Car Care: Fix the smoky tailpipe blues
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Smoke signals trouble
Have you ever noticed excessive amounts of blue smoke belching from the tailpipe of the vehicle travelling in front of you, especially when travelling downhill and after the driver has accelerated?
There can be a couple of different reasons, but one of the main causes on a naturally aspirated (non-turbocharged) petrol engine, is excessive wear in the cylinder head valve guides.
On deceleration (closed throttle), a high vacuum is created within each individual engine cylinder, which can draw oil from the top half of the engine through the worn guides and into the combustion chamber.
Once the driver opens the throttle to increase the engine power, oil is mixed with the unwanted exhaust gases and is emitted out of the vehicle’s tailpipe.
On older vehicles not fitted with catalytic converters (designed to trap harmful emissions), the amount of blue smoke and smell can be excessive, depending on the time and distance the engine is driven with a closed throttle.
It’s a worrying sign because it means excessive amounts of engine oil is being used, which is not good for the environment, but also very unhealthy for the engine, especially if levels are not checked on a regular basis.
What can owners do about fixing the problem?
The obvious answer is to replace the worn valve guides, but often it’s not that easy. Many repairers get nervous about carrying out or recommending such repairs. Most of the vehicles with worn valve guides have many kilometres on the clock.
If a vehicle has been regularly serviced, engines tend to wear from top to bottom at a similar rate, which means most of the main components grow old together.
To replace the worn valve guides, the cylinder head(s) needs to be removed, completely stripped and fully reconditioned, including reseating the valves and machining the head surface.
This means once the cylinder head(s) is/are refitted, the engine can become a mismatch: half is in as-new condition while the other half is still running on old and tired legs and can struggle to keep up. The end result can be that excessive oil consumption continues but this time via the piston rings and not the valve guides.
Extra load can also be applied to the engine bearings because the top half of the engine’s internals are all sealed up nice and tight with virtually no compression loss.
It can become an even worse scenario if an engine was to blow a head gasket and suffer a slight overheating problem.
The pistons can drag and damage the engine bore and the sealing rings can also lose their tension as a result.
If the cylinder head(s) is/are reconditioned as part of replacing the head gasket, then major problems can develop due to the increased load being applied to the damaged bores and pistons.
There is also a third option and one some owners of high mileage and old vehicles shouldn’t forget: if the engine is still running, then the vehicle retains some value as a trade. It’s likely the vehicle’s best days are over.