Emssions - Excessive Spark Knock at Part Throttle

87chevy09
GMC NUMBER: 86-T-139

SECTION: 6E - Engine Emission

DATE: February, 1987

SUBJECT: INOPERATIVE EGR VALVES

MODELS: 1986 S/T TRUCKS AND M VANS WITH 2.5L L4 ENGINES (VIN CODE E)

Figure No. 1 - EGR VALVE ASSEMBLY:

Some 1986 S/T trucks and M vans with a 2.5L L4 engine may exhibit excessive spark knock, at part throttle, due to an inoperative EGR valve. If during diagnosis for the above condition an EGR valve has been diagnosed as inoperative because it does not respond to a normal vacuum signal, remove the vacuum hose and check the orifice in the vacuum fitting for a plugged condition

(see Figure No. 1). Orifice inside diameter is .040" to .050". The orifice may have excessive, hardened hose assembly lubricant blocking it. If this condition exists, clear the orifice with a small wire, or pin, return the vacuum hose to the fitting and test the EGR valve for function. If the EGR is still inoperative, replace it.

Use applicable labor operation and labor time.