[68gmsee]

In most cases, white smoke denotes moisture. Some say that hydraulic fluid may burn whitish also. If he used a lot of additives it's possible that can cause it.

To eliminate the possibility of fuel contamination try this. (It's what I've done in the past).

Remove the rubber hose from the inlet side of the fuel pump and install a 4 foot piece of hose. With fresh gas from a 1 gallon contianer let it run at idle for about 5 minutes to burn any residual gas from the lines. Then check your exhaust.

If it still does it, it's possible it's head gasket. If you have an automtic it may be sucking in fluid from the modulator.

edit: I placed the container under the truck in front side with the hose in it...