Exhaust - Noise Diagnosis

NUMBER: 09-23-86

DATE: 10-09-86

APPLICABILITY:
ALL 1985 AND 1986 VEHICLES EXCEPT HATCHBACK AND BRAT MODELS

SUBJECT:
EXHAUST SYSTEM NOISE DIAGNOSIS

THE PURPOSE OF THIS BULLETIN IS TO ASSIST THE TECHNICIAN IN DETERMINING IF AN ABNORMAL NOISE EXISTS IN THE EXHAUST SYSTEM AND IF SO, HOW TO ISOLATE THE CAUSE.

All motor vehicles have certain noises that are characteristic of normal vehicle operation.

SUBARU NORMAL OPERATING CHARACTERISTICS

The rear air suction valve, found on 49-State Federal carbureted vehicles except 4WD, will produce a buzzing noise on deceleration above 3,000 RPM and during initial engine warm-up. The cause of the noise is the natural pulsation of the reeds located inside the valve assembly. This resonance can be transmitted to the rear catalyst cover but does not affect the normal operation of the vehicle.

DIAGNOSING ABNORMAL EXHAUST NOISES

A. Front or "Y" Pipe (EPF) Non-Turbo:

Rattling noise in the EPF cover shielding is most likely to occur during acceleration or deceleration from approximately 1,000 to 3,000 RPM at low to moderate vehicle speed. This noise is also apparent when the pipe is tapped with a rubber mallet at a point on the pipe before the EPF shield. This noise has a tendency to travel through the exhaust system, so careful investigation of the cause is important. If this noise is verified, use Service Bulletin number 09-21-86 released on 02-04-86 to repair the front catalyst shield.

B. Center Exhaust Pipe (EPC) Without ASV:

Noise in this pipe appears as a continuous resonance from the center of the pipe to the bend, and from an engine speed of approximately 1,500 RPM to 4,000 RPM after the engine is warmed-up. Repair the noise by replacing the center exhaust pipe assembly.

NOTE:
MODIFICATIONS HAVE BEEN MADE TO CENTER EXHAUST PIPES (EPC) To ELIMINATE THIS RESONANCE ON UNITS PRODUCED AFTER LOT NUMBER 5622. THE LOT NUMBER IS LOCATED ON THE REAR HANGER.

C. Muffler:

Rattling noise from the muffler is most likely to occur during engine warm-up with the engine speed between 1,000 to 2,000 RPM. This noise is audible only from the rear of the exhaust system. After verifying the noise, the muffler can be repaired by using Service Bulletin number 09-22-86 released on 05-21-86.