New rebuilt 5.3 - smokes at start up!
 

We just had a 5.3 vortex totally rebuilt by a machine shop to replace our 4.8 in a 2003 Chevy Silverado. We put the new motor in, did the break in period and had the truck Dyno tuned for the new engine. On day 3 it started smoking at startup (only at startup) and progressively got worst. After two weeks pulled the motor out and the machine went through it again, new rings, rod and main bearings. Put motor back in and on day 3 started smoking at startup again. Took it to Dyno shop, everything checked out. Took it back to machine shop, they replaced drivers side valve cover and a vacuum line. Still smoking! Machine shop says it's not the valve guides because he did the valve job and builds a ton of 5.3's. Any suggestions?

stock camshaft?
what about the valve stem seals?

Stock cam, stock everything, machine shop says the problem is not in the heads. They say the passenger side valve cover should be replaced next to fix the problem. I feel they did a bad job on the head.

Is the vehicle smoking from the tail pipe or is the engine smoking?
If the engine is smoking, possibly valve cover gasket.
If smoke is coming from the tail pipe when started and then stops, it is in the heads: bad valve stem seals, worn valve guides.
If the vehicle smokes at startup and continues to smoke while driving: bad rebuild, check compression in all cylinders. New rebuild after break in period should not have variance between high and low cylinder of more than 10%.

Smoking from tail pipe, Thanks for your reply Midwest Chevy.

And I assume it's oil smoke, not anitifreeze smell?

Oil smoke, you can see and smell it.

when the valve stem seals get real bad you will start seeing an ash deposit on the spark plug center electrode

Smoking at start up only
 

What Was The Problem ---Same happened to me -was told to replace Drivers side Valve cover(new baffle)have not replaced yet.
Let me know how you made out.Thanks

@TomZee BDPhoenix hasn't been to the forum in 8 years (viewable when you click his name). You're better off making a new thread than exhuming buried ones.