mark98ls40

Just got my 1st new black 98 LS400 & wonder why I waited so long for one. After Regal GS's & Cadillacs that just don't compare.

Anyway, I got the Colorado emissions test & just barely passed on the NOx part that says high temps or pressures in the combustion chamber. Don't know if plugs have been changed. 130k miles. Original body panels, paint & drives like new.

Would new sparkplugs bring the temp down in the cylinders? Or is there deposits in the cylinders that increase pressure?

Thanks for any suggestions?

LiCelsior

iTrader: (32)

What causes high NOx (nitric oxide)? High NOx is caused by high combustion temperatures and pressures. Problems that can cause high NOx include:

· An inoperative EGR system. The EGR system was designed specifically to reduce NOx; that's it's only function. The EGR, which stands for Exhaust Gas Recirculation, allows exhaust gas to enter the combustion chamber through the intake. The exhaust gas has a cooling effect on the combustion chamber.

· Over advanced ignition timing. The more advanced the ignition timing, the higher the combustion chamber temperatures.

· Overheating. If the engine temperature is too high the NOx emissions will go up.

· Lean air/fuel mixture. If the air/fuel mixture is too lean the combustion temperature will go up.

· Compression over specification. Normally when we think of engines aging we think of the compression going down. However, there are a couple of ways the compression can increase as the engine is used. One is carbon buildup on the tops of the pistons. When carbon forms on the tops of the pistons, the combustion chamber area decreases and the compression ratio increases. The carbon can be manually scraped off the tops of the piston to correct this problem, but that would require disassembling the engine.

· Mysterious reasons. Sometimes NOx is too high and there's no obvious cause: the EGR works, the timing and advance work normally, the engine temperature is within range, the air fuel mixture is good, and the compression is within spec.. When this happens you can try a little tweaking. If there are two thermostats listed for the car, use the cooler one. If there are two spark plugs listed for the car, use the cooler one. Retard the timing 2 degrees (the maximum allowed). Clear out the EGR passage even if the passage seems fine. Cross your fingers.

mark98ls40

Thanks LiCelsior for the very informative reply. I'll try the sparkplugs & possibly an additive to clean the combustion chambers. It appears (by the display) that I'm getting 21mpg around town so I may not change to a colder thermostat. Would a K&N drop in airfilter help?