I am assuming the truck is using a k21 or 25 engine if so check the propane converter pressure first take out the top plug at the Nikki converter and put a pressure gauge in there and turn the key to on and see if you you are getting 5 psi it should hold steady if keeps getting higher the converter is bad l have seen as much as 8 psi any more and will blow out mixture if OK then look at o ring at main injecter some time it breaks and could cause lpg to blow out mixter if all checks out l believe the body that the main injector sits in is bad there are small valves inside that stick and cause the lpg to blow out the air cleaner must replace complete holder hope this helps