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02 1.9Tdi130-Charge Pressure Negative (17964) code - Richard_Gomm
Hi all,

I have a problem with a Passat that some of you helped with previously, however it still exists and the garage have no idea where to go next.

To recap on the problem. I have a 2002 Passat 1.9 TDi 130 bhp, AWX engine code.

Intermittently (4 times in the past 4000 miles) the car loses a large amount of power, mainly struggling in 3 or 4 th gear. With the gas fully down in second there's no acceleration until around 4000 revs. Then its only a little boost.

Diagnostics so far:
Previously I have been able to resolve the issue by turning the engine off. Opening the fuel cap to allow air into the tnk and starting the engine. However it hasn't work this time.

The car has gone into the garage and they found a fault code:

17964 - Charge Pressure - Negative deviation in Turbocharger wastgate regulator valve.

The garage removed the hose leading into the regulator. They said it had a leak on the hose and replaced it. They also checked the voltage and impedance on the unit it connected into - they read 12.34 volts and 16 ohm so in range for the device. Next they deleted the code and tried the car out - still no acceleration.

The garage are now confused and not sure what to do next. Its been to two garages, one wanted to do a compression test as they felt it was misfiring. THe car does judder and jump when stationary indicating a missfire. However the second garage believe the misfire is due to this negative pressure.

On a side line when starting the car from cold a large amount of white smoke, smelling strongly of diesel (not oil) comes form the exhaust. Its enough to cloud the view in the rear mirror. It only happens on first startup from cold.

Any thoughts anyone? I've seen that some people on here have the same problem and are talking about a N75 - is that the unit I mentioned above??
thanks for all your help.


Edited by Pugugly on 12/06/2008 at 19:19

02 1.9 Passat Tdi - Charge Pressure Negative (1796 - Screwloose

You've got two problems; the idle miss/white smoke and the 17964 code.

The valve you mention is the N75 wastegate control vacuum solenoid; it one of the possible causes of the very common 17964/965 codes that are often flagged by a sticking, coked-up, vane ring in the turbo.

The negative pressure is underboosting and is nothing to do with the idle problem. First; do a compression test. No point in messing about with injectors and head looms if your fundamentals aren't correct.

Nobody seems to be interpreting live-data on either fault? There will be target and actual values provided for nearly everything - why are they not being used? Fault codes are only a tiny part of the info available.
02 1.9 Passat Tdi - Charge Pressure Negative (1796 - Richard_Gomm
Screwloose, thanks for the ideas.

After a day out on the car I've come up with something new - not sure if it means anything to anyone, but thought I'd post it.

I checked the vnt rod - it moves freely enough up and down so at least its not seized.

I've disconnected the MAF and took the car for a spin, it still has no power. The turbo doesn't seem to kick in till around 3000 revs (used to be a lot lower)

As a test I pulled off the vacum hose that goes into the vnt - I was expecting a hiss or sound of air escaping but I heard nothing... is that normal? I reconnected it and blew into one of the tube (very hard, almost passed out ;-) and I could hear a hiss of air coming out of somewhere.

I'm guessing that I have a leak in the vacum somewhere which I assume would give the negative pressure error??

thanks again for the help
02 1.9 Passat Tdi - Charge Pressure Negative (1796 - Screwloose

The ECU pulses the N75 solenoid whenever it wants to alter the vane ring position. There won't be any vacuum there at other times.

Frankly; without a scanner attached, it's very hard to test the system with any degree of certainty. Normally; you drive the N75 manually from the scanner and measure the vacuum level in the hose to the actuator to check that it's doing it's job. [Just because it's resistance is correct doesn't mean that it's not stuck.] Blowing down the tube proves nothing.

The MAP [thrust/boost/charge pressure] sensor must also be providing the right info to the ECU; or it will panic over a false alarm.

If that code is present; then the car will be in limp-home mode and won't run well. Negative pressure means that the boost is sub-optimal - it's not a vacuum-control level reference.
02 1.9 Passat Tdi - Charge Pressure Negative (1796 - Richard_Gomm
Hi, thanks for the quick reply.

That makes more sense, still wondering why I can hear a hiss thou when I blew into the hose?

The only garage near me is pretty useless - that have a diagnostic kit but have limited idea of usage (they can get codes but thats about it) - I've seen the machine working and it can measure all those values but they haven't a clue how to do it!

I'll order a new n75 and fit it just in case (looks pretty easy) otherwise I'll take the long drive up to a main city and find a main garage.

thanks for your ideas.

02 1.9 Passat Tdi - Charge Pressure Negative (1796 - Screwloose

In certain positions; the N75 dumps the applied vacuum to atmosphere. No good pulling something in if you can't let it go again.

Why not get yourself VAG-Com? That's got full live-data and dealer-level functions [if you want it to.] It's even free - though you'll need an interface box.

Your garage is fairly typical; unless they spend thousands on more kit and many days on training, then they'll never understand what it's telling them. Codes are dying fast - most are now unintelligible to the average mechanic - like your one. [Don't assume that the dealers are any better....]
02 1.9 Passat Tdi - Charge Pressure Negative (1796 - dieselnut
I had a similar problem with my sons Passat 115 PD.
One of the rubber vacuum pipes from the N75 valve had a leak.
The pipe they use seems to be very poor quality & easily becomes damaged, especially when it's a few years old.
The Turbo actuator normally has full vacuum applied until boost pressure rises.
Then as Screwloose says, the N75 valve is pulsed to dump the vacuum to stabilise the boost pressure.
The dump pipe is located near the air cleaner box, so I think when you are blowing down the pipe it will be exiting from the dump pipe.
If you start the engine & allow to tick over there should be full vacuum at the turbo actuator & by removing/replacing the vacuum pipe to the actuator you should see the actuator rod move slowly up & down by about 2cm.
Hope this helps.
02 1.9 Passat Tdi - Charge Pressure Negative (1796 - Richard_Gomm
Hi all.

I finally got a cable and shareware copy of vag-com. I've hooked it this morning and taken a load of readings from groups 001 - 025 at both idle and 3000 revs. Got tons of results but not too sure which areas I should be looking at.

Also the car has developed another fault code since I played with it the other day. I'm pretty sure thats whats causing the exessive smoking.

The faults as reported now are:

17964 - p1556 - Charge Pressure Contr. Negative Deviation
17552 - p1144 - Mass or Volume Air Flow Circ Open/Short to Ground

I'm assuming the second one is causing the white smoke on accerlation?? not enough air getting into the mix so the car throws out the fuel vapor - or does it not affect it that way?

Lastly I saw the group that showed glow plugs, with engine off but igination on I had two of the digits showing 1 (off) and all the others as 0. After the engine started they all went to 1. Am I assuming that this means I have two plugs out?

If anyone is interested in the figures vag-com throw out I'll paste them, if you let me know which groups (001 - 025). Or if you know which ones I should be looking at in particular?

If you actualy wnat to see the smoke and sick engine noise:


02 1.9 Passat Tdi - Charge Pressure Negative (1796 - dashdotdash
Richard, did you solve the problem?
I have 2003 sport 130 estate, same problem.
also bought vagcom,
Same code 17964 p1556

new air mas sensor,
new charge control valve,
new fuelfilter,
actuator rod has 1" movement in it when vacuum introduced to it.
Will check readings on the move tommorrow.

just hoped you may have solved it already!
02 1.9 Passat Tdi - Charge Pressure Negative (1796 - Richard_Gomm

sorry for the slow reply. Been working on / off the car for ages.

Finally fixed the turbo boost problem. It was the small silver box thing at the front of the bottom engine. The dealer called it a turbo boost pressure control but I think its also know as the wastegate. They replaced it and the car's been fine for a good 3000 miles nows.

thanks for everyone's help.


02 1.9 Passat Tdi - Charge Pressure Negative (1796 - 659FBE
VNT actuator?

02 1.9 Passat Tdi - Charge Pressure Negative (1796 - Richard_Gomm
just checked - reciept shows it as a turbo boost control solnoid (N75)

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