Differential diagnosis for slow with power on/off...

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July 13th, 2015, 20:43
Zorb

Differential diagnosis for slow with power on/off...

For the last couple of months, I have been having performance problems with my car.

Facts:

- Just turned 90000 miles.
- Timing belt and related done at 75000 last year.
- Spark plugs just replaced (NGK BKR7E-IX) which seems to have corrected some annoying roughness I had been experiencing, and literally added 2 MPG to the average.
- Fuel filter replaced at 80000 and then at 87000.
- Car has Eurocharged tune as of end of last year. Problem affects both tuned and non-tuned (stock program) operation.
- Car has aftermarket intercoolers because factory Volkswagen units leaked.
- No relevant codes on VCDS.
- No OBD2 codes at all.
- MAFs and TB perfectly clean.
- Factory SE exhaust.
- Air filters replaced 5000 miles ago.

Symptoms:

- Steadily slowed down over the last 3 months.
- Runs great cold. Start it, drive moderately for a few minutes until oil temp is showing something, and it's fast as normal. Let it heat up, it slows way down.
- Problem is intermittent but increasing. So far it has avoided my data logging efforts.
- I suspect a problem on the left turbo due to fairly significant (4 ounces in 15000 miles) oil in the intercooler while the right side has a few drops or a film.
- Strange power on/off thing going on at highway speed. Take an onramp at full throttle from a stop, boost comes in strangely abruptly and in "stages". If you stay on it, it falls on its face around 110 and feels as if the low-fuel "limp" mode has activated. Stay into it for a couple of more seconds and sometimes power comes right back to what it was before.
- Turbos sound "sick", whistle-ish and strangely noisy, rather than smooth and "melodious" like they used to.
- Almost feels like when my friend's Subaru was hitting overboost and it would cut his boost level way down for a few seconds and then give power back until it hit overboost again. Problem is that my slowdowns aren't preceded by increased power.

I have tried logging everything like MAP requested vs actual and a few other things. So far, the problem has not happened with the computer connected.

Any ideas where to start testing next?
July 13th, 2015, 23:21
Zorb

Add to symptoms:

- Very late turbo spool about half the time.
- judder/uneven power delivery above 4000 RPM.

My gut feeling is a bad turbocharger with a bearing problem and/or a problem with its variable geometry system.
July 14th, 2015, 00:19
nubcake

There's no variable geometry in stock RS6 turbos (I presume C5?).
Although it could be a problem with sticking waste gate actuator if you meant that. Or maybe it has something to do with N75 and control lines.

I'm puzzled as to why the problem is intermittent and/or heat related.
July 14th, 2015, 03:24
SteveKen

Might be the spark plugs. Try things with the stock ones back in.
July 14th, 2015, 04:35
Zorb

I was told by a VW engineer that the factory turbo used variable vanes.

This problem started before changing plugs.
July 14th, 2015, 05:03
DHall1

3 31 115

2500 to redline

3rd gear

Post the log
July 14th, 2015, 07:53
Zorb

Quote:

Originally Posted by DHall1

3 31 115

2500 to redline

3rd gear

Post the log

Will do. Last couple of times I tried this, it picked that moment to run like a bat out of hell, a.k.a. perfectly.
July 14th, 2015, 08:00
P1054

2 Attachment(s)

Quote:

Originally Posted by Zorb

I was told by a VW engineer that the factory turbo used variable vanes.

This problem started before changing plugs.

This won't answer the original question, but for the record I'd say the engineer was mistaken.

Here is the compressor wheel from my right side turbo when I was rebuilding it..
Attachment 16552

And here is the turbine wheel from the same turbo..
Attachment 16553
July 14th, 2015, 08:03
P1054

Regarding the original question, it sounds like the others have some pretty good suggestions. I haven't experienced anything like this (and hopefully won't) but if I do I would feel confident in solving it with the help of the other members of this forum (and a VAG-COM cable).
July 14th, 2015, 13:14
Other_Erik

Quote:

Originally Posted by Zorb

Add to symptoms:

- Very late turbo spool about half the time.
- judder/uneven power delivery above 4000 RPM.

My gut feeling is a bad turbocharger with a bearing problem and/or a problem with its variable geometry system.

Agree with bad turbocharger (ounces of oil blowby when the other side is just film/drops? yikes) - no variable geometry in K06's. Grab yourself a rebuild kit and hope the compressor wheel hasn't scored up the inside of the housing, chipping itself up in the process.
Couple that with possibly bad cam tensioners (requested vs. actual spark timing) and/or worn coil packs (random missing per cylinder, spark retard mode), and you've got yourself a recipe for the lowest-powered RS6 on the road.
Ever replace your N75 valves? May be sticking, fairly cheap/easy fix for this car.
Not trying to be a dick, when you replaced the fuel filter, you put it in correct direction, right? Monday morning detail that has been missed before by others.

Log as others have said here (3, 31, 115, Turbo mode, 3rd gear 2500-redline), keep doing it until it displays the symptoms.
One more thing to try - the "redneck reset". Key in, turn to "On" (don't start car), Gas to floor, hold for 5 seconds, Key to "off" and remove, release gas pedal, wait 7 minutes or more before firing up again. Puts power control back into learn mode.

O_E

PS - I hate seeing "runs great cold" - don't abuse the transmission when it's cold. At least wait until trans temp is 35C before pushing it! Don't need to hear about another $8k trans-grenade. Your gut instinct is probably a good place to start. You know the car well enough to know when the turbo doesn't sound right...
July 14th, 2015, 14:30
DHall1

No redneck reset yet

And its the sneaky reset. The redneck reset is pulling the battery cable which screws up the tps sensor calibration. :-)
July 14th, 2015, 17:16
ben916

I agree with all about the Turbo taking a dump (out of round and then it resets itself and is back "in line" and goes as designed).

MAFs clean = great! Still might want to check them AND the connections to the MAF as they seem to get stretched/pulled/"F"-ed with...

My rock solid guess for the "winner winner chicken dinner" is blown turbo. Final Answer.
July 14th, 2015, 17:43
Zorb

Quote:

Originally Posted by Other_Erik

...

PS - I hate seeing "runs great cold" - don't abuse the transmission when it's cold. At least wait until trans temp is 35C before pushing it! Don't need to hear about another $8k trans-grenade. Your gut instinct is probably a good place to start. You know the car well enough to know when the turbo doesn't sound right...

Please note that I did say after running and driving nicely for several minutes.
July 14th, 2015, 17:46
DHall1

Ahhhh

No

:-)

Quote:

Originally Posted by ben916

I agree with all about the Turbo taking a dump (out of round and then it resets itself and is back "in line" and goes as designed).

MAFs clean = great! Still might want to check them AND the connections to the MAF as they seem to get stretched/pulled/"F"-ed with...

My rock solid guess for the "winner winner chicken dinner" is blown turbo. Final Answer.
July 14th, 2015, 18:15
Other_Erik

Quote:

Originally Posted by Zorb

Please note that I did say after running and driving nicely for several minutes.

I did see that, but I also saw:

Quote:

Originally Posted by Zorb

Symptoms:

Quote:

Originally Posted by Zorb

- Runs great cold. Start it, drive moderately for a few minutes until oil temp is showing something, and it's fast as normal. Let it heat up, it slows way down.

May just be the way I drive, but "moderate" driving in this car (to me) is everything but flooring to kickdown switch / flooring from dead stop / brake-boosting. More than enough to set off the ticking trans bomb. I baby the trans from cold starts - nothing over 2500rpm, no jackrabbit starts, basically treat it like the initial 500 mile break-in period. Anyway, we may have hit on something here...

You've said you have aftermarket IC's - Wagner? Do you still have your secondary radiators for trans fluid cooling?
Another recommendation before the "sneaky reset" - log your trans temps. See if there's unusual spiking compared to load/ambient temperature/speed. If you see anything above (others will have to help me with this here- 115C? I could be way off on that number), it's time to find the issue with your trans fluid cooling loops. One each secondary radiator, one to the driver side of the main radiator (secondary tank)

In any case, blown turbo sounds like the consensus answer, but there's any number of things that could cause the symptoms you're seeing, including combinations of failures. Start with the known broke bits.

O_E
July 14th, 2015, 18:42
DHall1

I have never said blown turbo

i need the dang data log then I can speak
July 14th, 2015, 19:35
lswing

Is this just a bad case of heat soak? Symptoms sure sound like it. Zorb, have you had a hot summer in this car? The heat soak could be compounded by some of the performance issues such as failing turbos.
July 14th, 2015, 21:38
Zorb

Quote:

Originally Posted by lswing

Is this just a bad case of heat soak? Symptoms sure sound like it. Zorb, have you had a hot summer in this car? The heat soak could be compounded by some of the performance issues such as failing turbos.

When I brought it home from PA via Florida last July It was very hot. It ran far better than this, in hotter and more humid conditions. Also, it's running great today despite being warmer than yesterday.

Also, my first one never behaved in this way, though it admittedly had far fewer miles when I sold it.

And to answer someone else, I have the extra radiators installed still, yes.