Passat TDI - Odd Stumble at 1500 RPM

Just got back from a road trip in the Passat during which it developed a stumble around 1500 RPM. The car is a 2005 Passat TDI wagon with 160k km on the clock. At 80 kph and 1500 RPM I can feel a stumble - the tach oscillates rapidly over a 200 RPM range but the speedo holds steady at 80 kph. When I speed up above 1700 RPM or slow down below 1400 RPM everything smooths out and the car runs well. If I accelerate through 1500 RPM I can feel a hard stumble very much like a single cylinder misfire on a gasoline engine - again the stumble is gone around 1700 RPM. The engine idles smoothly and the car runs smoothly outside the 1500-1700 RPM range. The "stumble" is felt not heard.
I must admit I'm a bit puzzled. It seems like a "misfire" issue but I can't understand why anything that would cause it would be restricted to such a narrow RPM range. I would have thought a bad unit injector or a worn cam lobe would cause worse symptoms as RPM increased. The only thing I've ever come across with symptoms remotely similar was a '87 Olds Delta 88 with a bad lock-up torque converter. At 80 kph when the "lock-up" mode was triggered the TC would try - and fail - to lock, and the car shook like crazy as the lock cycled on and off.
Has anyone experienced a similar problem with their Passat? Any suggestions where I should start looking?

Thanks!

It's your torque converter slipping. You'll need to address it ASAP before serious transmission issues develop.

Here is my experience..
http://forums.tdiclub.com/showthread.php?p=4293241#post4293241


.

Thanks DeliveryValve.
I'll check the tranny oil level and scan for codes with VCDS, but I guess I'd better start sourcing torque converters.
Dang!

QZone said:

Thanks DeliveryValve.
I'll check the tranny oil level and scan for codes with VCDS, but I guess I'd better start sourcing torque converters.
Dang!

Click to expand...

If you didn't see this in my thread, you can verify the slippage using the graph function on the VCDS.



http://forums.tdiclub.com/showpost.php?p=4313176&postcount=15



.

Still hunting down this stumble issue. Pulled the fault codes with VCDS - no faults. Used DeliveryValve's suggestion to log the TC clutch state, TC slip speed, and engine RPM. There is no TC slip with the clutch Closed and sometimes there is slip in the regulation mode (TC Clutch Reg.) but from what I understand this is as it should be. There is slip in the Open mode as there should be. I don't see the nice regular slip DeliveryValve shows in his graph - mine is more varied in time and amplitude and only in Open or Reg mode. Sometimes there is no slip when I feel the "stumble" and at other times it shows slip when I feel the "stumble". Guess I'll have to get a buddy to ride with me to mark the log when I feel the stumble - still new to this VCDS stuff.

I'm wondering if it may be related to another problem I had a few months ago. I was on another road trip and the car was running fine. Stopped for supper and resumed trip - the car was way down on power. I'm talking '63 VW bug power levels - top speed 100 kph, 80 kph on hills full throttle. Otherwise the car ran fine - no CEK light and mileage was superb at 4.2l/100km. Eased it to my destination, next day it ran fine with full power. I checked the hoses to N75 & all was good, so I assumed the valve may have stuck and shut down the turbo. No codes for that either and I think it should have thrown a "low boost" code if that was what happened. Now I'm wondering if I have fuel delivery issues. I did notice a huge cloud of whitish-gray smoke when I gave it full throttle at 100 kph to see if the TC slipped when locked. The troubleshooting continues. Rebuilt tranny is $4500 at the dealer with a $1500 core...

Close mode, there should be no slippage at all regardless of throttle position.
Reg mode, there should be some slippage when adding throttle.
Open mode, there is a lot of slippage.

So if you hold the throttle steady at 80 kph and around 1,500 rpm, the slippage should be steady stream on the graph. If it's varied in time and amplitude as you say during this steady throttle period, then I think it's most likely the TC clutch is slipping too much and the computer is over or under compensating.

Thanks DeliveryValve. I'll have to do another run and try to get throttle opening in the log as well. I wasn't trying to hold the throttle steady on the last run - I was just trying to reproduce the "stumble", so the slippage variation I was seeing is likely from varying throttle inputs. If I read your response correctly you are saying at steady throttle in Reg mode the slippage should be constant with no variation in amplitude.
Do you know how the TC clutch limits slippage in Reg mode? Does it partially apply the clutch to create drag or rapidly cycle it on and off?

It partially engages the clutch during the Reg mode. I would imagine if it cycles on and off it would not be steady. Kind of like what happened to the worn clutch on my TC. It kept cycling to grip harder and release which cause the blips on the RPMs.

On page 8- 9 on this .pdf I uploaded, is the function of the TC. Here they called the Reg mode, Control phase.
http://pics.tdiclub.com/data/500/01v-transmission-description.pdf


.

Thanks for the pdf DeliveryValve! I was looking for something like this online but couldn't find anything.

My experience, that all these rpm related judder, misfire like stuff is related to #3 nozzle and needle lift sensor.. i had zero errors, but eventualy noticed, that plug and the socket are not rusty, but kinda oily, witch caused implausible connection, bad readings And it wasnt giving any error on vcds Clean up the oily connection, and problems are gone, no service could tell me why car was shuddering before. Also i have tryed to change #3 whole injector from other car, and then i got some juddering in 1700rpm range before and after car runs smooth. I think thats because its different wear, and opening presure is way off my rest nozzles, so its need to be calibrated. Cleaning #3 connection with some WD or smthng cost no money and no time worth to try.

Thanks poisas. I'll check those connections. Where are they - on the injectors under the valve cover? I'm not that knowledgeable on the TDI fuelling system, but I have a feeling I'm going to be getting a lot more familiar

Disregard that post, your car has no needle lift sensor.

From your description, I would change the CKP sensor.

Thanks oilhammer! Is this the sensor the manual refers to as the Engine RPM Sensor G28? It says it measures engine RPM and reference marks. What is involved in getting at this sensor? In it's typical fashion the manual just says "replace sensor" if faulty. The manual says that the engine will stop if the sensor fails with the engine running, so I'm assuming you suspect the sensor has gone intermittent and that is causing what I'm feeling as a "stumble". Is it common for these to get flaky without throwing a code?
Thanks again! I appreciate the advice!

Shudder Fixed W/O replacing TC

The general consensus was that the torque converter was in the initial stages of failure and would eventually need to be replaced.
I turned to my good friend Mr. Google in an effort to better understand the situation and this is what I found:
1. Other makes with "lifetime filled" auto transmissions equipped with lockup torque converters are also having this problem. It's generally called driveline shudder.
2. The "lifetime" in the lifetime fill is 160,000 km or 100,000 miles.
3. The shudder can sometimes be resolved without replacing the torque converter if caught early enough and the transmission oil is changed.
4. Transmission oil is no longer just a slippery fluid designed to reduce friction. It has friction properties specifically tailored to certain transmissions. When replacing fluid make sure to use the one specific to your transmission.
5. Modern lock-up torque converters have three operating modes: OPEN (low speed, clutch disengaged), LOCKED (high speed, clutch fully engaged), REGULATED ( medium speed, clutch partially engaged). The REG mode is intended to smooth out small abrupt throttle variations by allowing limited slip between the crankshaft speed and the transmission input shaft speed. The ECU measures slip in REG mode. If it exceeds 500 RPM it sets a code.
6.Rebuilders who work on bad Passat torque converters typically fine lots of clutch material left but the lining and the mating face is glazed (anecdotal).
7. A couple very good VW techs I know told me that they had seen failure of high mileage transmissions shortly after a complete oil change. If I wanted to change the oil they recommended a staged oil change.

I decided to proceed with a staged oil change. I drained 3 litres of oil and replaced it with VW transmission oil from the dealer. The drained oil was black and full of ultra-fine suspended metal particles. Under a light the oil looked a lot like liquid metalflake paint. With this metal load it's transitioning from oil to an abrasive fluid. I'm assuming the black colour was from suspended clutch material and the metal particles are from wear to the clutch mating surface - I dumped the old oil without any further analysis.
I drove the car for three weeks. The shudder issue was noticeably reduced but still present. I then drained the transmission oil and pulled the transmission oil pan. The bottom of the pan was coated with clutch lining sludge and the pan magnets were very "fuzzy" with metal particles. No large metal bits in the pan and the oil filter looked clean. I replaced the filter and refilled with close to 6 litres of new oil. I didn't flush the old oil out of the torque converter.
I've just returned from a long road trip in the car and am happy to report that there is no sign of the shudder problem. The tach no longer hunts at 1500 RPM and the drivetrain ran smoothly. I don't know if it will last but here's hoping.
It looks like it may be a good idea to change the oil on these transmissions every 80,000 km or so to flush out the suspended clutch material and metal particles.