How can you distinguish high temperatures caused by poor combustion in an internal combustion engine?

This is one of the occasional causes of high coolant temperatures in diesel engines. Because it belongs to a higher-level fault category and occurs infrequently, it’s rarely included in general forum posts — most ordinary mechanics find it difficult to diagnose, and for operators, it’s even more challenging.

Such faults have distinct characteristics, generally (and I’m not including shortcuts or “cheating” methods here):

1. Under normal load, the engine produces dense black smoke, sometimes accompanied by abnormal noises. During operation, coolant temperature gradually rises to alarm levels, and engine power is noticeably reduced.

2. Cold-start issues are common — long cranking times, even in warm weather, and heavy white smoke during startup. (Differentiate this from low-cylinder-compression issues; using starting fluid may improve cranking, but crankcase blow-by remains low.)

3. If the problem is fuel timing related, detonation may occur, producing knocking sounds. (Operators familiar with gasoline engine knock may understand this, though it’s rare outside carbureted engines.)

4. Often, there is a concurrent increase in oil in the sump.

5. If cylinder isolation (“cutting a cylinder”) does not solve the problem, the issue usually lies in the fuel supply system components or fuel timing.

Key characteristics of this fault:

• Symptoms are more pronounced under high load.

• Severity varies, and quantitative data alone (like injection pressure or timing deviation) is insufficient to diagnose without careful inspection.

This is therefore a high-level troubleshooting skill, typically requiring experienced, senior mechanics. You must also know the relevant specifications for multiple engine models — otherwise, identifying the root cause is very difficult.

Diagnostic essence:

• Use instruments to confirm whether excessive heat is due to over-fueling/poor combustion, or simply poor cooling performance.

• This distinction is subtle and difficult to master — which is why it takes years for mechanics to become truly proficient.

A Classic Example

During my time in a state-owned enterprise, we had a KLD85 Kawasaki loader (imported Japanese equipment, only available in official state units) with a Nissan diesel engine. The loader recently showed heavy black smoke, high temperature, and low power, and everyone said it needed a full overhaul.

In that context, a full overhaul would have worked, because in state enterprises, overhauls are thorough — almost all components replaced according to accumulated operating hours, so the machine would have been fixed.

However, an experienced mechanic, guided by the team leader, inspected the injectors and found low opening pressure: of six injectors, four were worn and poorly atomizing fuel, opening at only 17–18 MPa. After replacing all six injectors, the engine returned to normal operation.

This is a classic example of skillful fault diagnosis — something a senior mechanic immediately understood by experience. That technician was one of the very first batch to pass post-Cultural Revolution certification — 5 out of 1,400 candidates. Asking him how he judged the fault, he simply smiled and did not tell me. At that time, I was just a junior mechanic and could not engage in such discussion — operators should understand that not everyone can have this level of access; qualifications matter.

Applying the Skill

Using this kind of diagnostic skill by analogy, even before I worked for a dealer, I resolved a similar issue on a CAT 988B loader:

• The machine had been in an external repair shop for three months and returned looking pristine.

• Upon test run, the problem persisted exactly the same, despite 12,000 USD (in 1998) spent.

• Applying careful injector and fuel timing checks resolved the issue, saving further unnecessary repairs.

This shows that combustion-related overheating requires skill, experience, and careful measurement — it cannot be solved by guesswork or trial-and-error alone.