68 Z/28 Engine Smoking - Any Ideas?

[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

[1969z280]

I'm wondering about the machinist who would have cut an original bore 320 for sleeves in order to save $300 worth or pistons as well. It seems the sleeves would have cost twice that much and eliminated the originality. JMO Ed

[Lynn]

If you have already torn into the engine then disregard this. However, I would approach it this way.

First, I doubt it is valve seals based on this statement: “What it is doing is smoking occasionally, sometimes really badly. This seems to happen only at idle speeds.” Normally, an engine with bad valve seals will smoke on initial startup when cold.

Because it only seems to smoke at idle, I would suspect the carb is “loading up” a bit. Depending on how high the idle speed is set, it might not actually stumble before smoking. Could be a power valve, needle and seat, float. Eric can help you diagnose the carb. There is no one better with a Holley.


If you want to eliminate rings, valve guides and valve seals without tearing into it, take it for a spin, get it up to operation temp, lightly accelerate in second gear up to 5000 RPM, and let off the gas, but don’t hit the brake at all. Let the engine brake. If it has poor ring seal, bad valve seals etc. (or even a leak at the intake underneath so that it can draw in oil into an intake port, as someone has suggested) then it will billow gray smoke, guaranteed. If it doesn’t, you know it is not internal to the engine, and save a lot of tear down time chasing rabbits that aren’t there.

Hope this helps.