Why turbochargers fail, and how to prevent this from happening to your engine.
In this final part of our two-part turbo feature, our Technical Editor, Grant Spolander, investigates common causes of turbo failure and how to prevent them, and what to look out for when purchasing a used turbo-diesel vehicle.
In last month’s issue we dealt with the basic principles of turbocharger operation. We also touched on the subject of reliability, highlighting the fact that these high-speed units demand optimal running conditions. Generally speaking, turbochargers are dependent on two lifelines: lubrication and cooling. The first is delivered via an oil-feed pipe and the latter is provided via the engine’s water-cooling system. This brings us to the first major cause of turbo failure – insufficient lubrication. The general rule is that if you’ve driven your 4x4 at high engine speeds, or under load, you should allow the motor to idle for roughly 30 seconds before switching off the ignition. This gives the turbo a chance to lower its rpm rate and cool down before the engine is turned off. In some cases, a turbocharger’s impeller may still be spinning at 10 000 rpm or more when you turn the engine off. When this happens, the engine stops pumping oil – and serious damage can occur to your turbocharger’s centre shaft, impeller bearing and oil seal.
Examples of this are regularly seen on our national high ways. The scenario plays out with a hurried driver dashing into a petrol station for a quick loo-stop. He turns off his 4x4 immediately and rushes to the loo, unknowingly causing major damage to his turbo’s internals. Once he hits the road again, it’s only a few kilometres before his turbo goes bang and he’s seen on the side of the road with his bonnet open.
Unfortunately, the nightmare may not end there. Once a turbocharger has popped, there are several catastrophic events that could take place. If the oil seal’s damaged, engine oil can be drawn into the motor’s intake system and fed to the combustion chamber. This can result in a runaway engine, which is the very last thing you want.
A runaway motor can’t be switched off, as it burns oil as a fuel source; it sucks the sump dry until the engine burns out. The only way to put an end to this process is to suffocate the air intake system. You can do this with a cloth rag, but if you don’t have one close by, use the shirt off your back. An equally disturbing scenario takes place when a damaged impeller fragments into small metal pieces that are sucked into the engine, causing severe mechanical mayhem. Here the auxiliary benefit of intercoolers is demonstrated: they cool charged air, but they also stop debris from entering the engine should the turbo blow.
So, the lesson to be learnt here is always to allow your turbo time to spool down before switching off the ignition. This is particularly important if you’ve driven your 4x4 at high engine speeds or under load – uphill, or with a heavy payload. Fortunately, there are some aftermarket solutions that will do this for you. These devices are commonly known as turbo timers or protectors, and they function in various ways: some keep the engine running for approximately 30 seconds after shutdown, and others use a gravity-fed oil reserve to lubricate the turbo even though the engine is off. Also on the subject of lubrication: frequent oil changes, filter replacements and the use of high-quality oils are essential to turbo life. Accumulated deposits and water contamination will cause blockages in the oil-feed line, and lead to inevitable turbo failure.
Another prime cause of turbo failure is inadequate heat dissipation. This can be due to several reasons: poor coolingsystem function, blocked catalytic converters or excess carbon build up. A blocked catalytic converter can create air buffering within the exhaust system which will subsequently increase the exhaust-gas temperatures and lead to a cracked turbo housing.
What’s more, carbon deposits are often localised in one area, especially around the waste-gate valve. This build-up of carbon acts as a great heat conductor that fails to dissipate heat. The end result is a cracked housing near the waste-gate valve...
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