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 http://www.yenko.net/ubbthreads/images/graemlins/dunno.gif

[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

[68 RS Z/28]

I kind of thought that boring it out and going with oversize pistons would hurt the value - wouldn't really be a 302 anymore, that kind of thing. Anyway, I'll try replacing the valve stem seals and see if that helps. Thanks for the help!

[Salvatore]

Why would you think that it was not a real 302 being 30 over??? Who would really care about that? I get nervous when people talk sleeves for no reason IMO but that is just me. If you look at it by not being original because it would be bored.....its not original anyhow because of the sleeved motor. Hopefully a sleeve is not moving around in the bore. Try the umbrella seals. They don't need to have the head cut. But they do move up and down with the valve because they are not fixed to the head. Should be better than nothing though. Good luck! http://www.yenko.net/ubbthreads/images/graemlins/biggthumpup.gif

[SuperNovaSS]

Why not try a leak down test before chasing a ghost? It will give you a lot better idea of what is/is not going on. Didn't you say only one guy referrenced blue smoke? Is the engine using any oil?


Jason


Jason

[GreenLS6]

I'd do a compression test first-check balance between cylinders.Look for something obvious. Then a leakdown.
O-ring seals SUCK. Machine for teflon seals or at least get the good rubber sleeve seals(they stay put and last longer) Depends on the valve springs you have though(Internal diameter) If it's only smoking when you stop then check the needle and seat or spray carb cleaner around the intake gaskets. You have higher vacuum at idle a may suck oil from a warped intake . http://www.yenko.net/ubbthreads/images/graemlins/dunno.gif Although a MO doesn't have much vacuum anyways. But it sounds good doin it!! http://www.yenko.net/ubbthreads/images/graemlins/naughty.gif
Good luck

[njsteve]

On the warped intake idea, you can also have a leak on the underside of the intake that sucks oil from the lifter valley into the intake runners without ever giving any indication of a vacuum leak on the outside of the intake.

My vote is for the dried out rubber o-ring valve seals. After sitting a long time, those things can just fall apart on their own. They would also give an inconsistant oil down that would occur randomly and not all the time, depending on oil flow/engine rpm.

[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.

[SuperNovaSS]

Good advise.


Jason

[Hotrodpaul]

If the smoke is black, I would look at the carb, while a bluish smoke would be oil related. As suggested earlier, pull all the plugs and check the color. A dry black sooty residue indicates a rich condition, possibly a carb problem. An oily black residue indicates oil passing through the Intake Valve guides or Rings. Oil passing through the Exhaust Valve Guides will cause blue smoke but may not affect the plugs. If a particular plug shows these symptoms, look to that cylinder for the problem, such as rings, valve seals, or leaking Intake Manifold Gasket. Reading the plugs is the first thing I would do.

Paul

[king_midas]

Brake fluid?

[njsteve]

[ QUOTE ]
Brake fluid?

[/ QUOTE ]

Forgot about that rare problem: Once in a great while you'll find a master cylinder with a bad rear seal, combined with a vacuum brake booster with an equally bad front seal. The result is the large vacuum hose running from the booster to the intake sucks the brake fluid residue into the intake and burns it. Saw that happen once in 30 years but it was a wild one to diagnose!

[SuperNovaSS]

Come on..........Are you being serious Steve?

[JoeG]

Global Warming....... http://www.yenko.net/ubbthreads/images/graemlins/tongue.gif

[njsteve]

[ QUOTE ]
Come on..........Are you being serious Steve?

[/ QUOTE ]

Absolutely. One of those totally freaky, million-to-one things that will (hopefully) never happen to anyone else in the universe ever again. Discovered it by accident when they pulled the vacuum hose off the engine and noticed the paint curling around the fitting on the intake - you know how brake fluid attacks paint - that was the first clue.

[68 RS Z/28]

Thanks to all who have chimed in with an idea either here or via e-mail. I really do appreciate all the suggestions and have been trying to work my way through some of the possibilities. Today I found some time to do a compression test. Cylinders 1-7 pumped up nicely and read between 155 and 160 pounds. When I went to 8 however.....problems. The reading varied between 30 and 90 pounds. I don't have a leak down tester, but I did order one and it will be in on Tues or Wed so I'll see what that tells me, but there is something definately wrong with #8. I'm not sure its related, but for fun I pulled the valve covers and found that the rocker stud on the intake valve had snapped. The rocker was still on what was left of the stud, but the upper portion was just laying there. I was pretty surprised to see this since the studs were replaced when the engine was done. Also, there were NO funny noises - the car sounded good. Just prior to doing the compression test I had noticed that while all the other plugs looked more or less normal, the plug on 8 looked like it had never been fired - which I suppose makes sense if it wasn't getting the fuel/air mix. I did confirm that it has spark. So all in all, I think I've found a major problem, but I don't think its the reason for the low compression - with the valve stud snapped the valve should be closed and compression wouldn't be affected. I also don't see how this would affect the smoking issue. Anyway, it will be Wed at the earliest before I get the leak down results and then obviously the heads come off. Any comments/suggestions would continue to be appreciated.

[SuperNovaSS]

The snapped stud definitely accounts for the low compression.


Jason

[Plowman]

If number 8 cylinder has a broken intake stud, it would not let any air in to compress so compression should be zero. If it was tapping the valve at times that would tell you why you had what you have. Also, check for a broken valve spring.

[68 RS Z/28]

I guess the broken stud does explain the low compression - I hadn't thought of it that way, but of course if nothing is getting in then there is nothing to compress so that does make sense. With the broken stud I didn't want to run the engine again and so wasn't able to warm the engine, but I did do a leak down test anyway this evening before starting to really pull things apart. The results were between 9 and 13% on all cylinders. I'm assuming that on a warm engine these numbers might have been lower. No air was detected leaking out from problem areas like adjacent spark plug hole, bubbles in the coolant, etc. Do these numbers sound okay? I then started to pull stuff off the engine and when removing the right exhaust manifold I found a thin line of oil that had been leaking from the back of the valve cover and it looks like some of it had penetrated into the exhaust manifold near the number 8 cylinder. I think this is the cause of the ocassional smoking given that the leak down results didn't show any big problems. So, I think that once I get the studs replaced and put it all back together the problem should be solved. If anyone has any suggestions on other items to check while its apart, I'd be glad to listen. Thanks for the advice.

[Salvatore]

WOW, I think most of us here figured you got past any oil leaks or broken engine parts. So the rocker arm was laying in the head with a broken stud? Didn't miss a little?

[Plowman]

30% is bad rings,so you sound good. Paul

[quik9r]

This happened to me about 5 weeks ago will driving on the highway http://www.yenko.net/ubbthreads/images/graemlins/shocked.gif very luckey no damage, replaced stud and reset lash good to go.

kevin