: Splitting spark plugs

11-07-04, 11:29 PM
My father's '97 DeVille began doing a strange thing about a month ago. It consistently cracks the ceramic insulator on the number seven spark plug and only that one. After putting a new A/C Delco platinum plug in, the car will run fine on the highway but when driven in town or anywhere it will have to pull hard it will begin to skip, badly. An inspection of the plugs reveals carbon outlining a crack in the insulator on the spark plug in number seven. Replace it, everything is fine for about 10 to 100 miles, then a repeat performance. It has done this now to six plugs. He replaced that coil and all wires along with all of the plugs to no avail.
I've ran alot of different GM engines and I've cracked the insulator on a few plugs installing them, but I've never seen an engine crack them consistently and so quickly on it's own.

11-08-04, 12:42 PM
This is truely an oddity....!!!!!

The crack is in the porcelean inside the combustion chamber..???...or outside the chamber in the spark plug well??

Did you put antiseize on the spark plug threads when they were installed? If so, the installation torque may be stretching the plug shell excessively and causing the crack as the plug thermocycles...

Is the porcelean just cracked and no other damage to the plug such as a bend ground electrode??

Are you sure that it is a crack?? I have seen plugs that misfired repeatedly due to a sticking injector or bad plug wire where the porcelean was coated with carbon or fuel and the spark arced down the porcelean to ground. That would etch a slight groove in the porcelean that would then gather carbon and look exactly like a crack and cause a misfire as the groove would gather enough carbon and retain it even when the plug tip was up to temp and cleaned up otherwise that the groove still looked dark like a crack and would cause a misfire.

Look down the spark plug well with the plug out and see if there is any damage or discontinuity in the spark plug seat or something??? Could a plug have been crossthreaded at some point so that the plug is seating crooked and causing a non-uniform stress on the plug shell which then stresses the porcelean and cracks it.

Something had to happen or change with the engine because it is almost unheard of to crack a plug like you describe with the engine in operation and the Northstar does not have a history of that sort of thing. Plus, the car obviously ran fine for years without doing this and then started cracking plugs...???

About the only engine condition that could crack the porcelean inside the chamber is continuous heavy detonation. You would have to hear that level of detonation plus the Northstar knock control is very effective so it is unlikely that it could be detonating that heavy for that long but....??? Could there be excessive carbon buildup in the cylinders leading to detonation??

11-08-04, 02:57 PM
An AC aircraft sparkplug Rep., Snowy Coates, asked me that if I ever saw a lengthwise cracked insulator tip to give him a call instantly because it indicated a serious problem with the manufacturing process.

11-08-04, 09:53 PM
Could there be excessive carbon buildup

Sounds like it to me. Maybe time for some top engine cleaner in that cylinder? Can't have you going through $1200 worth of spark plugs a year ;) btw - if you did the antiseize, you're supposed to reduce the torque to 60-67% what the book says.