Hello,
I have an 87 5.7, appears to be all stock. 38K miles. Small puff of smoke when starting up, cranks good, runs/ drive good. Appears that I am getting a small misfire at idle. I have replaced the fuel filter, fuel pressure key on engine off is 42 PSI. Running is 35 PSI. Have done a complete tune up, new ignition module, new O2 Sensor, showing no codes. What would y'all suggest as why I am getting this small Misfire and Puff of White Smoke at start up.

 

Blue smoke is much more common. Valve seals tend to leak. White smoke is usually coolant. Few options. You could have a leaky injector, head gasket (not exactly my expected problem) or even an intake leak. Pressure test the coolant and see if you have a drop, see if you can find the leak. Those would be my initial suspects.

 

Might also be a problem with the 9th Cold Start Injector system.

 

My 86 has had a small random idle misfire since I've had it, after replacing literally anything anyone could name and it having good compression and mpg/power, no smoke, etc etc I've decided it's
just who it is as a person. Could be a valve not sealing well but the idle vacuum is rock steady, I've drowned the intake in flammable brake cleaner and found no vacuum leaks, if it's a worn
cam it'll tell me eventually I figure but it's quiet right now, meh.
Runs great otherwise, I've given up on it for now.