Diagnose me - Loss of power on any sudden decline in the road

[AlexRO]

So this has been happening for a while now (months at least) on one downhill section of road near my house.
I would be driving normally with my foot 1/4 on the acceleration and as soon as the car tilts to follow the road downhill all power cuts momentarily (as if you suddenly took your foot off the gas), looking at my boost gauge it would be on 1bar and when the car tilts downwards it drops close to zero and goes back up after a second, as the power comes back through.
Its weird, if I have my foot planted (WOT) there will be not even a hint of power loss as I go downhill, the car keeps pulling.

Im thinking it has to do with some sort of fluid shift possibly residual oil in my pipes triggering a sensor ??
The car has just been through a service, so engine oil etc are in good shape.
DSG oil hasnt been done yet, will get done next week or so, but I cant see how that could be affecting it

confused. :S

[AlexRO]

Ripping out my EGR valve and giving it a clean tomorrow to A) see if it fixes the problem and B) because it hasnt been done yet

[getjet]

Only on declines and only if not on the throttle? That's a weird one.

Do you have vcds?

Is the egr valve on your car electronic or vacuum operated?

[AlexRO]

Only on declines and only if not on the throttle? That's a weird one.

Do you have vcds?

Is the egr valve on your car electronic or vacuum operated?

Yeah only for a second as the car transitions from straight to decline with a little bit of throttle, if i take it my foot off i cant feel the loss of power as it wasnt accelerating anyway, and if i have my foot down it doesnt change a thing.

No, but Im not getting any warnings/lights ? would codes still be thrown?

Im pretty sure its electronic, has a plug that goes into a black compartment next to the Valve housing. Will check tomorrow when i pull it off

[getjet]

Yeah only for a second as the car transitions from straight to decline with a little bit of throttle, if i take it my foot off i cant feel the loss of power as it wasnt accelerating anyway, and if i have my foot down it doesnt change a thing.

No, but Im not getting any warnings/lights ? would codes still be thrown?

Im pretty sure its electronic, has a plug that goes into a black compartment next to the Valve housing. Will check tomorrow when i pull it off

I guess if there was enough buildup in the valve it might create a sticking point, but i don't think this will be your issue. Cant hurt to have a squiz, just be careful with the rubber rings.

Where is the boost valve getting its reading from? vacuum line or intake?

Unfortunately not all codes result in a warning light If you get a warning light, chances are you'll know what the problem is by how the car feels anyway.

[AlexRO]

I guess if there was enough buildup in the valve it might create a sticking point, but i don't think this will be your issue. Cant hurt to have a squiz, just be careful with the rubber rings.

Where is the boost valve getting its reading from? vacuum line or intake?

Unfortunately not all codes result in a warning light If you get a warning light, chances are you'll know what the problem is by how the car feels anyway.

I ended up taking the EGR out, had some buildup(90,000km) and cleaned it all out.. My intake manifold was in much the same shape but I couldnt see how I could remove it myself. Its the alloy one (not plastic like mk5's, and has all the fuel lines/fuel rail on the top).
Took about 2.5 hours start to finish lol dont wanna do that again anytime soon ..

Took it for a quick drive and went on that hill again, same issue.

boost is tapped on the rubber hose coming off the turbo, can get spikes from the stretching of the rubber but its pretty accurate.

Ps thanks for the heads up on the rubber rings !! I almost sliced them in half as i was twisting the EGR valve housing out

hmmm .. its just annoying, not something Im worried about as it seems people around the world are getting this with their tdi's and have taken them back for repair with no results.. would be nice to know whats causing it though

[gldgti]

I dont think this is an engine problem at all from the sound of it. Its more like its the ECU literally cutting fuel on purpose, which it does on overrun all the time.

Your car being a DSG lends more weight to this, as I'm sure there is some grade logic involved with the dsg control aswell (most modern automatic gearboxes are sensitive to grade).

To put the theory plainly - your accelerator is completely digital, and its calibrated. This problem sounds just like the 'dead zone' for zero accelerator position is too big.... not saying that that is the exact problem.

I notice you have VIEZU tuned in your sig - are you talking about just the dsg or the engine? If you have a viezu dsg tune it might be worth asking them about the problem, as it might be something to do with fuel saving measures incorporated into the new dsg mapping.

To write out the logic i'm talking about:

1. cruising steady state on flat ground, power request by driver 25%
(you crest the hill)
2. dsg senses grade decline, tells ecu
3. ecu knows fuel required to maintain speed will reduce, because you are going downhill now
4. ecu cuts fuel
5. vehicle speed starts to decrease
6. ecu decides more fuel is needed, and adds more fuel

ofcourse all this happens in a fraction of a second, and I'm simplifying.

[AlexRO]

I dont think this is an engine problem at all from the sound of it. Its more like its the ECU literally cutting fuel on purpose, which it does on overrun all the time.

Your car being a DSG lends more weight to this, as I'm sure there is some grade logic involved with the dsg control aswell (most modern automatic gearboxes are sensitive to grade).

To put the theory plainly - your accelerator is completely digital, and its calibrated. This problem sounds just like the 'dead zone' for zero accelerator position is too big.... not saying that that is the exact problem.

I notice you have VIEZU tuned in your sig - are you talking about just the dsg or the engine? If you have a viezu dsg tune it might be worth asking them about the problem, as it might be something to do with fuel saving measures incorporated into the new dsg mapping.

To write out the logic i'm talking about:

1. cruising steady state on flat ground, power request by driver 25%
(you crest the hill)
2. dsg senses grade decline, tells ecu
3. ecu knows fuel required to maintain speed will reduce, because you are going downhill now
4. ecu cuts fuel
5. vehicle speed starts to decrease
6. ecu decides more fuel is needed, and adds more fuel

ofcourse all this happens in a fraction of a second, and I'm simplifying.

Actually that makes a lot of sense, thanks for the reply.
The boost would drop if there was a fuel cut, which it does, and comes back within a second. Makes sense to why that doesnt happen on WOT too
although it is kinda silly to adjust power delivery based on gradient when the driver should be in full control .. eh.. i wont worry about it from this point on, its only ever happened on one specific portion of road.

Ive got the VIEZU vPerformance tune, with their 'light' indirect DSG tuning (just adjusting the input signals apparently), but this has happened before the tune was put on too.

cheers

[gldgti]

Although it seems silly at first glance in terms of control, VW (and most other car makers) have had load governing devices on diesel engines for a long, long time (30 years or more!) - Every vw diesel ever made (possibly not including the diesel beetle of the 50's that noones ever heard of

The mechanical pump diesels (prior to PD engines) use a flyweight governor in the pump to reduce fuelling as the rpm increases, for the same accelerator position. The flyweights work against a staged spring system connected between the accelerator and the fuel control collar - what this means is that under light loads, the fuel supply will change depending on engine load - so if you are cruising along and you start going up hill, the engine speed decreases and the pump starts adding more fuel to compensate. Similarly, as you start to go downhill, the engine speed increases so the pump starts to cut fuel to compensate.

The only difference between the old system and the new, is that these days its entirely electronic

[Mysticality]

Almost sounds like what mine does, going down hills... Accellerates enough and then drops power and begins to coast down. (0L/100km and 0% engine load)

A bit un-nerving, but I can replicate it 100% on a couple of specific hills in the area!!