Ignition - Poor Driveability, Carbon Tracked Spark Plugs
Bulletin No. T 85-12File In Group 6C
Number 88
Date Sept. 84
SUBJECT. IDLE ROUGHNESS OR POOR PERFORMANCE CAUSED BY CARBON
TRACKED SPARK PLUGS
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 plugs, 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 all 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. Pre-ignition 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.