Mitsubishi 4A9-series (4A91 / 4A92)
Inline · naturally aspirated · petrol · 2004–present
Also known as: 4a91, 4a92, 4a90, 4a9, mitsubishi 4a9, 3a91, 3a92, 3a9, 3a90
Mitsubishi's small petrol four — in the Colt, Lancer, ASX and Xpander.
It replaced the long-running 4G1 family and is a lighter, all-aluminium design with a timing chain rather than a belt — which removes the single most expensive scheduled item those older engines had.
Its recurring complaints are ordinary and cheap. A rough idle that clears as the engine warms is very often the idle air control or a vacuum leak rather than anything internal; the ignition coils fail one at a time and give a misfire under load first; and the timing chain tensioner rattles briefly on a cold start at high mileage.
The variable valve timing depends on oil pressure and cleanliness, so a flat top end or a rough idle on a neglected engine is an oil question before it is a mechanical one — the same rule as on Honda's i-VTEC and Toyota's lift-cam engines, and for the same reason.
It appears in cars sold under other badges in several markets, which is worth knowing when the model name finds nothing: the engine code is what to search.
Cars fitted with this engine
5 generations, across 1 make. From the authored catalogue — an absent car means the catalogue has not reached it, not that the engine was never fitted.
Information, not instruction
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- What actually fixed it — or that nothing did, which is worth knowing too.
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