Coupling/jerking

[Le_Combattant]

Hello all,

Hope you doing weel.

Since I get my car in July 2020, I get some coupling, engine jerking at low speed in traffic jam.

It happens when I'm in:

-1st, 2nd and 3rd gear
-Between 2K and 4k RPM

And only if I release the gas pedal more than one seconde and put my feet back on it.
Didn't appear when I release throttle and that I go over the gas immediately.

I already change the dog bount mount on the gearbox: nothing.

Sparkplugs have already been replaced by the dealer for the 90000 km overhaul.

However, coil igniter still from the factory.

The car has now 96700 km and no fault code (miss fire etc...)

What would be your advice?

[veteran]

Sounds like a clutch issue to me. Probably, the clutch plate (between flywheel and clutch cover) has become contaminated with engine oil that's seeped past the engine-to-clutch housing oilseal (main shaft). Thus, at low revs the linings on the clutch plate become sticky and you get juddering, and gearchanging is difficult, but as you increase the engine revs the contamination tends to burn off.

Failure of that oilseal seems to be fairly common on the 1.2 TSI, though this is something that can happen to most manual cars in the longer term, irrespective of marque or model. Sometimes, the issue can be aggravated by an overfilling of fresh engine oil on a service session, the oil in the sump then blowing past the major shaft seal(s) when the engine runs.

Look for signs of oil on the top side of the vehicle's undertray. Use your fingers to feel around where the engine joins to the clutch bell-housing. If there's oil getting into the clutch, some will eventually drip down the inside surface of the clutch's bell-housing and will exit through a finger-sized hole in the bottom of it and thence on to the undertray. If you've a means to raise the front of the car sufficient for you to temporarily remove the undertray, you'll be able to check this for yourself.

If this does turn out to be an oil-contaminated clutch issue, then you might be lucky in limiting the symptoms from this point on by just ensuring that the engine is never overfilled with oil in the future (keep to just over halfway on the dipstick marking). But usually if the clutch plate's become badly oil-contaminated then the only assured thing to do is to have the relevant oil seal(s) replaced and a new clutch plate fitted (preferably by a VW workshop). That would mean removing the gearbox and bell-housing and could be quite costly.

Sometimes, hydraulic fluid from a leaky clutch slave cylinder can find its way on to the main shaft in the clutch, eventually causing similar contamination. On the 6C 1.2 TSI, the clutch slave cylinder is mounted on the outside of the bell-housing, at the end furthest from the engine and so is easily accessible. Even a little bit of air trapped in the slave cylinder, or in its feed pipe, could conceivably make gear-changing and moving through the low revs a bit hit and miss.

[Le_Combattant]

Thanks to your feedback.

To be honest, the gearchanging is smooth. Nothing to report about that.

The oil level is stable since 6000 km, it didn't move at all.
And before that, nothing. No oil consumption.

Bottom of the engine is dry, even between engine and gearbox connection.

[veteran]

Okay, so the jointing between engine and clutch/gearbox is 'dry'. That's a good sign, as is also that the gear-changing is smooth.

You say the oil level is 'stable', but is it high, as seen on the dipstick? The difference between optimum fill and overfill is very small, maybe 50ml or so.

The fact that your Polo's not reporting any faults or errors on the dashboard suggests that the cause of this malfunction isn't ignition-related and is instead something mechanical and normally undetectable by the onboard electronics.

When you get the juddering, is the vehicle's Stop/Start on? If it's on, then it's possible to get the engine half cutting out under certain conditions. But I guess you've had enough experience with the car to appreciate that, as would also be the case with your workshop personnel.

Did this problem exist before that 90,000km overhaul? If not, then the workshop personnel may not have done the servicing 100% correctly.

[Bepis]

I get the exact same jerking and bucking between 1000-1200 RPM in 1st and 2nd crawling in traffic. More pronounced in 1st due to the gearing being shorter.

Just seems to be a polo thing as someone I know with the 1.4MPI has the same issue.

[amer6R]

I sometimes get the jerk when going down hill in second gear, with light throttle pressure on the tdi, pretty annoying.

[veteran]

"Just seems to be a polo thing as someone I know with the 1.4MPI has the same issue".

"I sometimes get the jerk when going down hill in second gear, with light throttle pressure on the tdi, pretty annoying".

Can this perhaps be put down to differing levels of fine clutch control between drivers? Or are amer6R and Bepis suggesting that the jerkiness has nothing whatever to do with the clutch at all but is maybe simply down to the built-in gearing? Can't say I myself experience any abnormal level of jerkiness with my 2017 very low mileage 6C 1.2 TSI.

If I've understood his description properly, Le_Combattant is saying that the problem didn't exist during the earlier period of his ownership of the vehicle.

[amer6R]

I'm not sure, I had a new clutch set put in and had the same issue.
I think it's something with the map in ECU, I also hate the anti stall it cuts in with slow at slow speeds in city driving. ( I think the revs drop too slow)

[veteran]

"I'm not sure, I had a new clutch set put in and had the same issue".

Well, could it have been that your jerkiness was the result of the clutch renewal (damage to or strain put on the output shaft's oil seal during the renewal, and so thereafter always some oil getting on to the clutch plate)?

[Bepis]

I get the jerking issue with the clutch completely engaged and just using the throttle to control my speed. I don't think I've ever experienced any jerking / bucking with partial clutch engagement.

With the 1.8TSI and MQ350 QQR gearbox, the minimum speed in 1st is 4mph and 2nd is 8mph.

If I'm crawling in 1st doing 5-7mph maintaining my speed, the bucking will start and continue until I either release the throttle or increase my speed. If I let the car continue to jerk and buck it'll eventually do it in a consistent rhythm which probably looks very odd the everyone around me .

In 2nd crawling at 9-11mph, the same thing will happen just not as bad. It feels as though the car's throttle mapping is shutting the throttle very suddenly in order to maintain the car's speed but causes very sudden engine braking and the lower the gear the more pronounced it is.

It'll sometimes happen in 3rd but not as consistently and nowhere near as bad.

With the anti-stall in stop-start and low-speed city driving, it'll happily go up to and maintain 4mph if I set off with just the clutch but if I'm rolling at say 2mph and get near full clutch engagement it'll shoot up to 7mph with no throttle input.

Other than this strange low-speed behavior, the leaking thermostat housing (fixed under warranty thank f**k) and now a leaking valve cover that needs sorting which likes to occasionally make it smell like I'm sat behind a 2006 Renault Trafic with a knackered cat. This generation of Polo with the 1.8TSI has many more positives than the previous examples of Polos I've owned .

Also regarding the dogbone mount. When that goes bad you'll get a lot of engine movement/shunting when downshifting accompanied by a muffled knock/thud. Pictures below of one from a 9n3.

[amer6R]

I've replaced the dog bone mount, still the same. I think its in the ecu map that cuts or add the throttle