Hello,
I have a 97 f-150 4.6 , with 185,000 and i love her. Problem is that cylinder 7 and 8 keep breaking plugs. plugs actually come apart ceramic shoots right out and wire melts. metal threads are still left in and tight. happens every couple days and its getting old. have most mechanics stumped. Any one here have any ideas.

 

Heard of plugs breaking during 100K change out but never in one or two days. I have the conventional push rod engine 4.2L. Have never worked on the 4.6 or 5.4 engines. Is the access pretty straight forward or is it tight and the plugs are damaged diring installation? Its deffinately not good or acceptable.

 

I had a similar problem with an old Mitsu Eclipse that I had. As it turned out, I had worn rod bearings on the #3 cylinder but it was only on the crank. The wrist pins and rings were still good causing only up and down movement in the piston but no lateral movement wich equaled no "piston slap" engine death noise.

Not to say that is your problem, but there are very few things that can cause that.

 

Good thought Lenn, I forgot these newer engine are zero clearance. Ouch, if you are right.

 

Yes access is okay. there doesnt seem to be any indication piston is hitting the plug only the ceramic comes loose. Also there has been plug nonfoulers in these 2 cylinders in the pst due to oil fouling the plug. Oil was coming from valve gasket from what we could tell. Would a cooler plug maybe help ? Seems like to much combustion or heat may be causing this

 

^ Couldn't hurt to try. It is one of the cheaper alternatives.

 

Mechanic did compression test cylinder 8 is at 180 he said others at 120

 

That compression pressure to too high....a real cause of detonation that could shatter the ceramic element of the plug. I'm surprised you haven't heard a lot of combustion racket from the engine! That sort of stuff has a bad habit of holing pistons and breaking rings....maybe you're just a little lucky, but not by much!

A possible cause is carbon build-up in the combustion chamber. The stuff might be cleared out over time with the use of a good fuel system cleaner coupled with the use of 93 octane gasoline...if you can get it.

Installing a water injection system is another way, but the scheme is rather involved; it's probably better to (dare I say it?) pull the heads to clean the crap out.