A3 TDI 170 won't rev past 3000RPM with no engine code?

[MA3TDIQ]

Hi guys,

So I was sitting at traffic lights and noticed slight rough idle in my a3 170 TDI (57 plate). Had occasionally experienced this before and thought I would rev my engine to see if it would resolve matters. However it wouldn't rev beyond 3000 rpm. Does this mean my car is in limp mode?

I have VCDS and scanned the car and there are no fault codes stored for the engine nor do I see any warning lights on my dash.

Any help as to what this could be and what I should do to rectify it?

Many thanks for your help!

 

[ayfreetee]

Check for boost leaks, intercooler hoses would be my first guess.

 

[Keram]

Lots of cars these days don't let rev too much when stationary.
Try turning off ESP then try again, or just gain some speed and then put neutral, you should be able to rev engine fully then.

 

[ch1z64]

OP didn't you post the same post a while back just it seems to ring a bell about someone with same problem

 

[MA3TDIQ]

pipes all seem fine. Strange thing is my car used to be able to rev right up to 6000 rpm.

Only now its been idling a bit rough it is not letting me rev beyond 3K revs.

I read elsewhere that it is actually the ECU that limits the revs to 'protect' the engine from damage if its not running right. But surely if the problem could potentially damage the engine it would flag up an error code with the engine?

On a side note - is there anyway to see how clogged up the DPF is with VCDS? I have driving 4 hours on the motorway last night - if the DPF was the problem surely it would have burnt off the carbon during that long drive?

 

[MA3TDIQ]

OP didn't you post the same post a while back just it seems to ring a bell about someone with same problem

I'd be interested in what his problem was and the potential solution!

 

[ch1z64]

Tip ( Audi-sport.net/ dpf) might be worth looking for through google

 

[ayfreetee]

If the dpf is clogged you will get a light on the dash and a code, you can do a forced regen with vcds, not sure how though, try the tdi section on here for information, try unplugging your maff to see if there is any difference, if so then you’re maff is a problem, though you should have a code, could be your throttle body, egr and inlet are full of crud, replacing the inlet with one from a 140 ps is a common mod to get rid of the swirl flaps, which leak out the left side of the inlet where the vacuum flap controller is.

 

[MA3TDIQ]

If the dpf is clogged you will get a light on the dash and a code, you can do a forced regen with vcds, not sure how though, try the tdi section on here for information, try unplugging your maff to see if there is any difference, if so then you’re maff is a problem, though you should have a code, could be your throttle body, egr and inlet are full of crud, replacing the inlet with one from a 140 ps is a common mod to get rid of the swirl flaps, which leak out the left side of the inlet where the vacuum flap controller is.

Thanks for the reply, I’ll check the MAFF sensor tomorrow.

I actually got a brand new manifold put on couple of months ago cause I was having an issue with the swirl flap, I was hoping that would fix the problem but it didn’t

 

[Rideen]

Strange thing is my car used to be able to rev right up to 6000 rpm.

If your car was hitting 6000 rpm as a diesel, one might think you broke something.

 

[MA3TDIQ]

If your car was hitting 6000 rpm as a diesel, one might think you broke something.

Chicken an egg scenario if you ask me, there was something wrong which made me rev my car to redline... not the revving causing the problem. I normally only Rev to 3000

 

[Daveyonthemove]

Fuel filter been changed recently?
MAF is also a good place to start.

I wouldn't expect it to be an intercooler hose as the leak would be prominent under boost, which you won't be when stationary. I'd expect it to rev normally when stationary.
It sounds air or fuel related IMO, but could easily be a sensor playing up somewhere.

 

[MA3TDIQ]

So I think I have diagnosed the problem. Seems to be a duff Mass airflow sensor.

With the engine running I disconnect the connector to the mass airflow sensor and the car keeps running. I read online that this is the symptom of a faulty sensor.

Still get no code though with VCDS, could the sensor still be broken without a code?

 

[ayfreetee]

It’s possible that it only plays up under load.

 

[Daveyonthemove]

From what I can gather, if you disconnect the MAF then the car reverts to driving to a predetermined set of parameters, so it should run fine.
When the MAF is connected it will send relevant info to the ECU and the car behaves accordingly. if the sensor is duff, then info is incorrect and the car drives badly as a result.

 

[Simonthepost]

Had same problem with mine, runnig rough not reving over 3000rpm (8p S3) took to garage to fix after a lot of diagnosing (DPF clean, new injector) turned out to be the turbo was not working properly, got refurb turbo and now runs like she should.
Might not be same as yours but worth considering.

 

[ayfreetee]

S3 with a dpf??

 

[Simonthepost]

My mistake, I’ve an s line tdi

 

[MA3TDIQ]

From what I can gather, if you disconnect the MAF then the car reverts to driving to a predetermined set of parameters, so it should run fine.
When the MAF is connected it will send relevant info to the ECU and the car behaves accordingly. if the sensor is duff, then info is incorrect and the car drives badly as a result.

Is there a way to find out if my MAF sensor is gone?

 

[Daveyonthemove]

Unplug it and drive the car. It should resort to a default setting and run ok. If the MAF is sending false signals the ECU reacts to them and the car runs badly, so disconnecting it should identify it.

 

[MA3TDIQ]

Unplug it and drive the car. It should resort to a default setting and run ok. If the MAF is sending false signals the ECU reacts to them and the car runs badly, so disconnecting it should identify it.

So does that mean the MAF sensor is not essential for the normal running of a car?

 

[Daveyonthemove]

It is needed, but when it goes wrong it makes the engine run worse than it does without it.
When it works, the engine runs better than it will without it.

It's not there just to fill space in the engine bay, if you plan to run without it you will probably do damage over a duration of time as the AFM will not be measured correctly and you risk running lean/rich.
it's only to be unplugged as a test to fault find.

 

[Rideen]

It is needed, but when it goes wrong it makes the engine run worse than it does without it.
When it works, the engine runs better than it will without it.

It's not there just to fill space in the engine bay, if you plan to run without it you will probably do damage over a duration of time as the AFM will not be measured correctly and you risk running lean/rich.
it's only to be unplugged as a test to fault find.

I completely agree with the statements above. Make sure you do not drive a lot with MAF unplugged because apart from other things, your DPF can go bad due to the wrong air/fuel mixture.

 

[Keram]

Following that logic bad MAF also contributes in forming wrong mixture and that also can damage other engine components )
but in reality MAF sensor doesn't play THAT big role in TDI 170 fueling system since it also has lambda sensors and intake pressure sensor, which ECU actively uses too.
So I believe you would get annoyed by the Check Engine Light (because of unplugged MAF) faster than DPF get clogged (which is unlikely to happen for the reason stated before).
On the other hand you can easily check all the sensors readings, together with soot mass and other DPF related parameters with VCDS. Just reading trouble codes is not enough.