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Old 06-28-2001, 11:09 AM
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My '74 450sl runs on 2 kinds of fuels, petrol and LPG (which is gas and very commen in the Neterlands in classic cars). It runs very bad on petrol, it has no power at all and runs very rough idle. I went to a specialized garage and they have replaced 2 injectors but that did not solve the problem. Another garage tells me that the valves are gone and that I am looking at a very expensive repair.
What I do not understand is that it is running fine on gas. When you use gas it does not use some parts of the engine like the fuel pump or injection but it is using the valves. Why would it run fine on gas when the valves are gone?

Best regards,

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Old 06-28-2001, 09:26 PM
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I am not familiar with a dual fuel setup, but would guess given the information that you provided, that perhaps the fuel pump is the culprit, or more likely, you need to change the fuel filter. Have you changed the filter as of late?

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Old 06-29-2001, 03:29 AM
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Hi Steve

Thanks for your reply. I was afraid that no much people would know about the dual fuel system. It is used a lot in the Netherlands and in some countries in Europe. It is a liquid gas (something like where you cook on) You can build it in, in every car with a petrol fuel system, when you get the car from the factory or you can build it in at a later stage. In my car it was built in only 2 years ago. The disadvantage is that it get hotter than petrol and burned valves happens more than with petrol. It is to me however a mystery why it would run on one type of fuel and not on the other one.

Anyway back to your suggestion. The car it well maintained and the fuel filter is replaced very recently. I have also cleaned all injectors because that seemed to be a possible cause to me. The fuel pump is checked 2 times by 2 different garages and seems to be working fine.

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Old 07-01-2001, 01:59 AM
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This has got me stumped. To me it does not sound like a valve problem, I agree with you on that. I have sat and pondered this and thought of something else. From what you have told me about maintaining it, I can only guess that my suggestion has already been thought of. Different fuels burn at different temperatures, thus a spark plug that may be fine for gas, may not be fine for petrol. Have you read your spark plugs as of late? Do they have a light brown coloration or are they black or possibly white?

The other possibility may be the air/fuel ratio while running petrol. This would show up on your spark plugs as running lean or rich, or it could be so far off that they have fouled. This just makes me think also that the valve theory could be right. If you have oil leaking past your valves, it could be worse while running petrol.

Keep me informed. I don't know that I can do you much good, but it is quite interesting to me.

Though gasoline/LP gas hybrids are available in the US, the cost is a bit prohibitive and thus the concept has not taken off. I know GM puts this setup in some of its pickups, as other manufactures probably do to. I am just waiting for a electric car that performs, has a reasonable cost, and can be owned or leased, as opposed to lease only.

Good luck,

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