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Discussion Starter · #1 ·
Have owned my 2004 Touareg 2.5 TDi from new, currently has 159k on the clock. At around eighty thousand miles it occasionally went into limp mode and then for months after would be fine. At around 90K the engine shut down completely whilst on the motoway and refused to restart. After recovery the ECU was diagnosed as being faulty and was sent off to be refurbished. All has been well until recently when the engine went into uncontrollable "race mode" with clouds of white smoke, leaking injector seals had filled the sump with diesel. Thereafter the problem with limp mode returned with an overboost error P0234, and with the ability of only able to travel a few miles without having to switch off and restart the engine. The turbo was diagnosed as the issue and subsequently replaced. Whilst replacing the turbo has been a major inprovement, general driving on town and country roads is without issue but on an open road or motorway, particually with a slight hill, on opening the throttle up the engine goes into limp mode, managing only 38 miles mile per hour. Hoses have been checked the EGR valve replaced as well as the boost pressure sensor. Can anyone please offer some advice on what action to take next? 馃槳
 

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Have you plugged a scanner/VCDS in to look for codes? Unsure if it's the same on the Treg but I had a Passat that did this and the garages all immediately said new turbo - default answer! ;)
Then scanned the engine while out for a run and found that a MAF sensor was failing randomly and throwing the engine into limp mode. 拢18 for a new sensor and half an hour in time and it ran like clockwork.

Defaulting to limp mode should record an error on the ECU so scanning is the first thing. (y)
 
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Was the actuator replaced with the turbo and the related vacuum hoses? The original small 5/6mm vacuum hoses will almost certainly be perished by now.
I'd assume there's some form of boost pressure control valve connected to the actuator and fed with a PWM signal from the ECU. Failure of that would cause an overboost.

No idea how this system works but usually MAP and inlet temperature sensors are used as a fall-back for the MAF. Failure of either MAF, MAP or inlet temp sensors would cause problems.
 

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Discussion Starter · #4 ·
Thanks for the reply Calumski and fka. A scanner has been attached and gives the overboost error, not been scanned on a run though. Will give that a try. Turbo was replaced as a complete unit complete with actuator and hoses. There is a boost pressure sensor which relays to the ECU this has also been replaced. Sounds like the MAF sensor replacement will be a good and not a too expensive move.

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Aye, taking it for a spin and recording the pressures/temps should hopefully show some weird readings on something easily replaced. (y)
 
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This is the boost pressure control valve You can usually measure the resistance across it's coil but without specs you're pissing in the wind. If it's ticking with the ignition on it's a good indication its working. Not too costly if the MAF doesn't cure it.

If you follow the hose from the actuator it will go to this valve. A second hose will go from the valve to the pressure side of the compressor somewhere. Check both those hoses are in good shape.

If you can get pressure to the actuator check its moving it's full stroke.
Disconnect the actuator from the waste gate and check the gate is moving freely and not binding. Although not binding when cold doesn't mean it's not binding when hot!

I love this generation of turbo system. Perfected by Saab and a work of art but a twat to fault find on..
 

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Have owned my 2004 Touareg 2.5 TDi from new, currently has 159k on the clock. At around eighty thousand miles it occasionally went into limp mode and then for months after would be fine. At around 90K the engine shut down completely whilst on the motoway and refused to restart. After recovery the ECU was diagnosed as being faulty and was sent off to be refurbished. All has been well until recently when the engine went into uncontrollable "race mode" with clouds of white smoke, leaking injector seals had filled the sump with diesel. Thereafter the problem with limp mode returned with an overboost error P0234, and with the ability of only able to travel a few miles without having to switch off and restart the engine. The turbo was diagnosed as the issue and subsequently replaced. Whilst replacing the turbo has been a major inprovement, general driving on town and country roads is without issue but on an open road or motorway, particually with a slight hill, on opening the throttle up the engine goes into limp mode, managing only 38 miles mile per hour. Hoses have been checked the EGR valve replaced as well as the boost pressure sensor. Can anyone please offer some advice on what action to take next? 馃槳
Thank you for posting this.

I also have a 2004 R5 and have EXACTLY the same problem/symptoms. Mine has 159,500KMs (appx 99k miles)

This all recently started when I noticed that if I push the pedal to the metal a little more than usual (say 3k RPM +), the dipstick would pop out and oil would start coming out, and I read all kinds of horrifying stories about compression valves, engine failures looming etc, so decided to change the PCV valve first as that's very cheap and very easy to change (sits on top of the engine and comes out by hand). That did not do anything so then I put my VCDS and it gave me an over-boost error below. (VCDS is worth its weight in gold).

Driving around town under 2000RPM cars runs like a swiss watch, so quiet I can barely hear the engine (surprisingly quiet for a diesel). As soon as I pop on the motorway and go to 2,500RPM +, the car goes into limp mode until I pull off and restart the engine, after that its fine again. My garage at home has a steep incline when exiting and the car struggles to get up it, showing 99.9L/100km so I understand this is an indication that the engine is pushing more diesel to the engine than normal (so possibly some kind of sensor issue) as usually ut would show 65-70L/100km.

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So thanks for posting this as I thought I was the only one and hopefully some of the comments will be ale to help me.
 

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Have you plugged a scanner/VCDS in to look for codes? Unsure if it's the same on the Treg but I had a Passat that did this and the garages all immediately said new turbo - default answer! ;)
Then scanned the engine while out for a run and found that a MAF sensor was failing randomly and throwing the engine into limp mode. 拢18 for a new sensor and half an hour in time and it ran like clockwork.

Defaulting to limp mode should record an error on the ECU so scanning is the first thing. (y)
I can run the VCDS and drive the Treg around? I actually did not know this. I am going to try this today. Thank!
 
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