Jetta 125TDi doing kangaroo impressions

[jeffa]

Hi everyone,

A couple of months ago I "went German" and bought my first VW, a Jetta 125TDi.

Unfortunately it has a problem, but being hard to reproduce, the dealers will not conduct any thorough investigation until they can see the symptoms. They did confirm there were no engine fault codes logged by the computer and at least tried driving the car several times during the day. I just thought I would ask here to see if anyone else has had this issue or can suggest things I can try to confirm what the problem is.

The problem is that the car "lurches", as if the engine is mis-firing or mssing, although it could be other causes. This can become very severe, and accompanied by a "clunck" from under the bonnet with each lurch. The problem is very specific: it only occurs with very low RPM (about 1200), and a very small amount of throttle applied. It also seems to be more prevalent at certain times of the day and rainy weather seems to make it much worse. It also seems to be more prevalent when the engine is warming, and the temperature gauge is at about 9 o'clock. At about this time, the idle can get rougher than usual. Applying no, or larger amounts of, throttle stops the lurching, and I have also found that if I swap to "manual" and select a lower gear, this also fixes the situation. Swapping back to the higher gear then causes the lurching to start again.

A you can see, there are a list of conditions to make this happen, but the annoying thing is that a couple of hills on the way home from work that pretty much always trigger the problem so I experience this virtually everyday. Unfortunately, these hills are no where near the dealer.

I am loving the fuel economy, but if it could refrain from jumping like a kangaroo at times, I would be much happier. Thanks for any information you an provide.

Best regards, Jeff.

[antone_86]

That's a tricky one Jeff - I'll have a think about it.

Do you feel it is more related to throttle position or RPM?

[jeffa]

Do you feel it is more related to throttle position or RPM?

My guess is throttle position. Maybe it does do it when travelling faster (higher RPM) and with little throttle, but in that situation the car would be coasting under it momentum and you may not tend to feel any surging.

Yes, it is a tricky one. My first thought was it is fuel problem, but it's hard to fully explain. I also thought maybe the electronics having an argument as to what gear the car should be in (continually changing up, then down, then up), but putting it into manual disproved that theory. I have read about oxygen sensors playing up in the past and that faulty ones gave a similar behaviour. That might make some degree of sense given that oxygen levels would be affected by air temperature and humidity etc.

Thanks for your interest.

Regards, Jeff.

[antone_86]

Okay, if it's throttle position it could be an issue with your fly-by-wire throttle signal that is sent to the ECU and that would, in turn, talk to your injectors incorrectly and muck about. This is a wild guess based on the information you can provide but I'd definately mention it to VW.

I would target the electronic throttle control module and the electronic throttle body if you feel it is more throttle position based rather than RPM based Jeff.

It could be a O2 sensor or similar playing up - I'd suggest running some diagnostics or making notes of changes in measurements from the CANBus using your Climatronic Unit --> See http://www.vwwatercooled.org.au/f15/...-out-8175.html

Instead of ECON use the AC button though, works the same.

You might find something wierd if you look for changes in sensor readings that may give you some direction in solving this mystery!

Happy to (try and) help mate.

[jeffa]

Okay, if it's throttle position it could be an issue with your fly-by-wire throttle signal that is sent to the ECU and that would, in turn, talk to your injectors incorrectly and muck about. This is a wild guess based on the information you can provide but I'd definately mention it to VW.

I would target the electronic throttle control module and the electronic throttle body if you feel it is more throttle position based rather than RPM based Jeff.

Well, I don't know. It only plays up with small amounts of throttle.

It could be a O2 sensor or similar playing up - I'd suggest running some diagnostics or making notes of changes in measurements from the CANBus using your Climatronic Unit --> See http://www.vwwatercooled.org.au/f15/...-out-8175.html

Instead of ECON use the AC button though, works the same.

You might find something wierd if you look for changes in sensor readings that may give you some direction in solving this mystery!

Wow! That wasn't in the manual! So is the CANBUS data on channel 20? Is that engine or passenger air data?

Thanks for the advice.

Regards, Jeff.

[antone_86]

Most of the data on the menu comes from CANBus - 19.0 is useful, km/h from CANBus shows how out of whack your speedo is!

Let us know how you get on with this mate.

[Russ59]

Could also be to do with the DPF and the regeneration process, my GT TDI 125kw was doing very similar only I don't recall a clunk. Could the clunk be to do with the traction control coming on due to the sudden burst of power to the front wheels and possibly wheel spin (especially in the wet) My GT TDI would run like an absolute dog when it started the DPF regeneration and was almost undrivable and very dangerous in traffic. I had it to VW and no error codes registered either. I have since installed the ChippedUK ST3 upgrade (Fast Road map) and unless I'm sitting in traffic when it starts I rarely notice it happening anymore. Even when it does start the regen. now it just runs a little rough for a few minutes and the idle goes up and down a few times, then it's done. Might be worth considering asking VW to check the DPF and even get them to manually start the process and see if the symptoms match up.

[anthony]

Make the dealer go with you on your usual drive to/from work...

[jeffa]

Could also be to do with the DPF and the regeneration process, my GT TDI 125kw was doing very similar only I don't recall a clunk.

I was under the impression that DPG regeneration was something that happened once every 1000 km or so (depending on conditions), not on a regular basis. I have had one instance where the car was acting strangely (revving higher and not changing to higher gears), and which I think was this process. There was no problem with the car during this time, but it was just unexpected the first time it happened.

Could the clunk be to do with the traction control coming on due to the sudden burst of power to the front wheels and possibly wheel spin (especially in the wet)

That's a possibility, but I guess anything could make a noise with sudden changes in power.

I can only try to see if I can get the car to play up on a road near the dealer.

Thanks for all the suggestions.

[antone_86]

Any update here Jeffa?

[jeffa]

Reprise

Hi all,

After about 18 months of driving, I am pleased to report that this issue I had with the engine stuttering seems to have corrected itself. In case anyone else is trying to find more information about this, it seems that the problem only occurred with the ambient air temperature was between, say 18 and 24 degree C, the engine was warming up, and you were applying very very light throttle. It was particularly noticable after a period where you have been applying no throttle, say going down a hill, and you were easing the throttle back after the road flattens out. When it happened, the engine would stutter, as if starved of fuel.

Over summer, the issue disapeared with the increase in temperature, but on winter's return, I found the problem was still present, but very much reduced. It very rarely does this now, and it is no where near as bad. So I'll guess we'll put this down to teathing problems.

Thanks to everyone who responded.

Jeff.