Intermittent Poor Running. Now Out Of Ideas

[Marshall1975]

I have a passat B6 170PD BMR. Over the past few months I started getting a problem with a strong diesel smell and low power below 1500RPM. I replaced the tandem pump and that cured the diesel smell. Next I replaced the EGR with an aftermarket one but this seemed to make the matter worse so I then replaced it again with a genuine Valeo OM one, but problem still there. This was followed by changing and anti shudder valve but this made no difference and finally I changed all the vacuum hoses which seemed to sort the problem for a few days but then back to not running right.

The issue is on say every 5th or 6th journey the car becomes sluggish and not very responsive and when accelerating a noise that sounds like the exhaust blowing which is it not. So if I stop and unplug the EGR the problem then goes away. This does not immediatly mean there is an EGR problem as im well aware but certainly something linked to that system. Now I dont mind leaving it unplugged and having an engine warning light but the EGR error codes cause a problem with the DSG box which gets very annoying lol.

During this whole process not one error code is generated when checked with VCDS except when I unplug the EGR which is to be expected. I have also tested the turbo actuator which works 100% perfectly and moves a good 20 mm freely. And to be fair if it was a problem with that then it would'nt just vanish when I unplug the EGR as once unplugged the problem never comes back. So sorry for the long post but i'm running out of ideas and avenues to go down.

[DMitch16]

Check the live data when the engine is running (Measuring Blocks under the Engine module on VCDS). Have a look at them all but especially EGR responses, fuelling, DPF loads (soot and carbon mass levels). Sounds a bit like you have frequent active DPF regeneration so is your DPF nearing a full situation (the DPF light comes on for excessive SOOT load not overall ASH collected).

[Marshall1975]

Hello mitch. Its funny you suggest that as after a while I was thinking the same as it was the only thing that I could think of that would change the exhaust note and raise the rpm slightly. I was under the impression though that a DPF regen was not supposed to effect the running of the car. Although I suppose with it being 11 years old and covering 100000 miles might make a difference. I will have a look tomorrow with VCDS at the current state of the DPF. Will let you know how I get on. The only slight niggle I have would be why unplugging the EGR would stop the issue.

[DMitch16]

One of the conditions of a regeneration is that the EGR is closed which is monitored by the ECU. Unplug it and the ECU most likely interrupts the regeneration as it will no longer have a valid signal from the EGR.

[Marshall1975]

Yeah that makes sense. Thanks mitch I will crack on doing some VCDS invest tomorrow.

[Marshall1975]

Hello mitch. Right have done a forced regen but im not sure of the numbers. Before I started the Load Coefficient figure was 32% and the Particle Filter Carbon Mass was 48%. Set off the regen with vcds and went driving. Over the course of 2 regens the Load coefficient slowly started going down until it reached 0% but the Carbon Mass figure never changed. Not sure if this is what is supposed to happen. Been driving around for a while with the egr plugged back in and it does not seem to be causing the same problems as I described.

[DMitch16]

Hello mitch. Right have done a forced regen but im not sure of the numbers. Before I started the Load Coefficient figure was 32% and the Particle Filter Carbon Mass was 48%. Set off the regen with vcds and went driving. Over the course of 2 regens the Load coefficient slowly started going down until it reached 0% but the Carbon Mass figure never changed. Not sure if this is what is supposed to happen. Been driving around for a while with the egr plugged back in and it does not seem to be causing the same problems as I described.

Carbon Mass is usually in grams not percentage and denotes how much ash has been collected by the DPF following the high temperature burn of the soot accumulated over many years or between 80 to 150k depending on driving cycles. Passive burning of the soot when doing long sustained faster journeys is more efficient than the active regenerations performed by the ECU when shorters journeys prohibit the combustion temperatures from reaching those high enough for automatic passive regeneration. Bigger particles of soot are combusted into very fine particles of ash and stay in the DPF accumulating overy time until the DPF is effectively full. Once a full or nearly full point is reached, regeneration becomes more frequent as the soot accumulation builds faster, exhaust gas flow diminishes and the engine starts to labour as it effectively starts to slowly choke to death. Legal options at this point are to have the DPF cleaned or replaced and the Carbon Mass figure reset (to near zero for a new unit or a few grams for a post cleaned unit - as cleaning usually is 95% to 98% effective, removing MOST of the ash stored but rarely all of it).

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48 grams is over the VW DPF replacement level but I have seen some higher with similar running issues and some a little lower. The ECU has a calculated value based on how the car is driven and therefore different engines reach a full DPF at different Carbon Mass levels and different mileages.