The 4M40 engine was installed in several models of CAT excavators as well as a few small models under the Sumitomo brand. This engine is now discontinued, but it was originally designed by Mitsubishi for the Pajero (Leopard) SUV. It was intended to be a high-speed engine, and not only CAT but also Sumitomo used it in small machines. However, since Sumitomo’s machines had a smaller market share, the issue was not widely noticed.

Under operating conditions in China, most operators prefer to run at low throttle, avoiding high engine speeds whenever possible. Unfortunately, that’s exactly what this engine is not suited for. Even Mercedes-Benz passenger car engines use timing chains, which also require replacement over time — so it’s not a chain quality issue. The reason chains are used is that they generate 3–5 dB less noise compared with timing gears.
I once repaired French Berliet diesel trucks, whose 5- and 6-cylinder engines were all chain-driven. But many operators simply lack the technical background to understand these differences.

This diesel engine inherently produces more vibration, and under low-throttle conditions, the chain lubrication pressure is low, which shortens its lifespan. After only about 2,000 working hours, inspection — and possibly replacement — becomes necessary.
To mitigate this issue, the Suzhou-produced models (as opposed to imported ones) were de-rated in power output, improving the chain’s lifespan somewhat. However, a far more critical flaw remained unresolved: the engines used domestically were not equipped with balance shafts.

This is a problem that virtually no operator has noticed. The omission was purely a cost-cutting measure, but it significantly reduced the engine’s durability. Since this was an obsolete pre-combustion-chamber type engine, although it had low noise and smooth combustion, its specific fuel consumption was extremely high — about 223 g/hp·h — which is why it was ultimately discontinued.

Strictly speaking, the imported C Series models (such as 305.5, 307C, and 308C) used engines with dual balance shafts, identifiable by their double-row timing chain, giving them a much longer service life compared to the D Series without balance shafts. Almost no operators have noticed this crucial difference — which reflects the general knowledge level in the field.

I became aware of this issue because of my broader exposure — I participated in a Mitsubishi technical investigation in China on the problem of short timing chain life. I raised this exact concern during the meeting, but the experts only smiled and said nothing. Later, a colleague told me privately that I had said too much — since it was all about cost control.
I even posted related discussions on professional forums, offering a free technical guide to help operators solve these problems completely.

As for cold starting difficulties in winter, that is not a design flaw. The real issue lies in poor maintenance habits. Many operators focus solely on operation and neglect regular maintenance, which inevitably leads to such problems. I have personally investigated cases of hard starting and found that they all stemmed from the same root cause.