When turbos go off boost

I have two things going on in my VW Caravelle TDI. Firstly, it just loses power after about four hours of continual driving.

I have to stop, switch the engine off and then start again, after which it is fine.

This has only happened once in stop-start driving conditions in town. Secondly, when I move off, the car makes a whistling noise. If I lift my foot slightly from the accelerator and then accelerate again, it goes away.

I have taken the car to Lindsay Saker and they say it's the turbo that needs to be replaced, for which I was quoted R24000.

I took the car to Barons for a second opinion, and they say there's a hairline crack in the manifold, which needs to be replaced (R7000).

Now I am wondering if I should have the manifold done or should I go straight to replacing the turbo? Both replacements are a lot of money. - Michelle

I discussed your symptoms with the owner of a workshop specialising in VW and Audi vehicles. He had very little doubt that it's the turbocharger that's on its way out.

By coincidence, they had just recently received a Caravelle TDI with exactly the same symptoms and in that case it turned out to be the turbocharger. He did, however, have a small consolation for you: the Caravelle uses a Garrett turbocharger which can be bought (for instance from TurboDirect, 011-3925195/011-3924071) for around R14000.

Installation will probably cost you another R2000.

I believe it's almost unheard of for the manifold on the Caravelle to crack but, to me, it's equally unlikely that Barons would have sucked such a story out of their thumbs.

There are sure-fire methods of identifying an inlet manifold crack. One involves spraying something like carb cleaner on the outside of the manifold and watching for a sudden fluctuation in engine revs or a puff of smoke at the exhaust.

I therefore suggest you take the car back to Barons and ask them to do such a test.

If your problem turns out to be in the turbocharger, the question arises:

why does a turbo fail and what can an owner do to extend its service life?

The main threats to a turbo are inadequate lubrication, dirty oil and excessive temperatures from heat-soak after the engine is switched off. Here are some helpful hints:

• Ensure that only the oil specified by the vehicle manufacturer is used, and change it at least every 10000km. Change the oil filter at every second oil change.
• Be wary of cleaning additives - they might loosen all sorts of debris.
• Let the engine idle for about 30 seconds after a cold start, to get the oil circulating around the turbo's shaft. Thereafter, be gentle until the engine heats up.
• Upon stopping, let the engine idle for a short period before switching it off.

A turbocharger is spinning at very high revs when the engine is working hard and the turbine is exposed to the full heat of the exhaust gases.

When the engine is shut off, so is the flow of lubricating oil around the shaft joining the turbine to the compressor wheel, unless the manufacturer has provided a special electrically-driven pump to keep the oil circulating after switch-off.

Without a flow of oil to carry away the heat soaking into the shaft and housing from the exhaust side, the oil around the shaft can literally be fried, causing chemical breakdown and the formation of carbon particles, which create problems all round.