ROUGH IDLE/POOR PERFORMANCE SPARK PLUG INFORMATION

MODELS AFFECTED: 1984 CADILLACS WITH HT4100 ENGINE 1985 FWD DEVILLE/FLEETWOOD WITH HT4100 ENGINE

Idle roughness, an unstable idle speed, possibly accompanied by a Code 30 or E30, and/or poor engine performance, particularly during acceleration and at the time of transmission shifts may be caused by spark plugs damaged by carbon tracking. Carbon tracking may occur when vehicles are driven extremely short distances in cold weather, such as during new car shipping. Fuel which is incompletely burned under these conditions may produce carbon tracking on the tip insulator, causing the spark to bridge across to the spark plug shell rather than across the electrode tip. Although the carbon tracking may not be readily observable, carbon tracked plugs are permanently damaged and should be replaced rather than cleaned.

Effective at the following breakpoints, R42CLTS-6 spark plugs were discontinued and replaced by R44LTS-6 spark plugs.

"C" 202700 "D" 098438 "E" 639017 "K" 820498

A limited number of after breakpoint vehicles may be equipped with R43LTS-6 spark plugs. While these provide less cold fouling resistance than the R44LTS-6 spark plug's, these vehicles will not be as susceptible to carbon tracking as R42CLTS-6 equipped vehicles.

Should a pre-breakpoint vehicle equipped with R42CLTS-6 spark plugs experience an idle roughness or instability or an unexplained engine performance condition or an unexplained Code 30 or E30, replace all eight spark plugs with R44LTS-6 spark plugs. R44LTS-6 spark plugs are the recommended replacement parts for an 1982-1985 HT4100 equipped vehicles. Do not adjust ISC/TPS on pre-breakpoint cars for an idle roughness without first replacing the R42CLTS-6 spark plugs.

R44LTS-6 spark plugs will not cause detonation or increase the susceptibility of detonation in HT4100 engines. Spark plugs with a hotter than recommended heat range should never be used, however, as the resulting preignition may cause immediate engine damage. Preignition is ignition of the air-fuel mixture before the occurrence of the spark at the spark plug gap. Detonation, on the other hand, is a secondary explosion that occurs after the spark at the spark plug gap. The knocking sound from detonation is usually regular in nature and is most likely to occur when the engine is under a load, such as when accelerating or climbing a hill. An occasional light detonation heard under these conditions is not a cause of concern and will not cause engine damage.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.