Mitsubishi Galant: 4G93 GDI Injector Coking & Cold-Start Oil Dilution

The 4G93 GDI 1.8L engine injects fuel directly into the combustion chamber at high pressure rather than upstream of the intake valve. During cold-start enrichment, excess fuel washes past the piston rings and dissolves into sump oil, causing measurable viscosity breakdown that accelerates bearing wear before the oil is changed. Crucially, because intake valves are bypassed by the injected fuel stream — unlike port injection — carbon deposits accumulate progressively on valve stems and seat faces, restricting airflow and causing asymmetric cylinder charging. High-pressure injector nozzle tips are also susceptible to coking that distorts the spray pattern geometry, generating uneven fuel distribution and misfire codes. These are inherent first-generation GDI design limitations compounded by short oil change intervals being observed on Japanese domestic market cars.

• Rough idle on cold start that improves as engine reaches operating temperature
• Black or white smoke on cold start, particularly in winter
• Oil smells of petrol or dipstick oil level rises between changes
• Misfires or hesitation under light throttle load
• Poor fuel economy relative to the 1.8L displacement

1. 1

   Check engine oil level and sniff for petrol contamination — a distinct petrol odour indicates oil dilution.

2. 2

   Request an endoscopic camera inspection of intake valve carbon deposits via the spark plug holes.

3. 3

   Check scan-tool short-term and long-term fuel trims for per-cylinder imbalance indicating injector spray pattern issues.

4. 4

   A chemical walnut blasting service of intake valves is the definitive carbon cleaning method.

Parts involved: Fuel injector.

$400 to $1,800 at New Zealand independent workshop rates. That is for this job on its own, not for the car.

An estimate, in NZD, for the repair as normally carried out. What a specific car costs depends on which part has failed and what it damaged on the way — treat this as the floor for a negotiation, not a quote.

• “How often is the engine oil changed and at what kilometre interval?”
• “Has the car had any injector cleaning or intake valve carbon cleaning performed?”
• “Does it smoke on cold start or run roughly until fully warm?”

The failure described above is documented in published sources — owner forums, technical service bulletins, recall notices and workshop write-ups — held in our fault database. The inspection instructions and the cost estimate are KickTyres’ own interpretation of them, and it is a starting point for an inspection rather than a manufacturer statement.