by David Finlay (27 Mar 02)

Reader John Moulding has been in touch to voice some concerns about cars fitted with LPG conversions. He is interested in going the LPG route on the basis of what he describes as "the 'it's cheap, it's clean, it's wonderful' school of thought", but he is also alarmed by reports of cars which have run on gas but then developed serious engine problems.

CARkeys has also heard of this, and the common thread running through every one of the failures has been the valves, which generally recede into the cylinder head as the valve seats gradually fail through overheating. This is very similar to what happens when unleaded petrol is used in an engine which was not designed for it, though the effect is greater with LPG than it is with unleaded.

We know of engineering shops which spend a large proportion of their time converting older engines to accept unleaded fuel. The job simply involves fitting stronger valve seats which can cope with the extra temperatures involved (most of the point of having lead in petrol is to keep combustion temperatures down.) These stronger seats are better able to cope with having the valves slamming into them several hundred times per second in what is already a very hostile environment.

Any engine designed to run on unleaded without modification - which means any engine in production within the last decade - already has stronger valve seats than would have been the case in earlier times. But these seats may not be able to cope with the still more difficult conditions of running LPG, which has far less cooling and lubricating effect than petrol.

Some engines can already cope perfectly well. Vauxhall, for example, has been suggested to us (by outsiders who have no connection with the company) as a very good example of a builder of engines which can, in most cases, be easily converted to LPG. This is in addition to the fact that Vauxhall offers a number of models with factory-fitted conversions.

Some To Watch Out For

The same is not necessarily true of other manufacturers. For example, although the Jeep six-cylinder engine seems to accept LPG without any trouble, we know of a Cherokee V8 which has caused one reputable conversion company endless trouble. The Ford Focus, Nissan Primera, Honda CR-V, some Peugeots and any Toyota using lean-burn fuel technology have also been known to suffer valve problems.

Where these cars are available with factory-fit conversions (as some of them are), it's reasonable to assume that special cylinder heads with stronger-than-standard valve seats have been used. These may be next to impossible for a private buyer to locate.

The list of cars mentioned above is in no way meant to be exhaustive, and the CARkeys recommendation would have to be to consult a specialist company and confirm that a specific model is known to be able to use LPG without suffering valve seat failure. Assuming this is the case, there should be no problem - after all, several taxi firms have switched to LPG because of the low cost of the fuel, and they would not do this with cars which can easily clock up six-figure mileages if there were a universal reliability problem.

Even if an engine can run perfectly well with a good LPG system fitted, though, it might not get very far with a bad one. The issue here is that you get what you pay for. Assuming a reputable company with a good history, correctly fitting a reliable and up-to-date system to a suitable car, there is no reason to expect any trouble. On the other hand, if a cheap system is fitted by people with no experience of the job, there's no telling what may happen.

A proper conversion may cost somewhere in the region of £2000, depending on the vehicle. But we've recently heard of someone who is buying a £500 system and fitting it himself. We're already waiting for the sob stories.

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