Crankshaft pulley bolt works loose or breaks (V6)

The single bolt through the middle of the crankshaft pulley backs out, then snaps. It is among the most-repeated complaints owners of the V6 first-generation Montero filed with the US federal safety regulator, and a repair database that names the engines affected puts the 3.0 V6 at the head of the list, and the reason it is filed here as a safety problem rather than an engine problem is what happens next: the pulley and every belt it drives leave the engine at once, so the power steering pump, the alternator and the water pump all stop together, without warning, at whatever speed the truck happens to be doing. Owners in the complaint records describe exactly that sequence, one of them ending in a loss of control. The repair is cheap while it is only a loose bolt and expensive once the crankshaft nose is chewed. Treat a knock from the front of the engine on this truck as this problem until you have proved it is not. This is safety-critical work: have it done by a qualified mechanic.

Gen 11990–1991USSafety-criticalCommunity consensus

Do not driveNo lo maneje

Safety notice — Engine.

This affects a safety-critical system. Reference material only: for safety-critical work, consult a qualified mechanic.

Esto afecta un sistema crítico para la seguridad. Material de referencia únicamente: en trabajos críticos para la seguridad, consulte a un mecánico calificado.

Community consensus. This entry is not backed by factory documentation — treat its values and steps as a starting point, not as if they came from the factory manual.

Consenso de la comunidad. Esta entrada no se apoya en documentación de fábrica — tome los valores y los pasos como punto de partida, no como si vinieran del manual de fábrica.

Symptoms

A knock or heavy rattle from the very front of the engine, loudest at low speed
The crankshaft pulley visibly wobbles or rocks as the engine idles
The steering suddenly goes heavy while driving
The charge warning light comes on and the engine starts to heat up at the same time

Likely causes

The pulley centre bolt has worked loose and the pulley is no longer clamped
The pulley centre bolt has snapped, usually leaving its shank in the crankshaft
The crankshaft nose thread or keyway is damaged, so the bolt cannot hold clamp load

Diagnostic steps

With the engine stopped, look at the bolt head in the middle of the pulley: is it still there, and is it pulled down flush?Points toThe pulley centre bolt has worked loose and the pulley is no longer clampedRules outThe pulley centre bolt has snapped, usually leaving its shank in the crankshaft
Start the engine and watch the pulley face from the side as it idles, looking for the pulley moving in and out rather than just turningPoints toThe pulley centre bolt has worked loose and the pulley is no longer clamped · The crankshaft nose thread or keyway is damaged, so the bolt cannot hold clamp load
Look under the front of the engine and on the skid plate for the pulley itself or for shed beltsPoints toThe pulley centre bolt has snapped, usually leaving its shank in the crankshaft
Take the pulley off and inspect the crankshaft nose thread and keyway for damagePoints toThe crankshaft nose thread or keyway is damaged, so the bolt cannot hold clamp loadRules outThe pulley centre bolt has worked loose and the pulley is no longer clamped

Fix paths

New bolt, clean threads, torqued to the factory figure

$–$$Difficulty 3/5

Caught while the bolt is only loose, this is a new bolt fitted to clean, undamaged threads and tightened to the figure the factory manual gives for this engine. Do not guess that figure and do not reuse a stretched bolt — this is the joint that just failed.

Extract the broken bolt or replace the crankshaft

$$$–$$$$Difficulty 5/5

Once the bolt has snapped in the nose or the thread and keyway are damaged, the job is drilling out the remains and repairing the thread, or replacing the crankshaft. Owners in the complaint records describe struggling to find anyone willing to do the extraction.