Exhaust backfiring

[SilverSHO]

My dad and I were discussing the possible causes of my SHO's backfiring problem (backfires through the rear exhaust manifold, irregularly but constantly), and I've been thinking that it's an exhaust leak. My dad, however, thinks there's at least one burnt valve somewhere on the rear bank. I ruled out the possibility of unburned gas igniting in the cats; it starts within a few seconds of starting the car.

The reason I think the backfire comes from an exhaust leak is because when my bro and I replaced the original Y-pipe (with another stocker, but hollowed out), he accidentally twisted off a stud on the rear exhaust manifold. Last time I pulled the Y-pipe, the donut gasket between it and the resonater crumbled.

I'm inclined to believe my dad, though, because I ran the KOER Cylinder balance test, and found that cylinders 1 and 3 were weak. This seems to make sense, as my SHO doesn't rev too smoothly and doesn't seem to have the oomph it should. Would loss of compression due to a burnt valve unable to properly seal the combustion chamber cause backfiring like this? If I left out any vital info, let me know.

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Nick
Lt. Titanium 90 136k
Coneless, ACCEL 8.8 stainless steel wires,
SHO Shop torque limiters, Cherry Bomb Turbo II mufflers, snazzy blue valve caps
AIM: loudog6491, TitaniumSHO; feel free to IM me about anything SHO-related

 

[sdpatt]

You could have a spark leak from the plug wires causing an out of sequence firing. How old are the plug wires? A backfire is most likely due to an electrical problem. A burned valve on a SHO? Haven't heard that one before. The valve springs are good to 8,500 rpm. Only if a valve was gapped much less than the minimum spacing (like zero)would the valve not seat and be allowed to have seat erosion. There could be a rare broken valve spring. Very rare. An exhaust leak won't cause a backfire, but it will let you hear it better.


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Scott

1991, 254K miles, glass hood, police grill, SVO shifter, Catz fogs,
K&N, 73mm MAF, Superchip, PP Y-pipe, Borla cat-back, 190 lph pump
Eibach/Tokico/polyurethane, SHO Shop front & rear strut braces,
16x7.5" Moda R1, 225/55ZR16 Bridgestone RE730, -1 deg camber x 4,
6 disc CD, Class II hitch, Silver award at SHOklahoma Car Show

 

[jimtash]

I had a burnt exhaust valve in my SHO and it would backfire through the intake manifold. If it is backfiring through the exhaust, then I woudd check for a vacuum leak around the intake or if you find nothing there, then I would check the lash on the valves.

 

[sdpatt]

How can a "burned" exhaust valve vent through the intake? It's only connected to the exhaust port, not the intake port.

Scott

 

[jimtash]

Then what would cause a backfire through the intake at light throttle? Finally the cylinder would not fire at all. A valve.

 

[sdpatt]

A backfire through the intake would require that an intake valve is not closed while the mixture is ignited by the spark plug. That situation could occur if either the valve timing was off, the valve seat was damaged or the ignition sequence was off.

Are you sure the cylinder is not firing? Is the engine only running on 5 cylinders? If the cylinder is not firing, it is probably not getting a spark. If the spark was still happening then there would be a smooth running engine or a constant backfire through the intake (assuming the spark firing sequence was off or the intake valve was damaged).

Scott

 

[jimtash]

Scott,

I do know the cyliner had spark. Something in the valvetrain was messed up. I can only assume it was a valve because the backfire lasted about 6 months before the cylinder finally went dead. Anyway this is no concern of mine as I replaced the motor.

 

[sdpatt]

I'll bet that when the valve covers are removed you will find a condition similar to the one Fred posted about Heather's car. A damaged tappet and a missing shim. As to the cause, that is a good question.

Scott