The Mitsubishi S6S is one of the most trusted industrial diesel engines in the forklift and heavy equipment world. Built as a 6-cylinder, naturally aspirated (and turbocharged in its S6S-T variant) diesel, it's known for its durability, straightforward serviceability, and long service life when properly maintained. You'll find it under the hood of Mitsubishi, Caterpillar, and several cross-branded forklifts, since Mitsubishi has long supplied engines to multiple lift truck manufacturers under shared platforms.
Where the S6S Is Used
The S6S shows up across a wide range of mid-to-heavy-duty counterbalance forklifts, typically in the 4,000-8,000 lb capacity class and above. Because Mitsubishi and Caterpillar have historically shared engine platforms, you'll commonly see S6S-powered trucks badged under either brand, along with the turbocharged S6S-T variant used in higher-capacity applications.
Common S6S Issues to Watch For
- Head gasket failure — often shows up as coolant loss, white exhaust smoke, or oil/coolant mixing, especially on higher-hour units
- Injector wear — causes rough idle, hard starting, or excess smoke under load
- Turbo failure on S6S-T units — listen for whining or whistling noises, and watch for a drop in power or excess oil consumption
- Worn valve seals and guides — blue smoke on startup after sitting is a classic sign
- Crankshaft and rod bearing wear — usually the result of delayed oil changes or running low oil pressure for extended periods
Repair or Replace?
Because the S6S is such a heavily used platform, most shops have a clear line: if a rebuild is going to mean pulling the head, boring the block, or replacing the crankshaft, it's often more cost-effective and far less downtime to drop in a complete engine rather than piecing together a full rebuild. This is especially true on higher-hour trucks where multiple systems are wearing out at once, since a complete engine resets the clock across the board instead of just addressing one failure point.
We stock a complete new Mitsubishi S6S engine, in stock and ready to ship same day if ordered by 3PM. View the Complete Mitsubishi S6S Engine here.