Cherokee 2WD L4-150 2.5L VIN P MFI (1991)
Connecting Rod: Technical Service Bulletins
Engine - Piston Knock At Idle
NO: 09-08-94
GROUP: Engine
DATE: Apr. 22, 1994
SUBJECT:
Piston Knock at Idle
MODELS:
1991 - 1992
(MJ)
Comanche
1991 - 1994
(XJ)
Cherokee/Wagoneer
1991 - 1994
(YJ)
Wrangler
1993 - 1994
(ZJ)
Grand Cherokee/Grand Wagoneer
NOTE:
THIS BULLETIN APPLIES TO VEHICLES EQUIPPED WITH A 2.5L OR 4.0L ENGINE.
SYMPTOM/CONDITION:
The engine may produce an objectionable piston knock sound at idle (either cold or at operating temperature).
DISCUSSION:
The piston knock at idle condition may be due to one or more of the following:
A.
Incorrectly classified cylinder bore(s).
B.
Negative piston skirt taper which may cause wear at the top of the piston skirt near the oil control ring.
C.
Flaking of the piston skirt coating on a small number of pistons.
ITEMS OF SPECIAL INTEREST:
1.
The new style piston-connecting rod assembly can be used to service engines having pistons with the old style tin-plated skirts.
2.
The old style tin plated piston cannot be used as a replacement for the new piston-connecting rod assembly.
NOTE:
WHEN PERFORMING THIS BULLETIN, USE ONLY THE NEW STYLE PISTON-CONNECTING ROD ASSEMBLY AS THE SERVICE
REPLACEMENT PART.
3.
If it is determined that a piston requires replacement due to piston knock at idle, all four/six pistons must be replaced with the new piston-
connecting rod assembly.
4.
The coating on the new style piston-connecting rod assembly is black in color. The coating material is applied to the piston skirt after the final
piston machining process. The coating may vary in thickness. Therefore, the Service Manual procedure for measuring the outside diameter of the
piston to determine size or piston-to-bore clearance is not recommended.