NOTE: I've learned over tightening has nothing to do with the condition of the plugs... see motorcarman's post ( #9) for the real cause!

This is why spark plugs should not be over torqued...

I've had my Jag for a few months now and it began running a bit rough; check engine light came on and the scan tool gave me a P0420 & P0430.

I tried a few things to clear the codes, but new plugs did it. Check out the pics...you can see streaks on the ceramic that apparently are from exhaust blow by due to the ceramic body seperating slightly from the metal base...over-torquing!

 

AAAGGHHhhhh, that's just frightening! I've never seen that before...and on every single plug too. Popeye did your previous service.

 

The far right plug, looks like the cylinder is burning some oil. The other plugs look fine.

 

In all my years (and I spent a few changing sparkplugs before my diesel days) I've never seen sparkplugs end up like that. And that's overtightening? I was always a bit careful as the head was usually ally, and you had to compress that copper washer just enough. And I never put anything like grease on the threads. You live and learn!
Leedsman.

 

Someone will most likely say I am off the planet, but I have never seen over tightening to cause this.

Badly installing, and cracking the insulators, yes.

Even if you over tighten, providing that you don't skew the socket you will not touch or crack the ceramic.

 

I've never heard of it either, to be honest. I've always thought the discoloration was combustion chamber by-products leaking past the seal between the metallic section and the ceramic.

<shrug>

I'm willing to be re-educated, though, on how over-tightening can cause this.

Cheers
DD

 

Really? I routinely use anti-seize on spark plug threads if the engine has aluminum heads.

Cheers
DD

 

What planet are you on??

Actually that's exactly what I thought, I'm not sure why overtightening would cause this problem.

 

Old TSB has the explaination for this. It is a common fault and we were instructed NOT TO REPLACE for this condition!!!!! We used to replace the plugs but we got backflagged for replacement after the TSB was issued!!!!!!

bob gauff

 

I've learned something new for the day with time to spare: I still have 4 hours left before bedtime! Sometimes it gets a little dicey and I have to wait till the very last minute :-)

Cheers
DD

 

Corona stain, color me educated. Thanks professor.

Seems the misfire and rough engine was not due to this 'cause' but more likely that plug that Rick pointed out. Check that valve cover.

 

Wow, thanks for the info Bob, always great to learn something new. Never even heard of Corona Stain before!

 

My bad... Thanks for the info from all.
Wish I could change the title of the thread! I read somewhere about over tightening causing the ceramic / base seperation. Thought that was the cause.

Thanks for setting me straight. Great info motorcarman! I will remember that for the future.

Cam cover replacement is under consideration. I have the spark plug seals & cover gasket but notice what I've seen referred to as the 'mumps' on the cover and in most plug wells. Since the oil seepage is minimal, I'll be looking to get the 'mumps' resolved prior to replaceing the gaskets.

 

Bob thanks for the heads up on the corona effect!

Although I like my corona, icey cold with lime!

 

Speaking of spark plugs, I learned something new to me on Motorweek TV today.

When changing the spark plugs on a car with aluminum heads:

1. You should first loosen all of the plugs 1/8 of a turn.
2. Re-attach the coils and start the engine and give it a few bursts of the accelerator and shut it down again. This will break away the carbon build-up around the spark plugs threads.
3. Loosen the plugs. If there is a lot of resistance, tighten/loosen back and fourth until you remove the plugs. This should allow the threads on the aluminum heads to stay intact.
4. When replacing the plugs, put a dab of anti-seize on two sides of the plug at the base of the threads. Re-insert the plugs by hand with the wrench extension or a length of fuel line hose. Do not re-insert the plugs with the socket wrench handle attached as you could easily strip the threads on the heads. Use no more than 5ftlbs of torque to reset them.

 

Mailshack
We changed hundreds of 'sparking plugs' because they looked faulty. We did not know if it was a torque problem or some defect with the RC12YC Champion OEM plugs. I always thought it was a sealing fault in the ceramic to base area, but after getting the TSB we quit replacing them under warranty. (especially after getting labor and parts docked from the paycheck!!!!)

bob gauff