Hi from Kent, UK!
I've had a Golf Mk4 TDI for about 6 months and getting limp home mode due to positive deviation almost every time I drive it now - seems to occur at high throttle openings in top gear, usually going up a hill. VNT rod appears to move freely and the capsule isn't leaking; have cleaned out the EGR valve (although it didn't look too bad) - that didn't help the limp home, but did seem to clear up a slight stumble at light throttle around 2000rpm. I've just changed both valves (N75 and EGR) but again no difference, so it is looking more like the VNT, but hoping to find something else before investigating that, as from the outside it appears to be working OK!
I'm using a registered version of VDCS-Lite, so have been looking at logged data - actual intake pressure is going up to ~3000mBar when spec value is ~2350! (and actual MAF is 1250 when spec value is 850). Limp home mode is triggered after 5 or 6 seconds of overboost, but can be avoided by not using full throttle; sometimes the fault code is described as intermittant, so I guess that is triggered after several shorter periods of overboost? (rather than one long one!). With a vacuum tester on the capsule (with turbo still on car) the lever appears to move as freely as in your demo video and the capsule seems to hold vacuum OK.
I have a couple of questions to try and understand how the VNT works (sorry if already answered elsewhere!):
1) which way round does the lever work? (i.e. does vacuum to the capsule give low or high boost?) When the engine is started the lever moves, so it seems that full vacuum gives max boost and venting it to atmosphere gives low boost, but just wanted to confirm this
2) what returns the VNT vanes when vacuum is removed? (i.e. is there a spring in the capsule)
I'm an electronics engineer and used to design motorsport ECUs, so I'm quite comfortable using VCDS and looking at logged data; not so keen to take my turbo apart though! :ugh
Any help gratefully received and of course I'm more than happy to post whatever help I find
I've had a Golf Mk4 TDI for about 6 months and getting limp home mode due to positive deviation almost every time I drive it now - seems to occur at high throttle openings in top gear, usually going up a hill. VNT rod appears to move freely and the capsule isn't leaking; have cleaned out the EGR valve (although it didn't look too bad) - that didn't help the limp home, but did seem to clear up a slight stumble at light throttle around 2000rpm. I've just changed both valves (N75 and EGR) but again no difference, so it is looking more like the VNT, but hoping to find something else before investigating that, as from the outside it appears to be working OK!
I'm using a registered version of VDCS-Lite, so have been looking at logged data - actual intake pressure is going up to ~3000mBar when spec value is ~2350! (and actual MAF is 1250 when spec value is 850). Limp home mode is triggered after 5 or 6 seconds of overboost, but can be avoided by not using full throttle; sometimes the fault code is described as intermittant, so I guess that is triggered after several shorter periods of overboost? (rather than one long one!). With a vacuum tester on the capsule (with turbo still on car) the lever appears to move as freely as in your demo video and the capsule seems to hold vacuum OK.
I have a couple of questions to try and understand how the VNT works (sorry if already answered elsewhere!):
1) which way round does the lever work? (i.e. does vacuum to the capsule give low or high boost?) When the engine is started the lever moves, so it seems that full vacuum gives max boost and venting it to atmosphere gives low boost, but just wanted to confirm this
2) what returns the VNT vanes when vacuum is removed? (i.e. is there a spring in the capsule)
I'm an electronics engineer and used to design motorsport ECUs, so I'm quite comfortable using VCDS and looking at logged data; not so keen to take my turbo apart though! :ugh
Any help gratefully received and of course I'm more than happy to post whatever help I find