Smoke coming out from front of intake manifold on 1990 F150 truck?

If you aren't hearing a lot of exhaust noise and you're just getting smoke, you have either an oil leak or antifreeze leak that is getting on the exhaust manifold. If you're getting oil on the manifold you have a fire hazard and should take care of the problem immediately. If you hear exhaust gas leaking around the manifold you need to replace the gasket or possibly the manifold is cracked and you need to replace the manifold.