VW Diesel No Start

A diesel has a P0299 and it takes six hours to change the turbo--how can a technician be sure?


The new VW turbo diesels appear to be less robust than previous models and it is quite common to see these vehicles radically under performing, if not failing to start altogether. When this occurs concurrently with the PCM storing a P0299 Turbocharger Underboost Condition DTC, it is time to suspect that a turbo failure is at hand.

After scanning for DTCs, start checking OEM technical service bulletins (TSBs) to see if there are specific ways to address it.

When looking at TSB 11-08 (See Fig. 1), it is going to have the technician look for vacuum leaks to and from the turbo, as seen in Figs. 2 and 3, page 12. Apparently, during the construction of these vehicles, many hoses were kinked or not connected properly. The plastic boost pipe that runs from the intake side of the turbocharger to the intercooler also tends to crack, and even if this pipe is undamaged, it is good to replace it if the turbo ends up being replaced.

Using a smoke machine to find intake and exhaust leaks is critical for turbo diesel driveability problems. Small vacuum and exhaust leaks can render a vehicle unable to start. Regular smoke machines operate at a maximum of 0.5 psi or so. Quite often, 0.5 psi is not enough to find a leak that can cause a driveability problem on these vehicles. A turbo-specific smoke machine that operates as high as 40 psi is a better option.

The best spot to smoke a turbo on the exhaust side is at the tailpipe, provided that the vehicle is cold, as in Fig 4. If it is hot, smoke test the turbo itself, looking for an access point on the engine side of the cat.

To do a full boost pressure test on the intake side, the fresh air intake on the turbocharger is a good spot to test from if the technician can reach it. Smoking the intake from other spots can work, but at such high pressures, new leaks can be created.

Dye can help find leaks on turbos because at the higher pressures these smoke machines operate, the smoke might not be easily visible. Oftentimes, where there is no smoke, dye can be found at the point of leakage. Unique to turbocharged engines are performance robbing boost leaks at cracked or broken hoses, connections or components which can be hidden from detection without the use of dye.

Technology that creates proportionately higher smoke content when increasing air pressure exists, but it is not out to market yet. (Go to VehicleServicePros.com/11223346 for more information on the Vacutec WV711.) Even with improved smoke machines, there can be internal leaks and spots that do not lend themselves to visual inspection where components will have to be taken apart. Since these components cannot be tested once taken apart, dye in these instances may be the only way of finding the leak.

After checking for leaks, the TSB has the technician check the turbo’s alignment. Generally, if the turbo is misaligned (and the TSB details specifically what it would look like if this were the case), the TSB recommends replacing it.

If the turbo passes all the checks, the TSB asks the technician to take a vacuum pump, such as one used to test an EGR valve, and disconnect the hose to the turbo charger vacuum diaphragm. A vacuum of about 600 mbar should be created, and if the vacuum decays, the diaphragm in the turbo has been compromised.

If the vacuum drops, the TSB recommends replacing the charge pressure actuation position sensor with the diaphragm.

If the vacuum holds and the DTC is a P0299, it is time to find out if there are PCM updates. However, if the vehicle is towed in and not starting, it is unlikely that software is the problem at this point.

If the vehicle passed all the previous tests, check the charge air pressure sensor (it might be called an "IAT" on some scan tools) and the charge pressure actuation position sensor parameter identifications (PIDs) to see if they are active. A sensor dead-in-the-water can cause driveability problems, as modern turbos’ driveability are heavily regulated by sensor feedback.

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