Audi A6 BLB engine 2.0 TDI vibration

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rudolf
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Audi A6 BLB engine 2.0 TDI vibration

Post by rudolf »

Good morning!

My personal car, spec Audi A6 2.0 TDI engine, code BLB, emmits a powerful vibration in the chassis between 1.600 and 1.800 RPM.
I did a cylinder balance test at idle, which I have attached to this post. I have searched the forum for explanations on how to interpret the results, but now I'm even more confused than before.
How can I detect which injector(s) is(are) faulty?
I have read about a method of using the crankshaft sensor signal, but I still require assitance.
Also, I could only measure cylinder balance at idle. As I press the gas pedal, i get an error.

Any advice is hugely apprectiated.
Thank you in advance!
Attachments
180721_CylinderBalance_001.pddata
(17.3 MiB) Downloaded 580 times

Steve Smith
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Re: Audi A6 BLB engine 2.0 TDI vibration

Post by Steve Smith »

Hello Rudolf and thank you for the post

Trying to determine the offending cylinder using the Cylinder Balance Feature of Picoscope is not possible as we have no synchronization signal.

The cylinder balance feature serves as a pointer during the initial diagnosis stage where you have confirmed at least one cylinder to be under-performing.

Unfortunately using the Alternator Ripple to obtain cylinder balance data is challenging with modern vehicles/charging systems however, it does work well with 4 cylinder diesels with a single offending cylinder. Increasing the engine speed will change the formation of the charging ripple from that at idle speed,(especially when the misfire takes hold) hence the error when pressing the gas pedal

You mentioned "injectors" Rudolf as the cause but have we confirmed relative compression via Pico Diagnostics first?

Next you are correct, I would switch to PicoScope, connect the crankshaft sensor to channel A and number 1 injector current to channel B.

Run the engine at idle speed (capture one buffer) then increase engine speed to obtain the misfire, then allow to idle.
You will have captured at least three buffers using the settings file below:
HALL EFFECT CRANK SENSOR.pssettings
Hall Effect Crank
(4.85 KiB) Downloaded 618 times
We then need to apply a math channel to graph the crank sensor signal. This will reveal our offending cylinder.

To apply a crank math channel could you take a look here?

topic11511.html

If not could you post your captured psdata file with and without the misfire present and we can apply this math channel for you.

I hope this helps, take care......Steve

rudolf
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Re: Audi A6 BLB engine 2.0 TDI vibration

Post by rudolf »

Hello Steve and thank you!

I will look into what you suggest. In the meanwhile, after my first post, before reading your suggestion, I have made a couple of measurements at different RPMs.
They are labeled and attached.

Thank you!

Edit: forgot to mention, the strongest vibration is at roughly 1700 RPM
Attachments
Audi_A6_2005_from 880 rpm to 2200 rpm.psdata
(19 MiB) Downloaded 584 times
Audi_A6_2005_aprox 2000rpm 50ms.psdata
(19.56 MiB) Downloaded 563 times
Audi_A6_2005_aprox 2000 rpm 10 ms.psdata
(16.78 MiB) Downloaded 533 times
Audi_A6_2005_1700 rpm 50ms.psdata
(19.18 MiB) Downloaded 568 times
Audi_A6_2005_1700 rpm 10ms.psdata
(16.5 MiB) Downloaded 532 times
Audi_A6_2005_880 rpm.psdata
(17.52 MiB) Downloaded 544 times

rudolf
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Re: Audi A6 BLB engine 2.0 TDI vibration

Post by rudolf »

Dead Steve.

The cylinder head was rebuilt, because 3 months ago, my turbocharger gave up and pieces of it, got into the combustion chamber.
The injectors were sent and tested on a bench and they said that injector #4 is faulty and it got refurbished.

I wanted to make the test since you asked for it, but I was sure that everything was fine, but now I am baffled.
Here is the file from the compression test. I repeated the test 2 times, because I couldn't believe my eyes.

I have a wps500x, how should I do the compression test, so it is accurate as possible?

Thank you!
Attachments
180723_CompressionTest_002.pddata
(105.21 KiB) Downloaded 514 times

Steve Smith
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Re: Audi A6 BLB engine 2.0 TDI vibration

Post by Steve Smith »

Hello Rudolf, that does not look good my friend but lets not jump to conclusions at this stage

Was your compression test taken when the engine was hot after a run? If so this is not good!
If tested when cold, allow the engine to run for a while and retest.

If you have a WPS 500 you can now carry out an Absolute Compression test by placing the WPS into cylinder 1

You will need to compensate for your compression hose and dummy glow plug with approx 8 ml.
Absolute compression test
Absolute compression test
Lets see what happens here and if you find a low compression, there after I would use PicoScope to measure each cylinder individually so we can evaluate the four stroke cycle. (Ensure the battery is fully charged here as cranking speed will hinder cylinder comparison)

Looking at your psdata files I am confused with the formation of your crank shaft sensor.

Could you confirm channel A is connected to the crank sensor?

If so the formation is incorrect, could you confirm you were not connected across the crankshaft sensor. Use only the test lead signal wire to the crank sensor signal and the test lead ground connected to chassis ground.

If this vehicle utilizes an inductive crankshaft sensor you may be able to reduce the input range if the amplitude is low at idle speed

I have applied the frequency math channel and cylinder 4 acceleration after injection (assuming injection took place) is low.
Cylinder 4
Cylinder 4
Please could you double check your crankshaft sensor signal before we commit?

I hope this helps,

Take care......Steve

rudolf
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Re: Audi A6 BLB engine 2.0 TDI vibration

Post by rudolf »

Hello Steve,

I have compared the crankshaft sensor waveform to the Audi SSP waveform and indeed it didn't have the right form. I had a bad electrical contact.
Today, I did a compression test with a manometer.
The result:
-cyl #1: 25 bar
-cyl #2: 25 bar
-cyl #3: 18 bar
-cyl #4: 25 bar

The measurements were made with a cold engine, but the compression difference can be observed with a cold engine too.
Before taking the cylinder head apart again, I did a couple of measurements with the WPS500, but except the peak pressure, nothing worth mentioning.

I have tried applying a math formula a correction factor to the diagram, but PicoScop Automotive says there is an error in the formula. What am I doing wrong?

rudolf
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Posts: 18
Joined: Wed Sep 07, 2016 1:35 pm

Re: Audi A6 BLB engine 2.0 TDI vibration

Post by rudolf »

Dead Steve,

In the end, I decided to take off the cylinder head and then I found the reason for the low compression in cyl #3.
I have found traces of damage on the piston's head, which were piece of my turbocharger left over from 3 months ago, when we last repaired everything. Even though I washed the whole thing with high pressure water and air ( 100 bar ), it seems that some debree still remained in the intake channels. Now the cylinder head is being repaired again.

I wish to thank you very much for your advice and help on this matter. Your guidance was crucial!
Yours truly,
Rudolf

Steve Smith
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Posts: 1581
Joined: Sun Aug 25, 2013 7:22 am

Re: Audi A6 BLB engine 2.0 TDI vibration

Post by Steve Smith »

Hello and thank you for the feedback.

It's looking like compression for sure but I believe there would be additional value in testing compression from hot too.

This would present additional info to the diagnosis as an increase in pressure when hot could be attributed to piston ring sealing /movement and hydraulic priming with regards to valve activity.

I say this because I recently attended an NVH issue with what I thought was a compression issue because the compression was low from cold but once warm compression stabilized. (This was due to hydraulic inactivity due to prolonged "standing")

Could you send over your psdata file for the concern you have with "PicoScope Automotive says there is an error in the formula"?

Take care.......Steve

rudolf
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Posts: 18
Joined: Wed Sep 07, 2016 1:35 pm

Re: Audi A6 BLB engine 2.0 TDI vibration

Post by rudolf »

Dear Steve,

I've been trying to recreate the problem, but the formula works now. It seems that I was tired and must've made some mistakes. :oops: Apologies.
In the meanwhile, the valves that were damaged, have been changed, the car is up and running and I'm testing it on the road after submitting this reply.

Thank you very much once more for the help!
Yours truly,
Rudolf

Steve Smith
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Posts: 1581
Joined: Sun Aug 25, 2013 7:22 am

Re: Audi A6 BLB engine 2.0 TDI vibration

Post by Steve Smith »

Great news and thank you for the feedback.

If you could post any pictures of the valves/head that would be great.

Take care......Steve

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