My 91 fordf150 with a straight six won't start.

How long since a tune-up? Any applicable trouble codes? If it cranks good but won't start, have a helper crank it, while you visually check for spark at the spark plugs. If you have spark everywhere, use a gage and check fuel pressure.

What is engine litre size? Do you have a distributor on this? I know ford had lots of problems with tfi module, where applicable. Don't replace that without narrowing the problem down. If you have dis ignition then then you don't have a tfi module.