white smoke on startup

Check your coolant level. Coolant exhausts as white smoke. After the engine is shut off all oil pressure falls to zero while the cooling system retains pressure until it has cooled off completely, or leaks. In other words, if there is a cooling system failure such as a headgasket leak, then after the engine is off the residual pressure forces coolant into the cylinder(S) where it collects until the next startup, or there is not enough pressure to leak coolant any longer.

 

White smoke is low compression, and fuel not firing be it from coolant absorbing the heat of compression. You should tell us the model and serial prefix of the engine, the simplest and most efficient way of finding out which cylinder is to remove the exhaust manifold, let it sit till cold, then see which cylinder smokes. Run for 10 seconds, do an injector swap with another cylinder than did not smoke and if you have a dummy injector do a Cylinder leak down test, on both cylinders, with the piston at TDC and BTC.
Just a thought!

 

Take a heat gun that registers temperature when pointed at something, check each cylinders exhaust temperature whileit is smoking. Bad cylinder will have lower temperature reading.

 

Yep, an infrared thermometer is an excellent tool to have.

 

This could be due to your injector seats being warped. I am finding this a lot. Cats only repair for that problem, is to replace the head. I have a repair for it.

 

Don't want to be the bearer of bad news, but sounds like a head gasket fire ring going south. Usually around #5, or between #4 and #5 cylinder. Go back to your "reputable" shop, let it cool down, and when you refire, have them immediately do a cylnder cut out and watch what cylinder when cut, clears up your white smoke.

 

I guess the next place I would go is what JohnP3 suggested earlier and pull the manifold and identify cylinder or cylinders that are popping white smoke. If cylinder cut out didn't make an iota of a difference (they usually won't clear completely up instantly, but you should be able to see the one that makes a difference), you'll have to track it down the manual way to identify the cylinder(s), that's not firing correctly.