Questions and Answers
Ford F-150 Backfiring
Q. Vincent, I have an 1984 Ford F-150 4x4 with a 4 speed manual transmission and a 300 straight six. The carburetor is a one barrel. The engine is a brand new remanufactured long block with about 300 miles on it. I put it in myself. It got a new pilot bearing, a new throw out bearing and a new clutch plate.
I couldn't get the thing timed right and it sounded like it had a vacuum leak so I took it to the mechanic. He timed it, fixed a vacuum leak in the carburetor and manifold. Everything works fine except for one thing. It backfires very loudly on deceleration especially when going down hill. As long as I give it a little gas it doesn't do this. What causes this problem and how can I fix it?
Thank you,
ZacharyA. It sounds like you have a bad Air Bypass Valve (ABV) or as it's commonly called, an Anti-Backfire Valve.
When you take your foot off the gas and decelerate, a very rich mixture forms in the exhaust manifold. This mixture is hot enough to ignite on its own, if there is air to support the ignition. Normally there is not. To insure there is no air going from the air pump to the exhaust manifold the ABV is used to turn off the air going into it, preventing the backfiring you are experiencing.
So either the valve itself is bad or the control system is malfunctioning. This is where you have to look to correct the problem.
Additional Information provided courtesy of ALLDATA
