Spark plug diagnosis chart

[Frank K.]

What causes compression leakage ( Spark plug has burnt looking band close to were plug wire attaches)

[Michael W.]

Re: Spark plug diagnosis chart

Do you mean this:

http://www.ngkntk.co.uk/index.php/technical-centre/spark-plugs/corono-stain/

[Frank K.]

Re: Spark plug diagnosis chart

Exactly! The rear plug (No7) Thats were the intake manifold has a problem on vacumn leaking on the china wall. Common problem on Late C4. The oil in the air attaches to the plugs porcelan.

[Terry M.]

Re: Spark plug diagnosis chart

Exactly! The rear plug (No7) Thats were the intake manifold has a problem on vacumn leaking on the china wall. Common problem on Late C4. The oil in the air attaches to the plugs porcelan.

On LT1/LT4 engines the EGR tube, that comes from the passenger side exhaust manifold to the EGR valve at the back of the intake manifold, heats the RTV on the rear china wall and deteriorates the RTV. The heat/cooling cycles also loosen the right rear intake manifold bolt. The combination of the deteriorated RTV and loose intake bolt results in oil/vacuum leaks at the #8 cylinder. The solution is re-gasket the intake and properly torque the intake fasteners.

[Keith B.]

Re: Spark plug diagnosis chart

On LT1/LT4 engines the EGR tube, that comes from the passenger side exhaust manifold to the EGR valve at the back of the intake manifold, heats the RTV on the rear china wall and deteriorates the RTV. The heat/cooling cycles also loosen the right rear intake manifold bolt. The combination of the deteriorated RTV and loose intake bolt results in oil/vacuum leaks at the #8 cylinder. The solution is re-gasket the intake and properly torque the intake fasteners.

this seams like a problem that never can be fixed. a job that will be required forever.

[Terry M.]

Re: Spark plug diagnosis chart

this seams like a problem that never can be fixed. a job that will be required forever.

Just one of the many tasks that are routine maintenance for driven vehicles with GEN II LT1. I manage to do it, along with Optical distributor, spark plugs and wires and EGR about every 100K miles. Water pumps, serpentine belt & tensioner and oil cooler hoses more often, on an as needed basis.
As someone once said: "If you are going to golf, you have to buy golf balls."
I doubt there are too many C4s that hit those kinds of mileage numbers, but there are probably a few.