Can anyone help with mystery intermittent fault

henrik1971

Hi there
Not sure this is the right place to post such a query, but throwing it out there in the hope someone might know something:

I have a 2005 Saab 93 diesel - same mechanically as Vectra 1.9TDi.
For the past 6-9 months my car will ocassionally (perhaps once a week, sometime less, sometimes more) do this strange thing where I'll be on my way somewhere and the engine will start running slightly rough, the tickover will increase from normal 750 rpm to about 950 rpm, and it will start surging a tiny bit, and it will start kicking quite a bit of smoke out the back and I get a strong burning smell. There is no obvious pattern to when or where it happens.
The funny thing is, if I pull over (or even just stop at a junction or traffic lights) and then quickly turn the engine off and on again, the car is back to normal again.
The car has been in for diagnostic checking with a Saab specialist a couple of times, they can find no error codes and nothing wrong at all. They've checked all the hoses and cleaned out the EGR and the throttle body, but it doesn't make any difference. Because the issue is intermittent I can't get it to duplicate the symptoms when I'm there.

Any ideas? I'm starting to lose the plot with the car at the moment...

Iceweasel

Probably not a fault or problem at all - that is what it is meant to do every now and again.

It's the DPF (Diesel Particulate Filter) regenerating itself.

Be thankful that it does that. - most folks with modern diesels have problems in that their short journey type of usage means that the car doesn't get a chance o perform a re-gen and eventually the DPF chokes up and needs replacement at considerable expense.

Have a google for DPF problems and you'll understand the procedure better.

By the sounds of it you don't have a problem at all.

But don't turn it off and then back on again as that cuts the re-gen short - better to let it run it's course.

motorguy

Iceweasel wrote: »

Absolutely not a problem at all - that is what it is meant to do every now and again.

It's the DPF (Diesel Particulate Filter) regenerating itself.

Be thankful that it does that. - most folks with modern diesels have problems in that their short journey type of usage means that the car doesn't get a chance o perform a re-gen and eventually the DPF chokes up and needs replacement at considerable expense.

Have a google for DPF problems and you'll understand the procedure better.

By the sounds of it you don't have a problem at all.

But don't turn it off and then back on again as that cuts the re-gen short - better to let it run it's course.

+1

I was trying to type something similar when my chrome session crashed.

Quite normal

captainawsome

No expert on Saab or Vauxhall by any means but an 05 car wouldn't have a DPF would it?

Iceweasel

It is just possible that the car is not getting long enough journeys to finish a re-gen and is attempting to re-gen more often than it should.

There is an Honest John thread about such a case.

http://www.honestjohn.co.uk/forum/post/index.htm?t=83390

[Deleted User]

OP, rule out if your car has the dreaded DPF filter on it. If the exhaust points down as it exits the car there is no DPF filter. If its straight.. there is.

Strider590

sacsquacco wrote: »

OP, rule out if your car has the dreaded DPF filter on it. If the exhaust points down as it exits the car there is no DPF filter. If its straight.. there is.

You can't really go by this, what's to say it's not had a new exhaust?

henrik1971

Thanks all for your replies.

The car does have a DPF, and about a year ago I was having trouble with a flat spot between 1700-1900 revs where the turbo normally kicks in (as well as this current problem). The Saab specialist forced a regen of the DPF (using tech 2?) to see if that would help, but it didn't.
After more investigation, turned out the flat spot was down to the swirl flat actuator not working correctly.

I will check the links provided about DPF regen.What you say does make a lot of sense as I can go a couple of months at a time between a motorway run of any length. I suppose the thing I'm surprised is why it never did this in the first 4 years I owned the car.

The exhaust was replaced 18 months ago and is a bit rattly at the moment so I'm getting that looked at on Thursday. Thought the problem might be CAT related, so DPF isn't a million miles away.

I think I can live with it doing this, but it does set my teeth on edge that the car almost feels like it could pack up at any moment! The amount of smoke it kicks out can be mortifying...

bigjl

captainawsome wrote: »

No expert on Saab or Vauxhall by any means but an 05 car wouldn't have a DPF would it?

Zafiras with the same 1.9cdti engine had the DPF fitted in 2005.

Exact same characteristics. The exhaust on those would get extremely hot aswell.

And the exhaust note got kind of "boomy"

BeenThroughItAll

henrik1971 wrote: »

I think I can live with it doing this, but it does set my teeth on edge that the car almost feels like it could pack up at any moment! The amount of smoke it kicks out can be mortifying...

If it's producing that much smoke, it suggests the DPF may be fairly clogged when regeneration is taking place. That being true, when it happens do not turn the engine off, or stop driving, until the regeneration is complete - if you do, you'll end up with badly diesel-contaminated engine oil which will not be positive.


The Z19DTH and Z19DT equipped vehicles don't actively regenerate unless DPF saturation is quite high; under normal circumstances they will passively regenerate during long drives, when constant higher engine speed will allow the exhaust system to get hot enough to regenerate the DPF.

If the car's doing active regenerations, that indicates either you're not doing long enough trips often enough, or there is a fault with the DPF or associated differential pressure sensors.


To put this into context, in 18 months and around 30K miles, my wife's 9-3's DPF saturation maintained a level of ~40-50% and never once did an active regeneration; instead passively regenerating once a week during her normal travel pattern.

mcpitman

sacsquacco wrote: »

OP, rule out if your car has the dreaded DPF filter on it. If the exhaust points down as it exits the car there is no DPF filter. If its straight.. there is.

Inaccurate twaddle.

Anyhoo, agree with everyone else, sounds like your DPF doing a regen.