Due to the ever increasing cost of LPG and the unsuitability of most modern petrol engines from around 2006 onwards most now suffer from severe exhaust valve and valve seat recession when converted to run on LPG.
This is caused by cheap and poor quality materials being used in the manufacture of the exhaust valves and seats, and also the discontinuation of Manually Adjustable Tappets, Self Adjusting Hydraulic Tappets and engines with VVT Variable Valve Timing.
In most modern multi-valve engines in a bid to reduce prices and ease of manufacture, the makers have opted for fixed clearance or solid tappets which cannot be adjusted without extensive work and often the removal of the cylinder head. These components and materials are fine when running on petrol only, I will explain in more detail below.
This is where the problems with valve seat recession starts,
LPG once vaporised is a pure vapour and has no lubricating properties at all, this then passes through the inlet valves and into the cylinder where it is ignited by the spark plugs, combustion takes place and the hot spent gasses exit through the exhaust valves.
Because these gasses are very hot and contain no lubricants or additives they very slowly start to burn the valve face and valve seat of the exhaust valve away, this process (called recession) over time eventually causes the valve to recede in to its seat eventually reducing the valve or tappet clearance to zero. Once this has happened it is only a matter of a few miles before the valve head starts to disintegrate causing loss of compression and misfire. This will also lead to catalytic converter failure if the vehicle continues to be driven with misfires by way off unburnt fuel passing through the exhaust system to the catalytic convertor.
On some cars such as:
All V6 Jaguar / Ford Engines
All Freelander 2's
All V8 Discovery 3's
All V8 Range Rover Sports with super-charged and normally aspirated engines
All V8 Range Rovers manufactured after 2006 with super-charged and normally aspirated engines All the above Land-Rovers now use Ford engines.
Also most Fords
All Vauxhalls after 2006
Subaru, Hondas, Toyotas, Lexus and Nissans can suffer in less than 10,000 miles,
even with lubrication systems such as Flashlube fitted these oil injection systems help a little but will not prevent wear entirely.Pre 2006 L322 Range Rovers with BMW engines and Older P38 4.0 and 4.6 V8 Rover engines are fine and will easily cover large mileages
Caution must be used with Flashlube or similar systems as excess dosing will severely damage oxygen sensors and catalytic convertors. In the past we have experimented fitting hardened valve seats at great expense to Vauxhalls, Nissan X Trails and Primeras along with Flashlube kits and all have suffered valve recession damage within 20k miles the Vauxhall Zafira in fewer than 12k miles
Figure 1 ?show a recessed exhaust valve
?Figure 2 shows a Recessed Exhaust Valve and Seat in a cylinder head
Figure 3 shows an Exhaust Valve in a 2006 Vauxhall 1.8 Zafira cylinder head with a large portion completely burnt away before covering 12,000 miles running on lpg.
Figure 4 shows 2 exhaust valves beginning to burn away.
Figure 5 shows an exhaust valve as it starts to burn away in a few miles it will look like
http://www.amrautos.co.uk/index.php/lpg-systems
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