LPG and Valve Seals

[levymetal]

Recently I've quit starting my dual fuel VN on petrol and then switching it to gas because I'm pretty sure I have a petrol leak. Now I just start it on gas and use gas full time. On cold mornings it can take a bit to start it, but it always gets going. I've heard that gas damages valve seals/seats(?), but how much of a problem is this? How can I diagnose that my valve seals are rooted, and if they are how much would it cost to replace them? I can't be bothered mucking around with petrol any more...

[tyrefryer]

damaged valve seats are a big problem, starting ur dual fuel car on gas is not a good idea as the gas doesnt run well when the car is cold, the petrol normally helps the cold start problem.

You could get ur car fully converted to run only on gas which is safe as the motor gets optimised for gas.

Cheapest option would be to fix that fuel leak

[Cheap6]

Recently I've quit starting my dual fuel VN on petrol and then switching it to gas because I'm pretty sure I have a petrol leak. Now I just start it on gas and use gas full time. On cold mornings it can take a bit to start it, but it always gets going. I've heard that gas damages valve seals/seats(?), but how much of a problem is this? How can I diagnose that my valve seals are rooted, and if they are how much would it cost to replace them? I can't be bothered mucking around with petrol any more...

No, no problem starting and running exclusively on gas except for the extended cranking time you've already found. If you don't use the petrol though, it, and the fuel system, deteriorate so that could be an issue if you later decide you want to use it again. It's not a complete solution but I would disconnect the fuel pump to prevent the possibility of it running dry (and dying) if you don't want to use petrol now but may wish to at a later stage.

LPG is harder than petrol on both valve seats and valve stem oil seals because it doesn't have any capacity to cool the valve (and consequently the valve stem) when it is introduced into the engine as a vapour. It's only likely to be an issue at high load and engine speed ie. when there is a lot of heat being generated. It's not somethin to be particularly concerned about during normal driving.

tyrefryer is right in that it may be easier to fix the leak. Where is it from? Even a fuel tank is cheap and easy to swap when you can empty the fuel out just by driving around.

Checking the valve seats are sealing adequately is done with either a compression test ie. the engine is turned over on the starter (with the throttle wide open) and the pressure in the cylinder measured, or with a leak down test where air under pressure is fed into the cylinder through the spark plug hole (with inlet and exhaust valves closed) and the rate at which the air escapes (the rate at which air has to be supplied to maintain pressure) measured. Generally one valve will fail before the others and the result will be an uneven idle ie. engine miss. Higher than usual inlet manifold vacuum at idle is also an indication of a valve leak.

Poor valve stem oil sealing is diagnosed by looking for oil smoke out the exhaust (= the valve stem seals letting oil past and into the inlet manifold) under conditions of high inlet manifold vacuum, such as when closing the throttle with high(ish) engine speed (engine braking).