Exhaust gas warning lamp

[DSGMAn]

I had the same thing happen to mt brand new 2 tdi comfortline with only 750km on the clock (still the first tank of diesel). The exhaust gas light came on and stayed on with no apparent changes to the way the car drove. It ended up being a sensor unrelated to the exhaust. It had a faulty MAF (air sensor,intake) which was replaced and now the light has gone out. I did notice that before the air sensor was replaced I could hear the turbo and blowoff valve working more easily. Now I can hardly hear the turbo or bov at all. Driving with the new sensor fitted I could feel slighty more torque down low in the rev range as well. It seems that the sensor was probably faulty from new.

No blowoff valve either from what ive read.

[Logzy]

Looking at the current Golf brochure on VWA website, only 125 KW TDI listed as having DPF so Mischa appears to be correct.

Cheers

George

Double Correct.

[niss_man]

No blowoff valve either from what ive read.

It certainly sounds like a blow off valve (don't most new turbo vehicles have one). Every time I let off the accelerator I could hear the distinctive BOV noise though I can hardly hear it now that the MAF sensor has been replaced. Could someone correct me if I'm wrong, preferably someone with factory manuals to look at. Does the 2.0TDI have a BOV and also what effect would a faulty MAF sensor have on the vehicle. I am just speaking from my own personal experiences. Before the sensor replacement I heard the turbo spool up easier and then a typical BOV noise when lifting off the accelerator (maybe excess boost pressure due to faulty sensor but it didn't have more power though). With new sensor installed I can hardly hear both turbo spooling up or BOV noise. Could someone with some answers please respond.

[gregozedobe]

OK - "WARNING - technical content follows".

Diesels don't have Blow-Off-Valves (BOVs) because they don't need them and they would reduce engine response/power if you did fit one (I'll explain why later on in this post).

BOV's are needed on turbocharged petrol engines because they have throttle/s (moveable plate/s in the intake tract that restrict the amount of air going into the engine and thus the power the engine can make).

The one exception to this design that I know of is some BMW petrol motors that achieve the same result (controlling air flow into the motor) by restricting the lift on all the intake valves (it is supposed to reduce pumping losses at part throttle or something)

If the turbo is pumping hard then you suddenly back off the accelerator pedal the throttle plate closes off, and the air still being pumped by the turbo now has nowhere to go, thus creating a high pressure spike in the intake. This spike is not good for some engine parts. Most factory petrol turbos have a BOV that vents this excess pressure to the exhaust, but some people like the sound an externally vented one makes, hence the demand for aftermarket BOVs. The BOV can also be used to limit boost (and consequent power and high temps) at full throttle to stop and engine from damaging itself.

Diesels don't have a throttle (the amount of power they produce is controlled by how much fuel is injected), so all the air (regardless of boost) just flows through the intake sytem unimpeded all the time, hence no high pressure spikes and thus need for a BOV.

If you did install a BOV on a TDI it would need some kind of manual activation mechanism, otherwise it would release the air every time boost pressure got above the pre-determined pressure limit (probably at just the time you wanted maximum power).

When you did activate the BOV it would release all the pressurised air in the intake system, and you would have reduced power until you de-activated the BOV and the turbo pumped more air in again.

Most VAG TDIs have turbos with moveable vanes, which are moved (controlled by the ECU ) to keep the boost pressure within the desired range.

Hope that helps understand some of the differences between how turbo petrols work vs turbo diesels.

I'm not sure why your engine sounds different since you replaced the faulty MAF sensor. Maybe the turbo was working much harder to produce the amount of power you were asking for due to the fault ?


Any errors or ommissions in the above explanations should be accepted as part of the "pay peanuts, expect monkeys" philosophy of life.

[niss_man]

OK - "WARNING - technical content follows".

Diesels don't have Blow-Off-Valves (BOVs) because they don't need them and they would reduce engine response/power if you did fit one (I'll explain why later on in this post).

BOV's are needed on turbocharged petrol engines because they have throttle/s (moveable plate/s in the intake tract that restrict the amount of air going into the engine and thus the power the engine can make).

The one exception to this design that I know of is some BMW petrol motors that achieve the same result (controlling air flow into the motor) by restricting the lift on all the intake valves (it is supposed to reduce pumping losses at part throttle or something)

If the turbo is pumping hard then you suddenly back off the accelerator pedal the throttle plate closes off, and the air still being pumped by the turbo now has nowhere to go, thus creating a high pressure spike in the intake. This spike is not good for some engine parts. Most factory petrol turbos have a BOV that vents this excess pressure to the exhaust, but some people like the sound an externally vented one makes, hence the demand for aftermarket BOVs. The BOV can also be used to limit boost (and consequent power and high temps) at full throttle to stop and engine from damaging itself.

Diesels don't have a throttle (the amount of power they produce is controlled by how much fuel is injected), so all the air (regardless of boost) just flows through the intake sytem unimpeded all the time, hence no high pressure spikes and thus need for a BOV.

If you did install a BOV on a TDI it would need some kind of manual activation mechanism, otherwise it would release the air every time boost pressure got above the pre-determined pressure limit (probably at just the time you wanted maximum power).

When you did activate the BOV it would release all the pressurised air in the intake system, and you would have reduced power until you de-activated the BOV and the turbo pumped more air in again.

Most VAG TDIs have turbos with moveable vanes, which are moved (controlled by the ECU ) to keep the boost pressure within the desired range.

Hope that helps understand some of the differences between how turbo petrols work vs turbo diesels.

I'm not sure why your engine sounds different since you replaced the faulty MAF sensor. Maybe the turbo was working much harder to produce the amount of power you were asking for due to the fault ?


Any errors or ommissions in the above explanations should be accepted as part of the "pay peanuts, expect monkeys" philosophy of life.

Thanks for that handy explanation. I do love a good technical explanation and this one makes sense.

[VW Convert]

OK - "WARNING - technical content follows".
etc.etc. abbreviated

Nice post, explains things very clearly.

Cheers

George

[Greg Roles]

I agree on the BOV "type" of noise, especially on backing off. What I have found is that with a pod setup, it's a lot louder, and is simply noise from the turbo inlet impeller itself. With an open pod housing, I can make the turbo make all sorts of noises back out the inlet, and in my opinion it's simply the turbo itself you are hearing. I've recently spent a lot of time ( and $$ ) on Erwin, and I haven't found any pressure regulation other than the vanes Gregoz points out. There is however a throttle flap "like" mechanism in the EGR inlet that both controls mixing of the recycled exhaust gas to lower the emissions, and to shut the engine down quickly without the typical diesel "run on" shudder. So it is a throttle of sorts.

Also at this stage the GT TDI is the only golf with a DPF, and the technology will be rolled out into forthcoming diesel models. It has been redesigned, in that it has a more distinct two stage format, to improve service life I figure. I read specs on the fact they will also all be common rail, but with the addition of some of the GT "hot-ups" lose no power from current with the 103kw still the same despite the "drop" back to common rail. Certainly be a lot quieter!

[benough]

negative on that.

Well I still have the spare key from it and it came with a set DPF instructions.

I must admit I didn't have it long enough to learn about all the features differences etc.

I have some photos of under the bonnet, will check them when I get home from work

[benough]

Ok. So here is a photo of the key that came with my first car.

DPF Key

and here is under the bonnet

Engine 1

Engine 2

because the motor has been pushed back it might not be visible.

Just not sure why VW would give me a DPF key and not have it on the car?

[Mischa]

Ok. So here is a photo of the key that came with my first car.

DPF Key

and here is under the bonnet

Engine 1

Engine 2

because the motor has been pushed back it might not be visible.

Just not sure why VW would give me a DPF key and not have it on the car?

id guess they gave you the little tag because the salesman probably thought there is a dpf in there lol. fortunately for non gt owners, they dont have dpf's, whether they're 05 or 08 models. the dpf isnt visible (i dont think) from the top like that, especially with the engine cover on. it sits in the downpipe right after the turbo like a cancer.

ps no offence to those who like their dpf's