... Is this because it is most likely to be the MAF, then the vacuum... or is it because it is the easiest things to test?
Thanks.

nicklas669 said:

Hey guys. Is this troubleshooting guide in order of likeliness or how easy it is to fix?
I mean, the guide mentions the MAF first, then the vacuum and so on... Is this because it is most likely to be the MAF, then the vacuum... or is it because it is the easiest things to test?
Thanks.

Click to expand...

A little of both

MAF tends to be a fairly quick test, though ruling in/out isn't necessarily a slam dunk (only if by unplugging it your engine instantly settles down).

UhOh said:

A little of both

MAF tends to be a fairly quick test, though ruling in/out isn't necessarily a slam dunk (only if by unplugging it your engine instantly settles down).

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I tried running without the MAF and the engine was running absolutely normal in both startup, idle and acceleration, except that I seem to remember the turbo got worse and didn't kick in at all.. so since the turbo got worse, I guess the MAF works?

I didn't get a CEL but I read somewhere that TDI has some default fallback value when no MAF is detected?

RE: limp mode.
I have a 2004 Passat with a PD. Still, my experience is relevant.

Car would start fine, accellerate, then buck and blow smoke under load and go into limp mode. Turn off and back on and it would run normal and repeat the cycle.

Codes were 0209 - underboost and 0101 - MAF.

1. Replaced MAP.
2. Cleaned MAF.
3. Cleaned EGR.
4. Ran decarbonizer in tank for a week.
5. Cleaned turbo in place with oven cleaner. Interesting process.

None of the above made a difference.

After step 5, I disconnected the EGR to prevent Easy Off from getting into the cylinders. Car ran normally. Thinking I had fixed it, I reconnected the EGR. Problem returned. So, I have high confidence that the EGR is faulty.

Disconnecting EGR is easy to do and may need to be added to the troubleshooting steps. It will throw a code, but better to run the car this way while I wait for the part.

Me too. My car was left around for a while and the EGR was sticking and that led to the limp mode. Disconnecting it worked for me as well.

Looking for a little help regarding overboost/limp mode (deviation positive) please.


Things I've done/eliminated to date.

Cleaned intake and egr cooler.
New vac lines (all)
Replaced turbo actuator (original had pin holes and was intermittently sticking)
Disconnected vac line at N75 and Ran a vac test, actuator begins moving at 3-4 in/hg with linear movement up to 17-18 and holds. Does not bleed down.

This is where I'm really stumbling.
Performing the key on test, the actuator does NOT move.
However, it does as soon as the engine is started. I'm also hearing a distinct turbo whistle at low rpm/load.
The only N75 tests I can perform with out vagcom per my Bentley service manual is a resistance test and voltage test on the vehicle side harness. My N75 tested at 16 ohms (manual states 14-20). When grounded to the battery pin 1 in the vehicle harness shows battery voltage. The yellow wire shows a tick over 3 volts with key on, I'm assuming this is likely the signal from the ecm and voltage changes with demand?
Anyhow, not sure what else to do and knowing I was wasting my money I bought a new N75. Actuator still does NOT move at key on. I will say it DOES run better but after several wot runs through the gears I was able to reproduce the limp mode (deviation positive). It is less frequent and the car does seem to run better but I've repeated the limp mode twice since the n75 replacement. The part that troubles me is, why does my actuator not cycle with the key on? Any ideas why this is?


Thank you kindly for any and all suggestions.

How clean is your intake?

alphaseinor said:

How clean is your intake?

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Sorry I forgot to include that. I cleaned my intake a few months ago. Thanks

Did you replace, or just clean the VNT with new actuator?

Where does the actuator start and stop moving with a MityVac gauge (in inches/Hg)

Actuator is brand new. Have not cleaned the turbo yet, actuator movement starts moving right at 3 in/hg and is very linear and smooth up until around 17-18 or 19. holds vacuum from the outlet hose at N75 to the turbo so I'm certain I have no leaks. I also tested system vac with the engine running, topped out at around 22.

Sounds like you've confirmed everything is OK except for the internals of the turbo and the N75.

I hate to say it, but the N75s get wrongfully blamed quite a lot on mk4s. Not saying they don't go bad, but they don't go bad as often as the forum might make you believe.

Easy test is to use VCDS (I know, you don't have it... but still, it's the best way to do it), engine at idle, run basic setting 03. Put your vacuum gauge to the outlet port on the N75 and watch it move from 0 vacuum to system vacuum (ideally more than 20") every several seconds.

I have had slow moving VNTs due to a kinked hose on the "purge" side of the N75 that goes to the air box, but that's pretty easily confirmed visually.

99% of the time if the N75 checks out OK, actuator is adjusted correct and the vanes aren't sticking it's overboosting because there's simply excessive wear between the control ring and lever inside the turbo's VNT mechanism. Easiest fix there is to bite the bullet and replace the turbo.

Getting VERY common as these things age. City driven cars sometimes barely make it to 100k before they have boost control issues. Most people will do 150k+. Guys doing mostly highway have no issues going 300k on one of these.

More moving parts = more spots for things to wear...

N75 is brand new... replaced it against my better judgement... 4th and goal type situation.

May try to clean the turbo just for giggles, then try and find someone in the vicinity of me with vcds before I bite the bullet on a turbo (gulp)

Thank you for your time.

Matt is right on about using the VCDS. That's what it's for!
The N75 can also be bench tested for function and leaking.

Jr mason said:

Performing the key on test, the actuator does NOT move.
However, it does as soon as the engine is started. I'm also hearing a distinct turbo whistle at low rpm/load.

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This makes me think the vacuum system is "leaking down" when the engine is off (not holding vac), so no vac "pressure" is available to move the actuator unless the engine is running and driving the vac pump.

Can you tee a mityvac (or similar) into the vac system so you can see if it's holding vac or not when the engine isn't running? if you can, you can also use the mityvac to "pressurize" the vac system without starting the engine and see if it allows you to see actuator movement in the key-on test.

Is that true?
I think the engine needs to run to make vacuum. There would be some vac at the brake booster, but I don't believe the whole system holds. Could be wrong.

I think as long as you're not pressing on the brake pedal with the engine off, it should hold vacuum.

Last summer I had an N75 that would "sing" (read: make groaning noise) for several seconds after engine shutdown. I tee-d my mityvac clone into the system and found that while it was singing it was venting to atmosphere. I think in my case, the valve was fouled by debris sucked in from an actuator failure I'd had several years before; I started the engine, saw it build up to nominal vacuum in a couple seconds, then when I shut back down, I'd see the vac gauge drop back to zero within 5-10 seconds. a "touch test" (feeling things to find what was making the groan) was how I figured out it was my N75).

Once I replaced the N75 with a new one and repeated the test, the gauge didn't sink back to atmosphere upon engine shutdown. Oh, and the singing stopped too.

The actuator needs vacuum to move, and JR Mason's already replaced his N75 and vac lines (not sure if he's including the one to the booster in that, or the check valves); checking for vacuum leak-down isn't hard. Most of us know that leaks in the vac line to the booster can get hard-to-detect cracks, and of course the nipple on the vac pump itself can lose its seal.

To JR Mason - if you start the engine and get it to move the actuator, what happens if you immediately shut the engine off afterward and repeat the key-on test? does the actuator move then?

Alternately, can you splice the mityvac in, pump it a bit to "pressurize" the system, and see if/how long it holds vacuum. If it does hold, maybe try the key-on test again and see what happens when the system is confirmed to be holding vac?

Rob Mayercik, you may have just provided the missing link here. I do not have a check valve in line at the moment, it was blocked up and blocking all vacuum. So that would explain why the actuator doesn't move at key on? Is that check valve really necessary?

I would think the check valve is there to prevent the system "venting" when the vac pump isn't being spun by the engine; without it, I'd guess the system would lose vac pretty fast with the engine off.

if you have one of the two check valves removed I'd wonder if the vac system is pressurizing right. Can you get a new (or at least known good used) one and put it back in?

Do you need a vac system reference? If so, here's one for an ALH like yours: http://forums.tdiclub.com/showpost.php?p=4431965&postcount=4

I figured the check was just a way of keeping vacuum for the brakes if they were needed with the engine off. I have seen those check valves before at idparts, I can order one up.
I've got a Bentley manual and reviewed the diagram just to make sure everything was right when I repalced the vac hoses so I should be good there.

The check valve (the one going to the N75) is only used to keep vacuum in the boost circuit (which includes the vacuum "ball")...

My '02 TDI has not had any check valves for years.
It runs fine which tells me it's not your problem.

flee said:

My '02 TDI has not had any check valves for years.
It runs fine which tells me it's not your problem.

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I tend to agree with you, logic tells me the purpose they serve is more for when the engine is off than anything...
If anyone has any other insight I'd be very grateful, as of right now I believe I am going to attempt to clean my vnt out. Once I've done that I may have found a gentleman about an hour from me with vcds, hopefully he can help me further pin point wether or not my vnt is checking out.

Don't really think the turbo cleaning is going to help because the actuator begins moving at 3 in/hg and moves fluidly up until around 17-18. But it certainly can't hurt at this point.

Jr mason said:

I tend to agree with you, logic tells me the purpose they serve is more for when the engine is off than anything...
If anyone has any other insight I'd be very grateful, as of right now I believe I am going to attempt to clean my vnt out. Once I've done that I may have found a gentleman about an hour from me with vcds, hopefully he can help me further pin point wether or not my vnt is checking out.
Don't really think the turbo cleaning is going to help because the actuator begins moving at 3 in/hg and moves fluidly up until around 17-18. But it certainly can't hurt at this point.

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The N75 purges vacuum from the actuator on shut down anyway. And I have my doubts the ECU is even instructing the vanes to close down in the first couple seconds after start up (though we could always verify that with VCDS I suppose!).

A variable geometry turbo is neat not only for its drivability and efficiency, but it also serves as a way for the computer to create extra back pressure in the exhaust manifold under light load driving to help increase EGR flow rates for emissions reasons.

The check valve is there because the turbo operation is the most critical thing the vacuum system controls. Also hence why immediately after the check valve, you get a T piece that goes to a reservoir. Being that the reservoir is T'd in after the check valve, that means the reservoir is literally only there to ensure consistent operation of the turbo when everything is plumbed as the factory intended. I don't think it has that much to do with retaining vacuum for start up, as you're not going to drive the car away the very second the engine restarts anyway. And even if you do, you're not going to need the full boost position on the turbo instantly if you're driving like a normal person anyway.

I have also noticed you can better hear the turbo after the engine has been allowed to idle for a minute or so. It very suddenly changes operating parameters for the turbo after it sits idling for awhile.

Matt-98AHU said:

I have also noticed you can better hear the turbo after the engine has been allowed to idle for a minute or so. It very suddenly changes operating parameters for the turbo after it sits idling for awhile.

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Isn't that related to the operation of the EGR system?

Back to the checkvalve... it's there to ensure a stable "supply" of vacuum to the boost circuit... (and... a bad checkvalve can cause both occasional over-boost and/or under-boost errors...)

Yuri