Engine: TDI: Slow return to idle

[slimeslab]

Strange fault here - wonder if anyone else has experienced it.

My 2001 tdi (audi engine) when revving it and when approaching junctions etc (dipping clutch etc) takes too long to return to idle.

In comparison to other diesels I have driven, this particular engine seems to take far longer to return to idle.

I have scanned for fault codes and the only one I get very intermittently is a turbo airflow one. Can't remember off the top of my head but something like (actual value different to measured value)
This tends to happen at the same long hill stretch of the motorway when my foot is 3/4 down.

I'll scan it again tonight and post the findings.

I'm beginning to think that maybe the MAF is on its way out...reason being is that in the past I have read off the values of the expected MAF and actual MAF from VIDA and their values have been quite different, but is this the way its supposed to be?

Apologies for the ramble.. any help would be appreciated.

Thanks..

Martyn.



PS. I have adjusted the pump timing and it is within the tolerances shown in VIDA.

[green van man]

Could be throttle potentiometer or servo motor on the pump sticky, these are free checks and a bit of WD40 never goes amiss any way.

Only symptom I had when my MAF was failing was a hesitation as throttle potentiometer went between first and second resistor tracks.
Be warned these are £300 items from volvo.
I note you say you have adjusted the pump timing, this is something I shy away from myself as without the locking tools I am told it can be nigh impossible to get right though some on here do it with no problems but provided there was nothing obvious on the controls I would double check the timing.

regards Paul.

[slimeslab]

Hi,
Thanks for the reply,

With regards to the fault code I mentioned - it happened on the weekend (on the same long motorway hill) and I scanned it when I got home and ECM-0039 came back.

Whether this is related to the slow return to idle issue or not I am not sure...
No error codes come back in relation to the throttle potentiometer or the pump.

How do I go about lubricating the pump servo motor?

The timing is within tolerances according to VIDA and the same symptoms were present before adjusting it. The engine has plenty of power (more low down torque since adjusting the timing actually)

Thanks again

Martyn.

[gjd]

Hello

Originally Posted by slimeslab

Strange fault here - wonder if anyone else has experienced it.

My 2001 tdi (audi engine) when revving it and when approaching junctions etc (dipping clutch etc) takes too long to return to idle.

In comparison to other diesels I have driven, this particular engine seems to take far longer to return to idle.

I have had this, and posted here for some advice.

My mechanic took the car for a terrifying "Italian Tune-up", and we laid a smokescreen that a WWII Destroyer would be proud of.

This seemed to cure the problem.

[slimeslab]

I will certainly try giving it the beans and clearing out some black sooty crud.

I took a video of the slow return to idle.

Seems more prevalent when the engine has warmed up.

And it's the last 1000revs back to idle that are the issue.

http://youtu.be/UwsoF9ptMgc

Thanks

Martyn.

[slimeslab]

Just an update.
I reckon my maf is dying.

I just did some data logging with vida and measured the ECM actual air mass against ECM desired air mass and there was quite a difference.

Overall the actual air mass was consistently higher than desired.

For example at idle desired value was 330mg/stroke
Actual was 429mg/stroke

Am I on the right track?
Thanks.
Martyn.

[slimeslab]

Update on this...
I replaced the maf element with a quality new pierberg one and this improves the mpg slightly and the values are now looking right.

The original issue still persists though.

I'm thinking throttle position sensor which is showing 7% when I don't have my foot on the peddle (but this might be correct?)
Or maybe coolant temp sensor (apparently there are 2 sensors) the temp gauge on the dash seems right but if there are two sensors then mabye one is working and one is duff?

Any ideas?

Thanks.

[ajm]

Originally Posted by slimeslab

I'm thinking throttle position sensor which is showing 7% when I don't have my foot on the peddle (but this might be correct?)

The TPS value doesn't sound unreasonable to me - I'm pretty sure that mine says the same (it's not diesel but I doubt the TPS is very different.) In fact, I think I'd be more suspicious if it were reading zero!

Edit: ...although just because it gives an OK reading at idle position doesn't mean it isn't faulty of course! You should be able to check out its behaviour in other positions and situations quite easily though.

[brickman]

Throttle pedal should rest at zero, so would suggest that's your problem, either something's stopping it return to zero or the sensor is dirty/ Fowled.

Don't know if there are two teap sensors? My 98 Mesa 15.7 only has one.

[slimeslab]

Ok, Update on this.

I changed the injection pump belt and reset the dynamic pump timing and it was roughly in the middle of the values that VIDA requires -0.8deg IIRC.

I did some testing of various sensors too.

Regarding the throttle position sensor - I checked the values required in VIDA and it says it should go from 8% - 88%, so mine being at 7% with no pressure applies seems right, my pedal also goes to 88% so all good there.
I graphed the values of the Analouge and Digital signals from the TPS and for the most part the graphs were pretty fluid, e.g. no jolts/spikes and the analogue and digital signals matched each other exactly.
Every so often though there was a small jump in the graph, but I am not sure if that was the data logging or a fault with the pedal sensor.

I also checked the brake pedal sensor, which was working correctly and within the range of 37% no brake to 81% full brake

VIDA States
When the brake pedal is unaffected a scrolling value will display 35-40%.
When the brake pedal is depressed a scrolling value will display 70-80%.

So all good there.

I also checked the clutch position sensor - this was reading 33% no clutch to 80% fully depressed.

VIDA says:
Clutch position sensor
Self-adjusting
When the clutch pedal is not operated a scrolling value will display 9%.
When the clutch pedal is depressed a scrolling value will display 80%.

So, clearly this is broken somehow, when operating the clutch pedal whilst graphing the data the values jumped from 56% to 80%

My car does not have cruise control so I am unsure what the clutch position sensor actually does, but clearly its there for a reason.
I did a search on here and found: http://www.volvoforums.org.uk/showthread.php?t=166827

The last post on there is interesting.

"Not 100% on these cars but clutch switches were originally on cars to help tell the ecu when you were changing gear and hold the rpm momentarily to help change gear and also to stop the revs staying too high when coming to a halt and to help keep the idle stable.
I also believe on certain diesels it limits the engine when it the pedal is pressed.
A.F.A.I.K this was the clutch pedal switch's primary purposes before cruise control became common on cars."


Can anyone confirm this is the way the system works in V70s?

VIDA is a bit vague on this it says:

The clutch pedal switch informs the engine control module (ECM) that the clutch pedal is depressed. This prevents judder when the clutch is engaged and disengaged and racing when the clutch pedal is depressed while cruise control is on.

Any help or ideas would be fab.

Thanks

Martyn.