Mike,

Serial number would help with exact system. Take the hose off going to the mixer. Get a cup and fill with water halfway. Cranks engine while hose is in cup and check for bubbles. This will let you know how much fuel is going to mixer. No bubbles no fuel. Take off the top of mixer if you have bubbles and check for torn diaphragm. If the mixer has tamper proof screws, will need to order new mixer, diaphragm is not sold separate due to emissions. There is also a test port on the regulator for fuel pressure. Use a low-pressure gage and I believe a 1/4-inch adapter. Crank engine and check pressure. No more than 4.5 to 5 PSI should be going to the mixer. If the truck is newer, you may have a DEPR. This is an electronic fuel pressure regulator that will also clog with LPG debris. Unfortunately, with the emissions engines must meet, fuel specifications are very tight and cause multiple issues. Good Luck.