The throttle position sensor (TPS) may fail causing intermittent drivability problems like hard starting, stalling, and/or hesitation when accelerating. A failed TPS will require replacement, proper diagnoses should always be performed before replacing any parts.
Problems for specific Jeep Wrangler years:
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Most reported 1991 Jeep Wrangler problems
A ticking type noise may be noted from the engine area due to an exhaust manifold that has cracked where the pipes are welded; cracked manifolds should be replaced.
Due to warped front brake rotors or variations in the thickness of the rotors, brake pulsations may develop. The best method to reduce pulsations is to use an "On the Car" brake lathe (as long as the rotors are suitable for turning). "On the Car" equipment can correct variations (or "runout") in the rotor surfaces because it also corrects variations in other components (e.g. the hub).
Some models are susceptible to rear main oil seal failures in dirty or dusty conditions. As dirt and dust collect on the oil seal, the abrasion wears through the sealing surface. A spacer is used between the bell housing and engine that has a small slot in the 12 o'clock position. Filling this slot with RTV silicone and cleaning the seal area can help prevent damage to the seal.
The throttle body may accumulate an oily, black residue in the bore and on the throttle plate causing hard starting or possibly stalling when coming to a stop. The throttle body should be cleaned during regular maintenance but do not spray cleaners directly on or at the throttle position sensor (TPS) as this can damage the sensor.
The outer exhaust manifold studs can break and cause exhaust leaks, commonly noted as a ticking type noise. The outer studs, clamps, and nuts will need to be replaced and are now being tightened to a reduced torque value (20 Nm/180 in lbs).
The tailgate hinges can corrode making it difficult to open the tailgate.
If the clutch is slipping when the clutch disc is 2/3 worn or if the clutch slave cylinder dust boot is torn, there may be insufficient clearance between the clutch slave cylinder and the clutch housing. A 5mm shim is available from Jeep that is installed between the slave cylinder and the clutch housing.