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Random misfire through exhaust

Discussion in 'The Hokey Ass Message Board' started by Ralph, Jun 11, 2010.

  1. Ralph
    Joined: Jan 8, 2004
    Posts: 295


    My neighbor brought his 55 Chevy 396 bigblock over with a problem. It has a random misfire/popping through the exhaust at anything above idle. He says it sometimes seems to go away when it warms up. I'm not a mechanic, I just play one in my garage. I went through the HEI ignition, firing order's right, put in a new cap and rotor , wires are good. don't think its ignition related unless the coil's bad. I'm trying to think of probable causes. Vacuum leak - haven't found one. Could it be a broken valvespring? What else would cause this? What should I look for?
  2. carcrazyjohn
    Joined: Apr 16, 2008
    Posts: 4,846

    from trevose pa

  3. johnboy13
    Joined: May 1, 2007
    Posts: 1,073


    I had a very random misfire in my truck a while back. I couldn't figure it out. Then one night I was out for a ride, and I noticed that when it misfired, one of the directional arrows on the dash would flash just the slightest bit. It turned out that the wiring running back to the taillights had come loose from the clip on the frame rail and was hanging down and swinging. It had hit the exhaust and melted through the insulation. When it would swing, it would hit the exhaust and short out. I clipped the melted part out, spliced it back together and zip tied it up. Haven't had the problem since. My point is, look around for loose wires that might be swinging around and shorting to ground.
  4. aaggie
    Joined: Nov 21, 2009
    Posts: 2,453


    It sounds like the camshaft is going flat.
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  5. Ralph
    Joined: Jan 8, 2004
    Posts: 295


    Taillight wiring? Man, that's far fetched. Amazing you found it. Don't really think it's a wiring short, though. Any other ideas?
  6. havi
    Joined: Dec 30, 2008
    Posts: 1,872


    Distributor gear worn?
  7. sure sounds like electrical issue being its random. cam going flat would be steady backfire as being mechanical problem. might not be melted wires could be loose ones. i have had melted wires and they drove me nuts, had a ground wire and hot wrapped together and didn't realize it because some dipshit used a huge roll of red wire on 80% of the car.:eek::mad:
  8. brad chevy
    Joined: Nov 22, 2009
    Posts: 2,627

    brad chevy

    Check your valve springs and rockers real good 396s were notorius for having valve train problems,breaking valve springs,bending push rods and throwing rockers.
  9. Desoto291Hemi
    Joined: Apr 21, 2009
    Posts: 353


    I have seen a bent valve do this,,,,it will be a very random pop or misfire.
    Just an idea.

  10. gbones32coupe
    Joined: Jan 1, 2007
    Posts: 576


    try a new coil wire or wire set even if you checked em somtimes it is that simple.
  11. plym_46
    Joined: Sep 8, 2005
    Posts: 3,356

    from central NY

  12. Ralph
    Joined: Jan 8, 2004
    Posts: 295


    Ya know, I'm really starting to wonder about that distributor...? Think I'll replace it with a known good one, and see what happens. Hey, gotta start somewhere. Then maybe hose the edges of the intake down with something to look for vacuum leaks. Any more ideas?
  13. hot rust
    Joined: Sep 18, 2007
    Posts: 617

    hot rust

    check the voltage going to the HEI. someone may have left the resistor wire hooked up instead of replacing it with a regular wire. so voltage may be low and causing the random miss. hope this helps.
  14. bob308
    Joined: Nov 27, 2009
    Posts: 220


    i had a small block that did that found a crack in the intake port.
  15. Ralph
    Joined: Jan 8, 2004
    Posts: 295


    Some great help here, thanks! Sure hope it's not valvetrain related. I'll be away from the computer for the weekend, but Monday the search resumes!
  16. lowsquire
    Joined: Feb 21, 2002
    Posts: 2,564

    from Austin, TX

    Loose spark plug. oldest trick in the book. its amazing how bad a car will run with an only slightly loose plug.
  17. oface
    Joined: Aug 30, 2008
    Posts: 64


    Sounds like a burned exhaust valve to me, do a compression check on the motor.
  18. Ralph
    Joined: Jan 8, 2004
    Posts: 295


    Hosed the sides of the intake down with WD40, no vacuum leaks. None from the carb base either. Coil gets 12.4 volts, engine off. Compression check done. 145 psi everywhere except #7, at 135. Motors only one summer old, by the way. Didn't check #5, cause it looks like "step one - remove steering box". Reset valves to 1/2 turn preload, because they seemed tight. Car only shows 8 shakey inches of vacuum. Hey Plym_46, thanks for that site. #7 Cylinder has me worried. I've got a bad feeling about this.....
  19. Lobucrod
    Joined: Mar 22, 2006
    Posts: 4,074

    Alliance Vendor

    Did you check out the movement of the rocker arms when sitting the valves? Best way to tell if the cams going flat. Poor vacuum sounds like either burnt valves or bad cam if there are no other vacuum leaks.
  20. Von Rigg Fink
    Joined: Jun 11, 2007
    Posts: 13,432

    Von Rigg Fink
    from Garage

    I had a spark plug do that on my 327 last year,
    but with your vac. readings it could be a valve issue
    Last edited: Jun 24, 2010
  21. Ralph
    Joined: Jan 8, 2004
    Posts: 295


    Here's something.. did I mention it stops doing the snap crackle pop when it warms up? Kinda deletes the bad cam idea, that wouldn't cure itself. I'm running out of ideas. When it warms up the vacuum's still lousy, though.
  22. usmc50lx
    Joined: Oct 3, 2006
    Posts: 711

    from St.Louis

    exhaust leak letting air in with unburned fuel popping in the headers??? my zoomies did it it took a few months to find a pin hole in one of my welds!!!
  23. Fedman
    Joined: Dec 17, 2005
    Posts: 1,165


    Could it be Cam timing?
    Slipped chain or incorrect installation.
    That would produce the low vacuum #'s
  24. Ralph
    Joined: Jan 8, 2004
    Posts: 295


    OOH! I like the exhaust leak idea. It fits! Also helps with my inability to cure it...I'm really thinking cam and valvetrain issues as the root causeof all its troubles. Low vacuum leads to its way rich idle, which leads to snap crackle pop. I think we'll have to find somebody with a dial indicator to check the cam. I've kinda hit the outer edges of my mechanical diagnosis skills here. I'm not going to abandon this thread till we've fixed it though. Stay tuned for the next installment!
  25. Just had a customer that was complaining that the exhaust I set him up with made his car "Sound like crap" like it had a leak. Well it was misfiring and making a popping sound in the exhaust. I looked at the tailpipes and it was so rich I couldn't believe it. Told him to get it re jetted and tuned, he did, and voilà, the misfire/popping is GONE!
  26. Ralph
    Joined: Jan 8, 2004
    Posts: 295


    The local engine guru is doing a leakdown test on it early next week. That oughta tell us something.
  27. krackerjack88
    Joined: Apr 6, 2008
    Posts: 1,247

    from Fresno,Ca

    This is what I was thinking...
  28. OldBuzzard
    Joined: Mar 8, 2008
    Posts: 879


    Moisture (condensation) in the distributor cap can do this.
  29. running lean will do that also
  30. Ralph
    Joined: Jan 8, 2004
    Posts: 295


    Well, it's sure not running lean! I'm pretty sure when we cure the low vacuum issue, the rest will follow.

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