4.6 & spark knock
 

My Granddad has a 1999 Mercury Grand Marquis with the 4.6. He's asked me to scan his car with my scanner and look at it to see if I can figure out what's wrong with it. He says it has spark knock (pinging) which to me means either poor fuel quality, dirty MAF, or carbon buildup. What other things can cause pinging? I would like to find out as much about the problem before I look at the car so I know what to look for. I'm thinking it may be carbon buildup because as a Grandparents' car it NEVER gets driven hard thus the low RPM's. What do you think? Any help is appreciated!

4.6 & spark knock
 

Sounds about right. Could be a faulty knock sensor or an egr problem. Those would probably throw a code though.
Jimmy

4.6 & spark knock
 

Thanks for the responce. The engine light is not on so no hard codes have been thrown. I found out about carbon buildup causing pinging by my searches on the board, but I don't remember anyone explaining WHY carbon buildup causes pinging? If someone could explain that I'd really appreciate it! (These things really interest me!)

4.6 & spark knock
 

The carbon in the combustion chamber will get hot spots on it. Eventially, they get hot enough to actually ignite the air/fuel mixture before the spark plug does. What you hear is the flame front from the carbon fired mixture and the spark plug mixture colliding(spelling?) in the cylinder. This is why pinging is called preignition. Removing the carbon, removes the hot spot and the air/fuel mixture is only fired by the plug and no more pinging.
Higher octane fuel helps keep this from happening because the higher the octane, the harder it is to ignite the fuel.
Jimmy

4.6 & spark knock
 

Thanks for the explanation!

4.6 & spark knock
 

What's the best way to clear the carbon in the cyclinders? My wife has a 4.0L 92 explorer that pings like a fiend... I replaced the plugs and wires which helped for a while, but it came back last night on a long drive to and from JFK in NY.

I would like to get this fixed, but I also would like to avoid taring down the entire engine.

Thanks

4.6 & spark knock
 

One other thing carbon deposits does is raise the effective compression ratio. This coupled with the hot-spot problem gives you a two-for-one deal. Not a good deal, huh?
I'd suggest one of the high-end (more costly) fuel system cleaners. Chevron makes a good one, and I believe STP Super Concentrated is good. Other folks might have other suggestions.