Hi

Can anyone advise on these symptoms and VAG-COM codes?:

I bought this Skoda Octavia 1.9 TDI 2001 78k miles a short while ago with full service history.

It keeps going into limp mode when I put my foot down. Before getting VAG-COM on the job, I tried changing the fuel filter (due anyway) and the MAF sensor, but these did not cure it.

The VAG-COM codes that re-generate when it hits limp mode are either of these (each appears with approximately equal frequency):

17563/P1155/004437 - Manifold Abs.Pressure Sensor (G71): Circ. Short to B+

Possible Symptoms

• Limp mode
  

Possible Causes

• Connection(s) to/from Manifold Abs.Pressure Sensor faulty
  
• Manifold Abs.Pressure Sensor (G71) faulty
  
• Pipes/hoses not/wrongly attached
  

Possible Solutions

• Check wiring and connections
  
• Check / Replace Manifold Abs.Pressure Sensor (G71)
  
• Check pipes and hoses
  

Special Notes

• Someone with this dtc replaced the Boost Pressure Controle Valve (N75) and the dtc disappeared
  

or:

17965/P1557/005463 - Charge Pressure Control: Positive Deviation

Possible Symptoms

• Reduced power output
  
• Limp mode
  

Possible Causes

• Boost Pressure too High
  
• Hoses/Pipes incorrect connected, disconnected, blocked or leaking
  
• Charger Pressure Control defective
  
  • VNT (variable nozzle turbo): nozzles stuck
    

  [*] Solenoid Valve for Boost Pressure Control (N75) defective

Possible Solutions

• Check Hoses/Pipes to/between Components
  
• Check Solenoid Valve for Boost Pressure Control (N75)
  
• Check / Clean / Replace Charge Pressure Control
  
  • Check / Clean mechanism for variable nozzles
    

I did suspect the "sticky vanes" problem as the car was previously owned by a disabled person doing only 3,000 miles a year, who may not have been exercising the turbocharger frequently enough. However, with the other error message pointing to an electrical problem with the sensor, I am reluctant to conclude the turbo needs re-conditioning without trying other things first.

I have checked the vacuum hoses and the wiring visually and cannot see anything obviously wrong.

Could the overboost message simply be a false reading from a faulty MAP sensor? Changing that is a bit of a mission as it is tucked away down between the washer bottle and the front driver headlight, but less of a mission than unsticking the turbo.

There are also a few other minor electrical gremlins in the car, a temperature gauge that occasionally stops working, a bad ground on the radia antenna and a warning that the CAN bus for the comfort system is operating in single wire mode. These may be totally unconnected to the main problem.

Anyone know the connection between these DTCs to help me decide what to try next?

when I had the sticky turbo problem it seemed like the car lost of lot of the lag when the turbo first cut in, there did seem a notable difference when the turbo was cleaned out. That might help in determining if the turbo vanes are sticking.

May well be worth changing the map sensor before looking at the turbo, don't know how much one is though.

One thing about these cars is the lack of space under the bonnet on the turbo models.

May well be worth changing the map sensor before looking at the turbo, don't know how much one is though.

Seconded. Try the MAP sensor first.

If you have VAG-COM I think you can do output tests on the turbo VNT actuator, and watch it moving- though it's usually hard to see. Have a look at TDIClub.com.

Thanks guys

I'll have a go tomorrow at using VAG-COM to cycle the VNT actuator with good light and a cold engine - as you say it is a bit crowded at the back of the engine bay.

Following your guidance I also found this useful resource:

replace VNT turbo actuator

I guess if the actuator rod moves freely it is almost certainly the sensor (about £55 ebay), but if the actuator does not move freely it could be a bad N75, a vacuum leak, a stuck/rusted actuator rod/lever or stuck vanes - which I will have to eliminate one by one from cheapest to most expensive.

I will post what I find.

Well - I said I will post what I find, but it is not an answer....

As it is impossible to see the VNT actuator rod without getting under the car, I asked the local independent Volkswagen specialist (not the local stealer) for a diagnosis while he had the car for some work on the brake drums.

He ran the VNT actuation cycle with VAG-COM (13 cycles on residual vacuum before it stopped) to confirm the N75 solenoid valve and the vacuum lines were OK and then tested the actuator rod movement using the solenoid/vac, a vacuum pump and manually - all OK again (although he did say he was not sure that the rod moved absolutely totally the full distance it should).

He took a VCDS measured blocks log on the requested MAP v actual MAP at full throttle up the nearest hill, together with MAF readings - no limp mode and no error codes. The log did show some over-boosting but not massive (30% overboost was the worst deviation) and not consistent with the deviation disappearing almost immediately. He believes the limp mode problem may be being caused by an intermittent wiring problem between the MAP sensor and the ECU (not the sensor itself as it is sending variable signals) but is not sure the wiring problem and the apparent overboost are related as sometimes one error code shows up without the other. He suggests monitoring and logging the problem over time, If the wiring error becomes permanent or a pattern is established of the overboost and wiring errors both being recorded together when it goes into limp mode, then he would bypass the wiring between the MAP sensor and the ECU to eliminate it (cheap job). He believes an ECU problem would be "one in a thousand".

These diagnostics were done for half an hour labour charge (£15 - no stealer prices here). I ran the car at full throttle up a steeper hill - again no limp mode or errors - but then on a more extended motorway test run I got the limp mode again, this time with just the overboost error recorded in the ECU.

Anyone got any more ideas?? On this evidence I am not going to fork out money on a reconditioned turbo or on replacing sensors. I am just going to have to be careful overtaking with my foot down on single carriageways, until I get to the bottom of it.

I found that the problem with the vanes was very intermittant. It started after the car was left to stand for a couple of weeks. For a while it would do it say once a week or a maybe couple of times in one day, it did get a bit worse before I sorted it but even then it didn't do it all the time.

That's an amazing coincidence, Trundlenut. I bought the car off a trader who had taken it in part exchange. He was a bit troubled that the car was not running too well the morning he drove it to me for sale and said it might have something to do with it standing for 2 weeks as it had been running perfectly fine before.

He, I and a mechanic then took it for a test drive and it was fine - so I went ahead with the purchase. Of course, we had switched the ignition off in between and I knew nothing about limp mode at that time. I think the trader was honest (he has not walked away from the problem as others might, but is stumped on exactly what to do - just like me and the independent specialist) and the previous owner may also not have known anything was wrong - lived in Lincolnshire (no hills), was disabled and did only a few thousand a year.

I think I'll see how it goes. The US site says just giving it a good workout regularly may improve things. I could live with once a week but twice a day would get on my t*ts.

If I find it gets worse and I have to have the turbo dealt with, how much did you pay to have it removed, cleaned out and re-fitted? My research shows reconditioned turbos start at about £300 and then there is about 3 hours labour on top if I go that way.

It was about 3 years ago and I don't have the receipts to hand but I think it was about £200 to get the turbo stripped and cleaned (sent of to a specialist to do this) and then about the same again in labour plus an oil change to remove and refit the turbo - you'll lose some oil when you remove the turbo so the garage that did it said they might as well change the oil and filter while they were at it.

I think it may have been about £500 in total but I couldn't use my regular mechanic and so I ended up with the closest people who actually knew what the problem was and how to fix it, so I could probably have got it done cheaper, but hey I needed the car and it was driving me round the twist.

When the problem first occurred I did take onto a dual carriageway near my house and give it a serious blast in 3rd gear and a lot of soot came out the back but it didn't really help. I had gone from doing a lot of motorway miles to about 6 months of pottering around town then the car stood for 2 weeks after an accident and the first time it went into limp mode was when I drove it away from the repair place.

£500 going the reconditioned/independent fit route was about what I was expecting (new/stealer fix route £1000+).

I have a theory that the electrical fault message is a red herring and that the pressure is simply spiking above the range that the MAP sensor is designed to detect, due to sticky vanes.

Before I spend the £500 I am going to start a new thread to try and attract the attention of an auto electrician as it is a long time since I did electronics in school physics.

If you want to follow the discussion, just search for my posts.

Thought I would just post the final resolution of this. Tried a turbo cleaning kit from Innotec applied by local garage up the turbo's a*se - £90 in the money pit - both codes still there, still limp mode, still overboost showing in vag-com measured blocks. Tried MAP sensor replacement and all new wiring from sensor to ecu - £130 in money pit - as above. Tried replace all vacuum/air pipes and swap EGR with N75 (even though all had seemed OK on inspection) - £40 in money pit - as above. Tried exchange reconditioned turbo (nasty job for the fitter as no room to work with autobox gubbins in the way) - £600 but success - no limp modes, both codes gone, no overboost measured in vag-com. So for anyone who gets these 2 codes together, don't assume that the 17563 "map sensor short to positive" code is necessarily an electrical problem.