1969 Cadillac Deville - Backfire through tailpipe

[69Deville]

Hello friends,

I am tuning the 472 after replacing practically everything under the hood. The car is running great with the exception of a tailpipe backfire after engaging the passing gear or near WOT. I have tuned it per the FSM and here is where what has been done and replaced.

• Timing: 5 deg BTDC
• Dwell: 30 deg
• Plugs: NOS Delco R44N and gapped at .035
• New Delco Wires
• New Accel Condenser and Points
• New Dist Cap
• New Accel ignition coil 
• Idle speed in drive: 550RPM
• Idle screws: 4.5 turns out
• Performed lean drop per FSM and confirmed via vacuum gauge
• 18.5 vacuum at idle - Very steady
• New exhaust, muffler, and resonator
• New heat riser valve (functioning properly)
• New intake manifold gasket
• Rebuilt original Rochester carb
• Throttle dash pot set to FSM specs
• KN Air Filter
• Cleaned Breather element
• New PVC Valve
• All vacuum wires replaced
• New vacuum break unit on carb
• New valve cover gaskets
• New AIR Pump Gasket

I've seen a post on another forum about the AIR Pump as a cause of tailpipe backfire. Has any one encountered this?

Any advice to eliminate the backfire would be greatly appreciated.

-DM

[savemy67]

Hello DM,

Your post indicates a backfire when either you mash the throttle to engage passing gear, or near WOT.  In both cases it appears the engine is near the upper end of its RPM band when the backfire occurs.  Does this happen consistently?

On an engine without an AIR system, a backfire through the tailpipe would indicate that ignition is taking place while the exhaust valve for one or more cylinders is open - thus the ignition is occurring late.  This might point to a problem with the advance mechanisms of the distributor, the distributor shaft and breaker plate, and or a timing chain issue.

On an engine with an AIR system, the Air Injection Reactor pump's job is to supply air to the exhaust side of the combustion chamber in order to oxidize unburned hydrocarbons.  You might have too many unburned hydrocarbons due to a choke that is not adjusted properly.

Your AIR pump may have an anti-backfire valve.  Imagine hot exhaust gas laden with unburned hydrocarbons.  Add a high pressure stream of air to this and you may get a post-ignition combustion event, sort of like blowing on hot coals.  This is what the anti-backfire valve is supposed to suppress.  Your valve may be defective.  I don't have a '69 shop manual, but there may be an anti-backfire valve test procedure in the '69 manual.  A Chilton manual covering '69 Cadillac's has this procedure.  Forum members with '69s will have to chime in.

Even with an AIR system, your problem could be caused by a worn distributor or timing chain, or a choke not properly adjusted.  The simplest first thing to check would be the choke.  Next would be the advance systems on the distributor.  Then the anti-backfire valve.  Then the distributor and timing chain.  Good luck and let us know what happens.

Respectfully submitted,
Christopher Winter

[69Deville]

Your post indicates a backfire when either you mash the throttle to engage passing gear, or near WOT.  In both cases it appears the engine is near the upper end of its RPM band when the backfire occurs.  Does this happen consistently?

Yes, this happens every time and is the only way the car backfires.

Choke is set per the FSM, but will take another look.

I am able to check the check valve on the AIR system and it seems to be working properly. The only other components are the pump itself and the diverter valve. Thinking about replacing both since I've replaced everything else.

Thanks for your help!

[Dave Shepherd]

Diverter valve can definitely cause this issue.

[69Deville]

I found the symptoms of a bad diverter valve on another forum and it is spot on. Checked the Diverter Valve and it won't hold vacuum. So I have ordered the Diverter Valve and it should be in before next weekend. I'll be installing and reporting back.

[Scot Minesinger]

Only 1970 does not use an air pump on the 472 engine, so diverter is likely problem.

[69Deville]

To close out this thread, I replaced the diverter valve and the backfiring and popping stopped. Problem solved.

[Dave Shepherd]

Great!!