A Shout for help.

[Steve Smith]

Here is another one of those shouts for help with a case study that I have been working on for some time.

To cut a long story short, I have a SEAT Alhambra Diesel with hesitation and holding back (hot engine) during light throttle, no engine load, 3rd gear at approx. 2000 to 2500 rpm

Make – SEAT
Model – Alhambra
Year – 2008
Engine Size – 2000cc (TDI PD) 140 HP
Engine Code – BRT
Model Code – 7V9
Mileage –122000
No DTC’s or warning lights.
Complaint has been evident for over 1 year
PCM Software level updated as recommended by VM

The hesitation is caused during the scheduled operation of the EGR valve at 2000 to 2500 rpm during light throttle, minimal engine load, where 3rd gear generates the most pronounced hesitation, however, the issue is evident in all gears

The capture below highlights the fault condition where we have an extreme drop in MAF during EGR operation

SEAT Image 1

Such an extreme drop in MAF (2.178 V) does concern me and this has become the focus of the diagnosis.
I can confirm the EGR Valve is operating correctly and responding accordingly to the duty control
If the EGR valve is sealed, then the hesitation is cured!

Rather than focus on serial data and the translation of MAF voltage into pressure, I have added a pressure transducer to the inlet manifold and another pre DPF.

Here we can compare the pressure differential between intake and exhaust (Pre DPF)

SEAT Image 2

What strikes me here during the pulsation of the intake pressure is the pressure differential between intake and exhaust

Intake pressure during EGR operation (pulsation) 807 mbar
Exhaust pressure Pre DPF 126 mbar
Pressure differential 681 mbar

As we know, where there is a differential there will be flow but the flow here will be from the intake manifold into the exhaust when he EGR opens and not vice versa!

I have then focused on the throttle plate: If the throttle plate is momentarily commanded to close during EGR operation, we can create a differential pressure that favours the flow of exhaust gas into the intake manifold as the closed throttle will lower manifold pressure.

After further testing of the throttle plate, whilst it functions as expected (to prevent shunt) it does not operate during EGR operation (The throttle plate remains wide open)

At this stage I thought we were onto something, but after comparing against another vehicle (BMW 2.0 Diesel) I can confirm this throttle plate remains open during EGR operation too! See below

BMW Image 3

Note the intake and pre DPF pressures above:

Intake pressure during EGR operation 255 mbar
Exhaust pressure Pre DPF 37 mbar
Pressure differential 218 mbar

Whilst the pressure is still greater in the intake manifold (but lower than the SEAT) we must remain mindful of where I am measuring exhaust pressure (i.e. not at the exhaust manifold where the pressure pulsations are likely to be greater.)

The other issue here is that I am not comparing apples with apples as I don’t have access to a SEAT Alhambra for comparison measurements and this is where I need help as I am introducing unnecessary variables

Could anyone provide intake and exhaust manifold pressure values for the SEAT above (or similar VAG engine) during EGR operation, minimal engine load, 3rd gear between 2000-2500 rpm?

Ideally I need:

Engine speed
Boost pressure (MAP)
Pre DPF Pressure
Throttle position
EGR operation
MAF
Gas pedal position

As ever any help or feedback would be most appreciated

Take care…..Steve

[carl456]

Hi Steve, thank you for the very detailed post. While I am not sure I can help you in any way, I do have a question. In your first image we can see a corresponding drop in MAF as EGR duty increases, as you would expect. The last few EGR duty cycles however result in a lesser drop in MAF, I have put arrows on them to show where I am talking about. Is this normal? The two blue arrows point towards duty cycle, the yellow, the resultant dop in MAF. AM I wrong in thinking an equal EGR duty cycle should result in an equal drop in MAF consider the APP voltage and engine speed remains constant?

[Steve Smith]

Hello Carl and thank you for the feedback.

Good spot on the Duty v MAF drop (I had not focused on this area of the waveform)

One possible explanation may be "Pressure"

This is one of those occasions where I wish I had included MAP in the capture

You are correct, if engine speed, load and APP remain constant then an equal drop in MAF should occur during EGR operation assuming the manifold pressure remains constant and this I cannot confirm with this capture.

I do however have countless others and will pursue these and feedback.

Thank you again, take care.....Steve

[KimAndersen]

Hi Steve

I don't know if this will help you in your quest for a diagnostic answer to this Seat Alhambra 2.0 TDI.

A few years ago I uploaded a file to the waveform library over an exhaust gas pressure sensor (G450) - which is mounted in a VW Jetta 2.0 TDI - engine type BMM model year 2008.

In my opinion this engine is very close to your Seat Alhambra except for one thing - it doesn't have an Egr cooler changeover valve fitted - so almost the same engine type if you ask me.

This exhaust gas pressure sensor (G450) test is done with no load on the engine and with the car standing still. The RPM is raised from idle to 4000 RPM.

VW JETTA 2.0 TDI BMM EXHAUST GAS PRESSURE SENSOR G450

It would be interesting to see similar tests on your SEAT ALHAMBRA or possibly a road test to see various pressure differences in the exhaust system.

VW_JETTA_2.0_BMM_TDI_2008_G450_EXHAUST PRESSURE_TEST.psdata

VW JETTA 2.0 TDI BMM EXHAUST GAS PRESSURE SENSOR G450

(326.16 KiB) Downloaded 431 times

G450 EXHAUST GAS PRESSURE SENSOR.psprobe

VW EXHAUST GAS PRESSURE SENSOR G450

(1.32 KiB) Downloaded 476 times

Regards
Kim

[Steve Smith]

Hello Kim and thank you for the feedback, I remember the post well along with another post here called "The Search For Overlap" where FF drilled holes into an exhaust system to determine the various pressures along the entire exhaust system, its a revelation for sure!

Having compared the BMW with the SEAT peak exhaust back pressure (pre DPF) I have discovered some interesting peak values! Both vehicles can produce a momentary peak of up to 600 mbar and both vehicles are performing and breathing correctly.

I must add this is on the road, hill climbing at WOT approx 4000 rpm with approx 2.4 bar boost (absolute)

It would be great to plot exhaust pressures along the exhaust system from manifold to tailpipe as FF did in the forum post above, not only during WOT max load, but during the fault event on the SEAT when the EGR opens (light throttle, light load)

With that said, it is not possible and I don't recall a G450 Kim on this vehicle. It does have an EGR Heat exchanger/cooler but no By-Pass arrangement about this cooler.

Moving on with captures, I firmly believe Boost pressure (Intake manifold pressure) to be too high under EGR operation and so focus has to switch to the turbo charger and control (N75)

The Turbo Actuator moves seamlessly through its entire travel and we have no over-boost codes

Please see peak pressure under WOT below and note N75 activity

Image 1 A WOT

We appear to see a drop in N75 duty as the gas pedal is released, accompanied with a decrease in N75 current and a reduction in the negative pressure applied to the Turbo actuator, but is this correct?

I do not have a comparison for N75 current flow, duty control and vacuum levels under these conditions

Below we have the same capture at light throttle under the fault condition (Huge drop in air flow during EGR operation)

Image 2A Light Throttle- No Load

Here we can see the PCM lowering the duty control of N75 but initially there seems to be very little influence on the vacuum supply to the Turbo Actuator, suggesting minimal change in the position of the Turbo Vanes so resulting in no reduction of boost pressure!

These are my thoughts at present and if anyone could help qualify these results with a similar vacuum controlled Turbo Actuator via N75 that would be awesome

Thank you again, take care.....Steve

[KimAndersen]

With that said, it is not possible and I don't recall a G450 Kim on this vehicle. It does have an EGR Heat exchanger/cooler but no By-Pass arrangement about this cooler.

If the engine is fitted with a particle filter, there must also be an exhaust gas pressure (G450) mounted somewhere in the engine room.

Regards
Kim

[Steve Smith]

Thank you for the feedback Kim

Forgive my misinterpretation of G450 as I am not familiar with VAG terminology & abbreviations

I had interpreted G450 as as exhaust pressure sensor elsewhere on the exhaust system beyond the DPF Differential pressure sensor

The previous capture "SEAT Image 2" highlights my connection to G450 Pre-DPF

I hope this clarifies my confusion and reinforces the fact that DPF pressure under the fault condition to be normal

Take care......Steve

[Steve Smith]

Following on from the above forum post, I was hoping to complete this Case Study with captures “post fix” including images of the faulty component; unfortunately, this is not the case

Based on continued testing we arrive at Sticking Turbo Charger vanes (minimal load range) and given the cost of a VAG replacement, the customer has declined the repair

Whilst this is a real disappointment, I guess we must focus on what we have learned throughout.

Below we have a summary of values for 2.0 litre 4- Cylinder diesel engines under light throttle, minimal load @ 2000-2500 rpm (EGR Active)
N.B the SEAT intake pressure and differential values below are both in error (too high)

1. Typical intake manifold pressure should be approx. 255 mbar (BMW example) our SEAT = 807 mbar
2. Throttle plate remains fully open during EGR operation (Regen not active)
3. Typical exhaust back pressure pre DPF approx. 37 mbar (BMW example) SEAT = 126 mbar
4. Pressure differential between intake and exhaust Pre DPF approx. 218 mbar (BMW example) SEAT = 681 mbar

Below we have a summary of WOT full load values where both vehicles perform excellently & values are correct. N.B these vehicles have different power outputs: BMW 184 HP, SEAT 140 HP

1. Peak exhaust back pressure Pre DPF approx. 600 mbar both BMW and SEAT
2. Peak Boost Pressure BMW approx. 2.9 bar SEAT = 2.4 bar (absolute)
3. MAF (displayed via Scan tool) BMW 1360.18 mg/stroke SEAT = 1201 mg/stroke

Notes surrounding N75 Boost Pressure Vacuum switching valve (Turbo Charger Actuator)

1. A reduction in the negative duty cycle (PWM) of N75 results in a reduction in vacuum applied to the Turbo Charger and so a reduction in boost pressure
2. A rapid reduction in vacuum to the Turbo Charger is initiated when the gas pedal is released from light throttle (minimal load) to overrun
3. Without vacuum applied to the Turbo Charger there will virtually no boost pressure (Fail safe)
4. Pulsing of N75 (via PWM) results in a stepped decrease or increase of vacuum to the Turbo Charger (See first image below)
5. Continual/fixed PWM control of N75 results in a progressive decrease or increase of vacuum to the Turbo Charger (See second image below)
6. An increase in negative duty PWM results in an increase vacuum level applied to the Turbo charger, increased boost pressure & increase current flow via N75
7. Light throttle, minimal load @ 2100 rpm, N75 negative duty approx. 76%, current flow 606 mA
8. WOT full load @ 4255 rpm, N75 negative duty approx. 47 %, current flow 378 mA

The following images will help clarify the above

Below fault present at light throttle, minimal load @ 2100 rpm

Fault present and active

Below at WOT, full load @ 4255 rpm

WOT, full load

All in all, what amazed me about this vehicle was the absence of any fault codes and nothing to indicate over boost!

My thoughts here suggest that the PCM does not see over boost at minimal load (low rpm) as detrimental to the engine.With the EGR active we have "pressure release" similar to an internal "dump valve"! However, an over boost at 4000 + rpm at full load (EGR closed) could be catastrophic, a code would be stored & limp home activated. This vehicle did not have an over boost condition under WOT full load (2.4 bar absolute) and so the PCM would not display a code?

Food for thought as ever and thank you for reading and replying.

Thank you also to John Tinham (John Tinham Motor Engineer) for his invaluable support and assistance throughout and countless cups of coffee from Karen

Take care……Steve