2010 A4 2L Tdi erratic idle.

[dooroy]

A friend of mine has a 2010 A4 2l Tdi.
Some time ago there was a problem wih the manifolc swirl flaps and I eventually got one of those 'adapters' for the manifold to limit the flap travel.
To fit it I removed the inlet manifold and took the opportunity to clean the manifold , EGR etc.
Got everything back together and while the car is starting fine and driving well the idle speed is all over the place.
The owner said the idle was erratic before the work and we hoped it would be much improved but is still very poor.
I have a scanner which shows rail pressure as a graph and at idle the graph is going up and down rapidly - this evening for example I raised the revs slowly and when they reached about 1600 the graph was practically a straight line (which I presume it should be) and engine ran smoothly.
The 2 graphs show the fuel rail pressure at idle and and revs raised.
The fault code was P106500 Fuel pressure regulation - control Differences.
There are 2 sensors on the fuel rail - could one of them be responsible for the fluctuations ? If so can they be changed with the rail in place?ny opinions /suggestions appreciated as I'm DIY and not much experience in this area.

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[chippie]

A quick guess…,

Disconnect both pressure sensors….Determine which is the signal wires at each of the pressure sensor’s connectors.

Reconnect one of the sensors to it’s wiring…then add a jumper wire from the signal wire of that sensor to the wiring of the non connected sensor…,

Start engine and monitor fuel pressure, note what the pressure is doing….I’d expect to see the reading steady…basically fooling the ecu seeing two steady signals…the eml code should clear too…

Now swap over to the other sensor, with the same wiring mod….check again fuel pressure…if this sensor is faulty the readings should be different

Should help identify if the sensors are acting up…


Sounds long winded, but in the absence of new sensors, best I can come up with…

Where’s our diag guru?

[dooroy]

Thanks for reply.
Will try what you suggest when the car is available.
I don't think there are any other sensors involved - and I wondr why they have 2 sensors on the fuel rail.

[chippie]

Critical inputs to the ecu, usual to double up the inputs in case of failure and to ensure accurate control of fuel metering …..throttle pedal might have 2 potentiometers fitted for same reason…upstream and downstream O2 sensors for tighter control of emissions…l

Very complex…

[remmington]

Jun 23, 2023 23:31:08 GMT 1 dooroy said:

A friend of mine has a 2010 A4 2l Tdi.
Some time ago there was a problem wih the manifolc swirl flaps and I eventually got one of those 'adapters' for the manifold to limit the flap travel.
To fit it I removed the inlet manifold and took the opportunity to clean the manifold , EGR etc.
Got everything back together and while the car is starting fine and driving well the idle speed is all over the place.
The owner said the idle was erratic before the work and we hoped it would be much improved but is still very poor.
I have a scanner which shows rail pressure as a graph and at idle the graph is going up and down rapidly - this evening for example I raised the revs slowly and when they reached about 1600 the graph was practically a straight line (which I presume it should be) and engine ran smoothly.
The 2 graphs show the fuel rail pressure at idle and and revs raised.
The fault code was P106500 Fuel pressure regulation - control Differences.
There are 2 sensors on the fuel rail - could one of them be responsible for the fluctuations ? If so can they be changed with the rail in place?ny opinions /suggestions appreciated as I'm DIY and not much experience in this area.

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Even if you put new a swirl flap motor in the manifold you gotta do a "learn" with VCDS to get the idle OK. I done loads of these.

NOT sure with your limit plate you will ever get it right. Never done one - but local garage to me - aborted with a kit the car owner bought for them to fit - due to this...

There is filter on the pump metering valve on some of these (not all) that sometimes partialy block - which will throw the rail pressure switch and pump metering valve to "fight each other".

[valhalla]

Jun 25, 2023 12:32:23 GMT 1 dooroy said:

Thanks for reply.
Will try what you suggest when the car is available.
I don't think there are any other sensors involved - and I wondr why they have 2 sensors on the fuel rail.

Do you have an engine-code for this car?

It seems more likely that one of the sensors is a Pressure Control Valve, although I wouldn't rule anything out on a VW engine these days.....

Pressure measurement at the rail itself is a nightmare, as there are a lot of engine speed-dependent waves and reflections going-on throughout the rail volute. A fair bit of R&D goes into finding the "sweet spot" for siting injector-feeds and measurement points on these things, but there will always be a "problem speed" or two where the measurements are erratic.

The presence of one or more "lazy" injector/s can really make the signal bounce around, but something you need to be aware-of is that the measurement you are making over the scantool is also a (severe) compromise, and what you need to do is measure with a high-speed transducer directly at the pressure-sensor (or tap directly into the back of the sensor with a flylead) and feed that signal into an oscilloscope. The reason this matters is because the rail-pressure signal is filtered and conditioned by the ECU, and also what you are seeing is sampled at far too low a rate by the scantool - it might suggest that the "average rail-pressure" is fluctuating up and down, when all it is doing is sampling peaks and troughs in a normal cyclical waveform. Rail pressure is always noisy until it is properly conditioned by the ECU, but that can also be caught-out by the same problem if the filter cut-off frequency for its own sampling is too high.

If you have two things that are too close on frequency (source & measurement) then you get a problem of heterodyne freequencies - thing of the low frequency cyclical droning of German bombers in the 2nd World War, which comes from two high frequencies (the propellers whizzing around at high speed) that are slightly different from one another (the German pilots de-synchronized their engine speeds slightly). Same thing is used for FM (frequency modulation) radio signals.

[dooroy]

Thanks for replies.
Owner is working away from home all week so won't get to see it until the weekend to check on engine code etc.
I will check the fuel metering valve on the fuel pump as remington advised - not sure that the limiter made any difference ; the owner now thinks that a SH swirl flap motor was fitted some time ago when the EML began to appear. I presume that needed to be programmed in but most likely wasn't .
Measuring fuel presure accurately seems a lot more complicated than I imagined and is affected by a variety of factors
May try chippies suggestion if I can identify the wires .