backfire through exhaust cause burnt or bent valve

[junkyardjeff]

A friend has a 70 Torino with a 250 six and had some ignition problems which ended up blowing the muffler apart,got the ignition repaired with a electronic and now has a miss. 5 cylinders have 160 psi and one 70 so could a massive backfire through the exhaust cause a valve to burn or bend.

[wayne petty]

was the #5 cylinder spark dead... and the cross firing coming from the #3 cylinder.. then probably it was caused by the misfiring.


firing order..

1,5,3,6,2,4

if the #5 cylinder is not firing.. but the #3 is cross firing to the #5 spark plug wire .. the exhaust valve will just be opening when the #3 spark crossfires and the combustion will blow thru the open #5 exhaust valve face and seat like a blow torch..

six cylinders turn 1/3 of a crank turn between firing the next cylinders.


hard seats.. new valves.. new guides.. new springs.. and it will go another 200,000 hopefully..

[junkyardjeff]

Its #3 cylinder and has about 90 pounds less compression then the other 5 cylinders where all 6 had 160 shortly before the major backfire through the exhaust.

[Arnold]

was the #5 cylinder spark dead... and the cross firing coming from the #3 cylinder.. then probably it was caused by the misfiring.


firing order..

1,5,3,6,2,4

if the #5 cylinder is not firing.. but the #3 is cross firing to the #5 spark plug wire .. the exhaust valve will just be opening when the #3 spark crossfires and the combustion will blow thru the open #5 exhaust valve face and seat like a blow torch..

six cylinders turn 1/3 of a crank turn between firing the next cylinders.


hard seats.. new valves.. new guides.. new springs.. and it will go another 200,000 hopefully..

 ONE backfire through the exhaust isn't going to burn or bend a valve.
However..IF as Wayne mentioned..this is an ongoing crossfire(has been) then the damage may have already been done. There might have been a hair of seat left on the seat..either head or valve..and now it is gone. That hair of seat left may have been just barely enough to maintain a seal for a compression test.OR there was enough carbon on the seat to seal it..and the backfire blew that off.I'ld be looking at a head gasket.
 Could be that the backfire yanked a valve off it's seat and carbon or crap fell in there(or loosened carbon)
 Make sure that both valves are seated/closed
 Pressurize the cylinder and see where it is leaking.

[junkyardjeff]

The car sat all last winter after the incident and was noticeable in the spring after we stuck a duraspark electronic ignition on it and got it running again.

[Crosley.In.AZ]

never seen a back fire cause a bent valve.  I've had a couple back fires that blew open mufflers at the seam. Basic budget muffler with rolled seam  if you know the  design

[junkyardjeff]

It also destroyed the muffler.

[txturbo]

It could be a sticking valve or maybe the backfire dislodged a piece of carbon that is stuck between the valve face and seat. If it were me.... I would buy a quart of Marvel Mystery Oil....half in the oil and half in the tank and go drive it until it's hot. Shut it off and let it sit there for a couple days. Then start it and take it for a spin. This procedure has saved my butt several times. If it doesn't work it cost you less than $10. If it works it saved you lots of money and time.

[jaybee]

It doesn't solve your compression problem, but my experience with a muffler blown apart was triggered by intermittent failure of an anti-dieseling solenoid...a device which was very common on Fords of that era. Once in a while the engine wanted to die as you rolled up to a stop sign or not idle without feathering the pedal. I was playing pretty hard on county blacktop roads when I chopped the throttle. When I jumped back on it kaBOOM and it blew a big hole at the back end of the muffler. I figure it must have filled the exhaust with unburned fuel just like shutting off the ignition and turning it back on.

[Mac]

I'd say that "sitting all winter" has as much chance of being the cause as anything. Did you put oil in the weak cylinder when compression testing to ID ring problems?

I would start with non invasive efforts like: MMO or ATF in the oil to free a stuck valve or ring. Then maybe dribble water or Seafoam in the carb while maintaining a high idle to kick off carbon that might be propping open a valve.

[Mikej]

I had car that back fired because the valves were bad..