2009 3.0tdi lumpy idle.
 

I am looking for some help tacking down the problem with my A8.

At idle the car appears to be on 4 or 5 cylinders, and when engaging drive or reverse causes a lurching effect which is most disconcerting.

It began only when cold, and I changed the glow-plugs initially, to no avail. Since then it has now developed to happen almost constantly when the car is idling. It has been on VCDS and had a coolant temp sensor changed (think it would have faulted when I disconnected to check glow-plug circuit was operating).

The car drives perfectly from 1000rpm upwards, is still economical, and has no visible smoke (DPF may be masking this). It does seem to "smell" very rich in the garage when started and idling, but that's not scientific, it never normally gets started and idle for any time inside .

Anyone any ideas, or suggestions of garages with diagnostic experience in central Scotland. It's been on VCDS , but I don't believe they will have spent time on injector measuring blocks etc, although if injectors were at fault, would they not generate fault codes?

I have searched online to look for similar issues, and ruled out glowplugs, coolant sensor, and the air distribution system is clean from looking at the throttle body unit upstream of the inlet manifold Y-piece.

Any suggestions or help welcomed.

Cheers,

Iain.

Faulty injectors don't give codes in my experience, even when running with one completely unplugged. I had similar problems, in May, I was able to stop the engine and unplug an injector and then drive and observe any change, then reconnect and try another until I narrowed it down.

It is important that you stop the engine before disconnecting an injector's electrical connector as if you unplug it while open, it will stay open as they require reverse current to close them.

I think one of mine was lazy, as if it was stuck open, it would have stayed open even with the electrical plug off.

If you unplug 5 and each one makes it worse and another makes no difference, they it must be that injector/cylinder.

Someone on here had something similar - bad compression in one cyclinder due to some valvegear problem IIRC.

In vcds can you go into the injector readings , look at the injection quantity deviation mg/str .......milligrams/ per stroke , big plus readings is the ecm trying to get that cylinder to contribute, to keep the engine in its smooth running adaptations

As a test if you have big plus readings on one injector swap that to another cylinder to see if the fault follows it .

But big plus readings are not nessasarly injector faults .....they can be other cylinder issues .....compression ...valves and so on

Like Snap says do not unplug Piezo injectors whilst its running .

You could do a ohms check across the injector pins think it will be about 150 ohms , also do each individual pin to injector body ......should be open circuit .
If you get any resistance that is a Piezo crystal breech to ground =duff injector .

Would be god to get a compression test done also to rule out any losses

Lee

Thanks, I'll do the injector testing tonight that I can do without vcds. Should have bought VCDS years ago, but been lucky enough to have access to it any time I needed until now.

Cheers, Iain

Did the checks on injectors, all 6 show similar resistance, 190k ohms.No shorts to body on any and running with each disconnected makes little difference.
Spending some time under the bonnet, in the vee is a vacuum operated diaphragm which is cycling whilst it idles....can anyone tell me if that's normal.

Also followed all vacuum lines I could see , but no sign of any splits or leaks....

Any suggestions welcomed. I will get the injectors checked on vcds also.

Sounds like the egr vaccum acctuator I think your on about , should it cycle ?? Mmmmm not sure on that ..... I would say no , would of thought it would have a fixed duty cycle at start up .... I could be wrong though .... I digress .....

Just as a test .... un plug the vac pipe off the egr pod and put a bolt in it plug it up , the egr will not work now but for testing purposes at this stage it's ok .

When you switch ignition on or start it up do both the swirl flap motors do a sweep , are plastic rods from the swirl motors connected to the linkage on the inlet manifolds , you normally get codes for swirl flap issues but check any way .

If I remember correctly when at idle the swirl butterfly's will be closed , one side swirl the arm is down , the other side the arm is up .

What I'm thinking here where you say he car seems to run on 4 or 5 cylinders
I'm wondering if one swirl is open and one is closed giving a miss match in air flow per bank giving the missing cylinders feel to the engine , albeit you would think you would lose 3 cyls at a time but some thoughts anyhow

Still carry out the injector tests for the mg/str per injector it may hold a clue , the ohms tests can all be ok but injectors could still have issues , if the injector corrections all show one bank to be on the plus side this is a tell tale to a 'per bank ' issue .

With diag it's a case of ....carry out appropriate tests .... rule out .....move on to next test .....assuming makes a fool of me and you , or the fit parts till the customer runs out of money approach , I see this near enough everyday .....part fitting on the back of code tests .....no follow up testing to prove or dis prove whatever codes are present

Yadder yadder I'm rambling on here aren't I ......... ' computer says noooooo '

Just looking at pictures of 3L tdi engine , does yours have vaccum operated egr or is it the electric one , if it's the electric one the vaccum acctuator you say is cycling could be the egr cooler switch over valve .

Mine has a vacuum egr, that is what is cycling . Not sure it's related, but will blank and try.
Need to get a vcds on it again to check the injector flowrates.
Thanks for all the suggestions.
Any suggestions for quality diagnostics in central Scotland?

Cheers, Iain.

Is this like your engine

http://i618.photobucket.com/albums/t...pst3hovokm.jpg

Is the unit you can see cycling to the left of the green box ??

Hi, thanks for all the thoughts.
It was the diaphragm in the red box that is cycling but disconnecting the vac hose from it makes no difference.
It may be cycling due to the uneven idle?
The hunt goes on....