68 Z/28 Engine Smoking - Any Ideas?

[68 RS Z/28]

I have an intermittent problem I can't track down so I thought I'd ask if anyone has any ideas on this. The car is a bone stock 68 Z/28. The engine has about 1,500 miles since a complete rebuild. This work was done by a very reputable local machine shop. The crank was turned .010 over; each cylinder in the block was bored and sleeved to allow re-use of the original pistons; rotating assembly was balanced. New cam that is supposed to match the original profile. The car has the correct original heads with 2.02/1.60 valves, aluminum intake as well as the correct Holley 780cfm carb and dist. Both the carb and dist were redone by Camaro Hi-Performance. The A.I.R. system is functional and on the car. Overall it runs well. No funny noises, lots of power, etc. What it is doing is smoking occasionally, sometimes really badly. This seems to happen only at idle speeds. I had the car out yesterday for the final run of the year. I drove about 20 miles through the country at speeds up to 60mph. I didn't notice any smoke while driving or at stop signs/lights. As I got home and pulled into the garage however she started to smoke. One guy told me that he thought there was a blueish tinge to the smoke, although I'm not so sure about that given the recent rebuild. As I say, its only intermittent. Has me stumped. Any ideas you have would be appreciated. Thanks in advance.

[SuperNovaSS]

Primary float sticking?

[70 Forest Green Zee]

Since the engine was just rebuilt, I would suspect piston rings. Blue smoke would indicate some oil blowing by the rings and out the exhaust. Sorry , that's probably not what you wanted to hear.

[firstgenaddict]

How was the break in done? Doesn't sound like the rings are seated... I hope it didn't break a ring... eeeek!

[Hotrodpaul]

I would ask who did the rebuild and what type of piston rings were used, end gaps, piston to wall clearance, low tension oil rings or standard? Did they install valve stem seals when the heads were rebuilt, were the valve guides replaced, new valves, etc. How were the cylinder walls honed, what grit, on what type of machine?

Paul

[68 RS Z/28]

Thanks for the suggestions guys. The engine was done around 5 years ago so I doubt the engine builder will remember exactly what he did. I do know it was new valves. I'll check to see if valve stem seals were installed, but I expect so. I've been dreading rings as the problem....definately not what I wanted to hear to be sure. If it broke a ring, wouldn't there be other issues (noise, debris in the oil pan, etc)? If the rings aren't seated, is there any way to do this now without ripping the engine apart? I'm not a complete novice, but I confess I have no idea what the difference is between low tension oil rings and standard oil rings. Thanks again for the suggestions.

[Kurt S]

Do a cylinder pressure check and/or a leak-down test. That will tell you a lot.

[JoeG]

Check out the condition of the plugs..might be just be a valve stem seal....just a thought

[92646]

If the motor runs well when started then starts puffing blue smoke after it heats up I would suspect the seals or the intake. Pull the dipstick smell it and look at the color of the oil. If the rings are not seating properly they will allow some of the combustion mixture into the oil pan and you should be able to smell it.

[Salvatore]

[ QUOTE ]
Check out the condition of the plugs..might be just be a valve stem seal....just a thought

[/ QUOTE ]Do what Joe suggests. If the heads did not get the good seals (Perfect Circle) etc, the little O ring seals are probably brittle. The valve guides could also be worn if they were not replaced on the rebuild. Not sure I would spend the money or take a chance on an original block to have all 8 cylinders sleeved just to have/retain the original pistons. JMO