3.0 TDI Boost Issues. VCDS Log.

[Tommy Pickles]

Hi guys, so I'm having ongoing boost issues with my b8. I keep getting an Underboost cel. I've found if I drive the car hard it will overcome this and work for a while, then will come back after driving normally for a day or two.

I first thought it was a dpf issue as it goes away after driving hard (forcing regeneration) but I tried doing a regen in VCDS and it completes instantly. Maybe someone could help me out with that?

Anyway I logged actual and specified charge pressure and air mass and this is the result I got (see pic). Its definitely underboosting. Can anyone tell me if this is normal or is there an issue? Sorry for quality of picture, will try get a screenshot uploaded tomorrow. Also is there anything else I should log/check over? Thanks.

 

[bhodgkiss]

It does look to be underboosting as you say
What Peak boost is being requested and what is being achieved?

You're running a standard map?

Towards the right hand side of the graph, is that a 3rd and 4th gear pull to the red line?

 

[desertstorm]

What kind of pressure readings are you getting from the DPF pressure sensor ? Are you sure you don't have a boost leak somewhere. Have you checked all the boost hoses thoroughly for holes.

 

[Tommy Pickles]

It does look to be underboosting as you say
What Peak boost is being requested and what is being achieved?

You're running a standard map?

Towards the right hand side of the graph, is that a 3rd and 4th gear pull to the red line?

Thanks for the reply. The graph you see is only 1st through 2nd. I will be doing a propper run soon when I get the chance.

An example of requested and actual boost pressure is requested 2589hPa actual 2206hPa. I know they aren't going to be bang on but this seems a considerable difference.

Yes it is a standard map.

What kind of pressure readings are you getting from the DPF pressure sensor ? Are you sure you don't have a boost leak somewhere. Have you checked all the boost hoses thoroughly for holes.

Thanks for the reply. When I did the forced DPF regen I recall it saying 0 but will double check tomorrow when I am back in the car. The car itself is a very high miler and I would of expected there to be some pressure from the dpf. Can the dpf pressure be "mapped" out so I don't get a cel come through? Since buying the car I've found lots of hidden secrets

The car was smoke tested a couple months back for boost leaks but I have a feeling it wasn't smoke tested under pressure. In certain places I have found light mistings of oil around the boost pipes so I do think a boost leak is a probable cause. Any advice on checking for holes/leaks?

 

[desertstorm]

There should always be some pressure reading on the sensor. It can be mapped out if the DPF has been hollowed out and the dpf turned off. Possibly why the DPF regen completed straight away. Is the exhaust tailpipe clean. If the DPF is intact it should be spotless.

 

[Tommy Pickles]

There should always be some pressure reading on the sensor. It can be mapped out if the DPF has been hollowed out and the dpf turned off. Possibly why the DPF regen completed straight away. Is the exhaust tailpipe clean. If the DPF is intact it should be spotless.

Will double check the reading tonight. From what I was told the dpf is intact and no mapping has been done but you never know what's true when buying a used car... Haven't noticed excessive soot on the tailpipe and I never get any smoke out of it even when booting it so to my knowledge the dpf is doing it's job.

 

[desertstorm]

I don't have excessive soot and only get a very light haze when booting mine . If they are properly mapped or standard with no DPF you won't see anything. Just have a look in the tailpipe if you have any soot at all the DPF has been tampered with. When I cut open the rear silencers on my car at 75K miles you could see your face in the stainless it was that clean. The DPF does a good job of stopping soot finding it's way out.

 

[Tommy Pickles]

Ok, so currently sat in the car. Tried service regen of dpf - completes instantly. Dpf pressure is at 0hPa and doesn't change when driving or reving etc. So do you think a gremlin has been in and mapped it out or faulty sensor? Surely if sensor was faulty I'd get a CEL. Thanks for your help so far.

 

[desertstorm]

If it's been mapped out the DPF would have been removed. Stick your finger in the exhaust rub it around and pull it out. If it's black and sooty you have no DPF. I have no DPF on my car but the DPF pressure sensor still works correctly. There is a possibility the sensor is duff. Try disconnecting it electrically and see if it flags a fault code.