This is not your typical overboost story — and we think it is happening more often (or now that we are aware of it, we simply see it more).
Here is what we did to diagnose the issue.
Scan and record fault codes
PCM
- P0234 — Turbo charger over boost condition
ABS
- ECM and TCM invalid data codes
Test drive
Confirmed the fault code was real — boost pressure was going too high.
- Desired value: 17 psi
- Turbo hitting: 32 psi
Inspections
- Checked EGR and other accessories — numbers looked good
- Boost sensor relatively clean
- Turbo and turbo actuator rods / control checked — no dedicated turbo-actuator learn/reset on this setup; actuator is vacuum / solenoid type
- Vacuum operation checked OK
Reset fuel pump and injectors
Because there is no specific turbo-actuator learning reset, we cleared the learning values for the main diesel fuel pump and the pre-injection learning instead.
Then we carried out a drive cycle and confirmed learning was complete.
During that cycle, boost levels were good and no longer going wildly above desired boost pressure.
Second drive cycle
Checked turbo operation again on a second drive cycle.
- Desired value: 17 psi
- Turbo hitting: 17.5 psi
- Regulated just below a second later — no wild boost
The vehicle does not feel as punchy now — but it drives the way these vehicles should.
Book in
Similar P0234 / overboost behaviour on an Isuzu diesel? Call the workshop and we will advise what to expect for diagnosis time.