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? Ralph
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.
Taillight wiring? Man, that's far fetched. Amazing you found it. Don't really think it's a wiring short, though. Any other ideas?
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.
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.
rxhaust valve hanging up. Put a vacuum gauge on it and check out the stuff it can tell you at http://www.secondchancegarage.com/public/186.cfm
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?
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.
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! Ralph
Loose spark plug. oldest trick in the book. its amazing how bad a car will run with an only slightly loose plug.
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.....
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.
I had a spark plug do that on my 327 last year, but with your vac. readings it could be a valve issue
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.
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!!!
Could it be Cam timing? Slipped chain or incorrect installation. That would produce the low vacuum #'s
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!
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!
Well, it's sure not running lean! I'm pretty sure when we cure the low vacuum issue, the rest will follow.