Question: My wife’s 2007 Volvo C30 T5 suddenly stopped on the freeway. Upon opening the bonnet, oil was discovered all over the place as if the oil cap was not properly closed. The car was taken to a mechanic, who opened the engine and advised that the pistons had packed up. These have since been replaced, but now the car won’t start. Initially, it was thought that it was the battery. However, recharging the battery didn’t solve the problem. The engine shows signs of wanting to start, but does not.
Answer: Moloko, we can only speculate about what caused the initial incident. Was the eruption of engine oil the cause or the result of the damage to the pistons? And what exactly did the mechanic mean when he said the pistons had “packed up”? In the absence of answers to these questions, my best guess is that the engine ran low on oil, probably due to a serious oil leak not detected in time. This, in turn, resulted in the engine seizing.
The seizure was “soft” enough that the damage was mainly confined to one or more pistons. Seemingly, the mechanic decided to replace only the pistons. Whether this was a wise decision is debatable — when an engine seizes, there is usually at least some damage to the cylinder walls, and honing the cylinders should then be regarded as part of the minimum requirements for a lasting repair. But that is just by the way. In order to get to the pistons, the mechanic would have had to remove the cylinder head.
Replacing the cylinder head on that transverse, five-cylinder, 20-valve, turbocharged engine is never going to be a trivial job. It’s very easy, for instance, to get the valve timing slightly wrong, and incorrect valve timing could explain the starting difficulties you describe. There are, of course, other possibilities, such as a damaged or oil-contaminated sensor, or damaged wiring between a sensor and the engine control unit. An engine seizure can be an horrific experience. In severe cases, a conrod can break and the flailing stump can knock a hole in the engine block. Mechanics, with typical gallows humour, speak of the engine “putting a leg out of bed”.
Low oil level, overheating, tight clearances (especially on newly rebuilt engines), or serious wear in the crankshaft bearings (on high-mileage engines) are the most likely causes. One should also remember that a conrod, like any component subjected to stress cycles, is susceptible to fatigue. This explains why some motorcycle manufacturers, knowing that their engines will be revved hard and that the consequences of a broken conrod may be life-threatening, will insist on supplying new conrods together with new pistons.
-For all your motoring queries, please contact Gerrit Burger: firstname.lastname@example.org