Drive train shunt

[ChrisH2017]

Hi all, after a bit of advice. My 2003 rs6 has developed an odd issue, basically it feels like a hard-ish shunt forward at times whilst at steady throttle, worse at motorway speeds, and possibly worse on light throttle. I had a gearbox fluid and filter change, seemed to improve (fluid was low due to a leak on cooler pipe joint, but not burnt or particularly bad). Had joint repaired, fluid topped up, seemed ok again then has started the same thing. No codes when I last checked on vag, I am thinking tc un-locking then relocking, but oddly if I give it some throttle whilst it is doing it I don't get any rev flare - suggesting it's all still locked. Could be an engine missfire, but rev counter is solid, and doesn't feel like engine, too sudden. Maybe tc internal fault, but would have thought I would feel slip, maybe solenoid or loom? Any thoughts or suggestions welcome! Seems more likely to do it when hot,

[srichards]

If you use cruise control does it do it the same or differently? If it has only been when using cruise control don't use it for a bit on the same roads to see whether it's the same or not.

Think the TC locks at about 40 mph so if you bibble below and above that speed a few times and it's consistently doing it then it does point to a TC issue. If you do it a few times it might give you a fault code as well. You'd need to fix a gear and do it to rule out a clanky gear change.

When has the F125 switch been previously replaced?

[ChrisH2017]

Mary thanks, usually 60 to 70 when its obvious so well past when the tc should lock then. Don't know if the F125 has ever been replaced, I guess that could be a possible? I don't tend to use cruise so definitely not that. Cheers

[srichards]

Worth checking it at the same rev band in another gear or two to see if it does the same thing or whether it's just a weird thing at that speed alone.

If F125 has never been changed it's worth replacing on spec as it's a relatively cheap thing to do.

[ChrisH2017]

Just rescaned with VCDS lite, no transmission codes. Very strange, I guess if it was a failing solenoid or loom it would throw a code? Similarly if it was a tc issue? Got a couple of engine codes, one about the aux fan module (been an intermittent code for a while), the others I need to check but were both intermittent and I think they are O2 sensor speed of response.

[Shoppinit]

By shunt you mean hesitation? Or stuttering?

Could be a failing fuel pump. I had this once (on a different car) and it felt like a gearbox issue.

[ChrisH2017]

Hi its a bit like someone driving into the back of the car, although not as hard as that, but more abrupt than taking off the throttle and reapplying. I did wonder if it could be an engine missfire but it feels too sharp for that. I changed the sparkplugs, no change, engine has loads of power so.dont think its a fuel issue. I had an issue with the brake light switch a couple of year back, that cut the throttle occasionally but again not as sharp as this. This does feel drivetrain related.

[Shoppinit]

Inner CV joint can feel like a misfire.

When my fuel pump was failing I would have full power most of the time.

[ChrisH2017]

Ok many thanks, that's interesting as I thought I could.feel a slight knock on occasion when slowing to.junctions, felt like something catching on a front wheel. Could a bad cv joint give a hardish jolt when driving?

[Shoppinit]

Doesn’t seem likely at constant speed. I’d still suspect fuel pump.

To eliminate the TC and box, log it all when driving and out a marker down when it happens.

[ChrisH2017]

Ok many thanks, I need to figure out how to use the logging functions of VCDS!

[srichards]

Fuel pump relay is another likely suspect if it can be a fuel pump issue.

[ChrisH2017]

I logged the transmission, the car did the same jolting as before prob approx half way through the log file, I can't see anything odd in the log, in particular looking at the tc it locks when expected then goes to Reg when I lift off. No.slip when the tc is locked. Anything I am missing?

[ChrisH2017]

What's the best way to check for fuel pump issues, don't think VCDS can log fuel pressure?

[Shoppinit]

Yeah. It irritates me that the RS6 doesn’t have a fuel pressure sensor. You could remove the back seat and get someone to listen to the lifter pump. Might hear it cavitate, but it’s far from sure. If it’s the main pump then you’d need to insert a pressure gauge into the injector rail.

You could check the voltage to the pump from the fuel pump relay. It varies the voltage depending on demand but only between 10 and 12v IIRC.

You could check the fuel pressure regulator for leaks. You’d probably smell it if it was failing though.