wbrockstar

Unfortunately there are quite a few things that can cause backfiring on the Foxbodys.It could be something as simple as worn out spark plugs,spark plug wires,distributor cap/rotor,dirty fuel filter or the wrong firing order in relation to spark plug wiring at the distributor cap.The fuel filter is located under the car,on the passenger side framerail,a few inches in front of the gas tank.Its held on by a large hose clamp and the fuel lines are attached to the filter with one plastic clip on each line.A flathead screwdriver and needle nose pliers are the only tools you need for filter removal.New clips will come with the new filter,so dont worry if you break the old clips.The 86 model should have the HO 13726548 firing order,if its the original engine or ,at least,the correct one.People have been known to replace a HO engine with a Non HO engine if thats all they can find.So just to double check, see if the cap is wired 13726548 in a CCW direction on the cap.If its wired 15426378,youve got a Non HO engine or the cap is wired incorrectly. Swap the wires to the 1372 firing order,if its wired 1542, then see if its still backfiring.

The pip sensor,inside the distributor,can also cause backfiring and it will normally cause a no start condition once the engine reaches operating temp.

Another likely cause is vacuum leaks.Be careful when using this method and have a fire extinguisher or water hose closeby,but use a small handheld propane torch with the wand removed and a 1' long piece of hose attached then run the hose over any possible leak areas/components with the engine idling.Intake/throttle body/egr spacer & valve gaskets/vacuum hoses & fittings/injector orings & any other component with vacuum passing through it is a possible leak point.Vacuum will draw the propane into it & trigger the idle to surge,once you reach the part with the leak.It triggers a lean condition.

Check your grounds too.Make sure they're tight and corrosion free.The three main grounds are located:
Neg battery cable to engine block.
Driver cylinder head (rear) to firewall.
Neg battery wire to driver fender apron and from there to a plug in connector a few inches from the fender apron.

Check timing also.Mark the 10° btdc mark on the balancer using Witeout.With the engine at operating temp and a timing light connected,remove the spout plug from the ignition module wiring harness, pull the trigger on the timing light while aiming it at the balancer.The timing mark you made with the Witeout (10°) should be aligned with the timing pointer.(the marks posted at the end of this paragraph shows where the balancer should be marked) Sometimes the previous owner may have bumped timing up from 10° to 12 or 14°.This is fine as long as you run 91-93 octane fuel.So if timing isnt at 10-14°,loosen the holddown bolt and rotate the distributor to the correct setting. If timing is correct ,reconnect the spout plug and check timing with the light again to make sure the ecu is advancing it properly.Timing should advance from the 10° position to the 16-20° position once the spout plug is reconnected.If it doesn't, theres an issue within the spout circuit that needs to be trouble shooted.

----------------- 10 ' ' ' ' 0 ' ' ' ' 10* ------------------
^^ balancer view when looking down at it ^^

Check the egr valve to make sure its not opening when it should not be opening. The easiest way is,remove the vacuum hose from the egr valve, plug the hose then see if the backfiring has stopped.If it stops,that indicates the egr valve is sticking open either because:
1) the wrong vacuum hose is connected to the egr valve
2) the wrong vacuum hose is connected to the evr solenoid
3) the evr solenoid is defective
4) the egr valve is clogged with carbon buildup
The egr valve diaphragm can also develop a pinhole or tear in it,which creates a vacuum leak and lean condition.