How long will exhaust smoke after engine rebuild?

Discussion in 'Classic Mustang Specific Tech' started by oz, Jul 7, 2005.


  1. oz

    oz
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    I rebuilt my 302 recently. After ~150 miles there is still some smoke in the exhaust (both pipes).
    The block is bored +.030". I used flat top Speed-Pro pistons with coated skirts and chro-moly rings. All the ring endgaps were in the .016" to .020" range. I have checked the compression after ~50 miles (145 – 158 w/ stock ’69 302 heads) and leak down (all less than 6%). I had the heads gone through. All the valves and guides are within spec and I have new valve seals. I have the PCV routed from the passenger side breather (with foam inside, etc) rearward to the firewall, tucked up under the export brace bolts, down the drivers side next to the valve cover, and up to the front of my Edelbrock (Carter) carburetor.
    It’s kind of strange. I can drive, accelerate, decelerate and I don’t see smoke in the mirrors but when I stop and idle, after 20 seconds or so, it starts to smoke… I have not driven behind it to see if it is really smoking all the time so I am not completely sure that it is not...
    I have heard that chro-moly rings take longer to break in...
    Is the smoke due to the harder than usual chro-moly rings not fully broken in or do you think I have a problem?

    Thanks
     
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  2. 12sec67

    12sec67 Active Member

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    what color smoke is it?

    have you tuned you carb to the rebuild?
     
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  3. bnickel

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    i wouldn't worry about it much until you have around 500 miles or so, yes the moly rings do take longer to seat so that is more than likely the cause. if it is still smoking after that then you may have a problem.

    question, have you changed the oil yet? you really need to change the oil after the initial cam break in and again after 50-100 miles or so. if you haven't changed it yet do it as soon as you can. also don't run a synthetic oil until you have at least 500-1,000 miles it to ensure the rings are properly seated.

    the only other thing i can think of is worn valve guides.
     
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  4. oz

    oz
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    Thanks for the replies.

    It is most definitely oil smoke. Bluish-gray...

    I changed the oil after initial break in. I need to do it again now that I have a few miles on it.

    The carb is matched to the Edelbrock Performer RPM intake and cam I am using and is supposed to be pretty close off the shelf. That said, I got it from a friend who only ran it for 500 miles. I rebuilt it and had a shop tune the idle mixture. I have played around with primary needles and step up springs too. It seems to be in good tune. I am currently at 4% rich cruise and 2% rich acceleration with a light (~7"?) step-up spring.

    I thought of guides too but I had all the valves and guides checked by the machine shop. The ones that were out of spec were replaced. I need to look at the receipt to see exactly how many were reworked. The smoke does not seem to correspond with typical worn guide symptoms. I have intentionally accelerated and then let off the gas with it in gear to see if it blows any smoke. I have not seen any. My wife did comment when she was riding with me that it seemed to smoke when I shifted (which would indicate guides)... I have not noticed smoke when I shift. The rear window glass is so foggy that I can hardly see out the rear view mirror though - it's tough to tell what's smoke and what is window haze... :rolleyes:

    I am going to play around with the PCV system a little more. I am using finned aluminum covers off a ~'83 GT and I had to remove the oil baffles. I had trouble with this at initial start up... I had the PCV directly in the grommet and was sucking oil off the rocker into the carb.... had serious smoke then out of the left pipe.... I ended up getting a tall breather with a baffle and foam in it to help with oil control but there's a chance that a little gets into the hose and then is sucked into the carb when vacuum goes up at idle. I don't think it would take much oil to cause the smoke I have. This would explain why the onset of smoke seems to be delayed 20 - 30 seconds after returning to idle (oil is sucked into the carb, drips into the intake and slowly runs down the runners). Not sure how to fix this one without getting taller covers with baffles (to fit over the roller rockers)... A lot easier than tearing the engine apart I think. I will unhook the PCV and plug it and see if it makes a difference when I drive it.

    I keep crossing my fingers that the smoke will be gone when I drive it. I must admit that, in general, the smoke seems to be slowly getting better. It's pretty embarrasing though when you sit at a light or in the driveway and it looks like you are spraying for mosquitos (exagerating).... :(
     
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  5. dsmGST91

    dsmGST91 New Member

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    You say you had serious smoke before...could some oil still be in your exhaust, its just slowly burning off? When I blew a headgasket in my Eclipse, it sent coolant into the exhaust, and took about 2-300 miles to burn off.
     
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  6. brianj5600

    brianj5600 Active Member

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    Smoke only on one side? Only at high vacuum? If it is not pvc, check the plugs on the smokey side. My guess is possibly an intake gasket problem.
     
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  7. oz

    oz
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    It WAS only on the left side at initial start up but after I fixed the PCV situation (no longer sucking oil), it settled down and is now from both pipes equally. It's not a ton of smoke but enough to be noticeable.
     
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  8. oz

    oz
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    I took off the 3.5 foot long breather hose and let it hang vertical for a day or so and about a dozen drops of oil came out of it. I installed a regular breather in its place and started up the car. There was visible vapor coming from it (no PCV in place so all the blowby was coming out the breather) so I didn't drive it that way. I reinstalled the drainded PCV hose and drove it. I did not notice any smoke for the first 10 miles or so but then it started to smoke just after I shifted. It may just be a blowby/PCV issue...

    Does anybody make a good baffled PCV breather? My valve covers do not have the oil baffle in them. I got my generic breather at the local parts store. It has some foam and a baffle in it but I'm thinking better vapor control may go a long way toward eliminating the smoke.

    What do you think?
     
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  9. D.Hearne

    D.Hearne Banned

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    :D Ok, which rings are they? Chrome rings or Moly rings. They cannot be both. :D Chrome rings take longer to break in than Moly rings do. Chrome are faced with chrome on the top ring. Moly rings have the top ring, grooved and filled with a moly compound to help speed up the seating process, some also have this on the 2nd ring as well. There are NO "chrome-moly" rings. Chrome-moly is a steel alloy and as such is unsuitable for piston rings. :nice:
     
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  10. oz

    oz
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    sorry, I should have checked before I made the statement.
    They are chrome.
    thanks.
     
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  11. D.Hearne

    D.Hearne Banned

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    Like I said before, chrome rings take longer to seat. Give em time. I didn't think anyone still made them, they've fallen from favor due to their wearing the block sooner.
     
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  12. oz

    oz
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    After a few PM's to D.Hearne and a couple of experiments, I HAVE ELIMINATED THE EXHAUST SMOKE :D :D :D

    My breather cap was sucking the rocker arm oil splash up into the carburetor. I added a piece to convert my breather to push-in type (which raised the breather about 2" and out of the splash). Viola, no more smoke.
     
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  13. 12sec67

    12sec67 Active Member

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    we have to keep D.HEARNE around... he good for something every now and then! :rlaugh:

    good job D.HEARNE :cheers:
     
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  14. D.Hearne

    D.Hearne Banned

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    AW, Shucks, It weren't nothin. :D Same thing happened to me a year or so ago. Baffle-less valve covers and PCV systems are a good way to keep the valves lubed ( more than they need though) :rlaugh:
     
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