My '07 Base XF, stick, has a backfiring problem. About a month ago, about every third time I started it, (cold or warm) the car would seem to backfire. Not an ear shattering explosion, but definitely a moderate "bang". That was enough to wake me up in the AM.
Several days ago, I tried an experiment.
Rather than starting the car, I would turn the ignition to the "on" position only just to see what would happen. "Bang".
Incidentally, I've used the search program here but couldn't find anything that described my problem.
HAS ANYONE EVER EXPERIENCED THIS PROBLEM? IS THERE A SOLUTION?
Otherwise, the car is running beautifully. It is quick and fast, idles at approx. 1,400 RPM.
"Help me Obiwan Kinobi, you're my only hope".

 

Your car Idles at 1.4k Rpm and you call that running beautifully?

How does your car backfire when your car isn't even turned on?

 

How long after the ignition is turned on does the bang happen.
You are sure that it is in the exhaust pipe?

 

I'm in agreement that the idle seems a bit on the high side.

As per my Ultraguage, my idle after warmed up, will sit in the mid to high 700 rpm, and usually only vary 50 to 75 rpm.

And a bang with only the ignition on and no engine movement, eliminates quite a few things (engine wise that is).

It may be time to get another set of eyes and ears fixated on and under your car, and listening closely when you turn the ignition key...!!!

 

if it isn't running, it isn't backfiring. If there is a pop, after you turn the key on, then you have other problems. Maybe a fan sticking, something of that nature.

 

I have seen engines fire with just a turn of the key to the start position. You wuld have to a cylinder at the tiop of its compression compression stroke and a spark to occur with the turn of the key. Kind of hard to do with a electronic fuel injection but could be easier to do with the idle twice as high as it should be. I would first look for vacum lieaks. It would probably have to be larger leak like EGR hose into the elbow just before the throttle body. also check the brake booster hose. I bet if you get the idle under control the problem will go away.

 

Bob, he stated he just turned the key to the "on" position... can't think of anything that would ignite a backfire from the on position...as soon as he would turn past the on to ignition, I could see it...

 

I start my car wihout ever tuning past the "on" position but I do crank from a starter button. the coils and injectors are hot in this position. To make backfire happen you need fuel under compression and a spark plug to fire when he turns the key just to the start posistion. This is a really hard if not impossible thing to do with a with a EFI engine. the only way I can see getting the fuel in there is if a high idle was cuased by vacum leak and the car is thoughing in more fuel to compensate. turning the engine off from 1,400 rpm kills the spart but their may be some extra unburned fuel in the engine that could ignite if a spark plug fired at the turn of the key. I realize that this is a long shot and you would have to have a piston stop in just the right spot. but I have seen this happen before. So he could be hearing an actual backfire. I am confident that if he gets the idle right, he probably won't ever see this again.

 

If it's consistent I'm more inclined to think something fuel-system related - the fuel pump is located behind and down low where it could sound like exhaust to some drivers, and it *consistently* turns on when the key does.

While I'm sure backfiring might be possible by some coincidence when you shut the engine off and turn the key back to on, I highly doubt it'll happen consistently enough to cause this issue.

 

I am not saying it is his problem, but it can backfire by just turning on the key. I have seen it many times on propane fueled engines. A slow leak of fuel into the cylinders or intake and just at the right time you get a spark from the coil when you turn the key on. I can't see it happening everytime, but once in a while is very possible. As stated before you need to get your idle down to normal. Turning the car off at high idle may allow some gas vapor to sit in the cylinders, and then the next time the key is turned on, a spark is igniting the left over fuel. Maybe.

James

 

usually, it's when you switch the ignition off that you get a spark: when the field collapses or when you open the coil circuit. However, this probably only applies to pre electronic ignition systems.

 

I still have never heard of it but...I gave you the wrong info.