Questions and Answers

Ford Ranger Popping on Deceleration Q. I recently bought a 1986 Ford Ranger pickup with a 2.0 liter engine with a single barrel carburetor and 100,215 miles. It has been backfiring since I bought it. As you are driving down the highway and let off the accelerator to change gears, 5 speed manual, it pops, sometimes real loudly, in the exhaust.

The bigger problem is that during the past three weeks, as you are driving, it begins to buck and cut back until, with the accelerator on the floor, you are only going 30-35 mph. It clears up on its own. Sometimes after you pull over and rev the engine, sometimes just continuing on down the highway.

At first it seemed to only do this when it was "hot". Now, it will do it after only a mile or so. I have checked the timing, replaced the fuel filter at the carburetor, checked the EGR to see if it was sticking, and pulled the top off of the carburetor to check for trash in the bowl. Any suggestions?

Thanks!

A. The popping between gears is almost always due to a problem with the air injection system. The air pump pumps air into the exhaust manifold to reduce exhaust emissions. At high engine vacuum, the Air Bypass Valve (ABV) vents pressurized air from the air pump to the atmosphere in order to prevent backfiring. At other times it sends air to the exhaust manifold; on cars with a three-way catalyst, it sends air only to the oxidation catalyst when the engine warms up. It is also known as the "anti-backfire valve" or "diverter valve."

To confirm it is an air injection problem, disconnect the hose from the air injection pump, or remove the drive belt. If the popping and power loss goes away, you know it's in the air injection system.

There are Ported Vacuum Switches, Vacuum Control Valves, A Temperature Vacuum Switch (TVS) and Vacuum Regulators that control the ABV that could be bad, or the ABV itself is bad.