Does it start on petrol? If it does, once it's started, disconnect the MAF or if it starts on gas, disconnect it and then switch to petrol. Reason I say this is my 3.9 will start and run for 4 or 5 minutes and then stop and will not restart until stone cold again. If I disconnect the MAF once it's started from cold, it'll continue to run. It won't however start again with the MAF disconnected.