[Mohawk]

Good write up. The issue really is that this is NOT really the throttle body, as Diesels don't use air regulation like a Petrol does to control the engine. That is the air intake manifold & the flap is actually the EGR Vacuum flap. Such that when you lift off throttle which actually controls the injector duty cycle. The flap is actuated to create a vacuum in the inlet manifold & that sucks exhaust gases in through the EGR valve & recycles through the engine. If the flap sticks then when you throttle up the turbo creates boost, but the flap creates a momentary over pressure condition if it then opens or a proper over pressure if it does not.

These codes are stored & when stored the DPF regen is suspended because the ECU now believes the Fi is not working correctly ! That is a stupid failure condition as it could lead to DPF failure for no good reason.

In every video I've seen of people cleaning or filing that flap they are treating it like a throttle butterfly plate, its not it just needs to not stick when closed, file away enough but don't be precious about it, it does not require a perfect seal IMO.

 

[Mohawk]

So I put the OBD reader on the car at lunchtime & found it had recorded a "P0234 Turbo Over Boost" code. Didn't have any other engine related codes. It has not gone into limp mode since I removed the TDI Tuning Box, which was doing it for a past time. The codes were cleared then.

So thinking about the mystical Throttle Body (TB) flap fault, I need to check mine again, not done it for a year or so.
This got me thinking if the flap sticks closed how does the car detect the over boost condition?

This led me to work out how the SDV6 intake system works with its sequential turbo chargers, I couldn't find a video or online write up, so heres my take on it.

So each turbo has its own air filter box & MAF sensor in each turbo feed pipe. The exhaust manifold on each bank (left+right) is different. The left turbo is the primary low rpm turbo, it receives exhaust from both banks via the engine exhaust crossover pipe around the back of the engine, which feeds the right bank exhaust into the left turbo exhaust turbine at low engine rpm, helping to spin up the turbo quickly. This is achieved by using an exhaust diverter valve on the right turbo manifold. At a certain rpm/load condition the diverter valve closes off the rear feed pipe to left turbo & allows the right bank exhaust to act fully on the right turbo exhaust turbine, spinning it up to feed boost via the boost merge valve below the intercooler. It's not a switch as in on/off, it's more like a dimmer control, gradually switching exhaust from the left to the right turbo for a smooth transition & power increase.
The boost feed pipe from the right turbo has a MAP (SCOP) sensor in it. This is used to determine when to open the boost merge valve. This is also how the ECU detects a "Turbo Over Boost" condition when the TB flap is stuck, because the TB MAP sensor can't detect pressure it can't feel.

So from that I can only assume that my TB Flap is sticking intermittently.

It also showed me that the crankcase is positively (negatively) vented by feeding the vent hose from the oil vapour separator into the left turbo low pressure feed pipe, thus its exposed to vacuum pressures when the turbo is spinning. This also explains how oil gets into the boost side of the feed pipes & the intercooler & sometimes as far as the TB. So I intend to add an additional oil vapour separator to hopefully fully stop oil from contaminating the turbo intake/boost track.

I hope that all makes sense.

 

[Mohawk]

Checkout my new thread & beat all the bean counters !

https://www.fpaceforums.co.uk/threads/sticky-throttle-body-fix.8166/?post_id=90115#post-90115