Reading datalogs: what to log and what to look for
Which channels to record, how to get a log you can trust, and what the common problems look like in the data.
What a datalog is
A datalog records the ECU's live channels many times a second while the engine runs. It is how you see what the engine actually did, as opposed to what the tune asked for. TunerStudio records MLG files, and MegaLogViewer is the usual tool for reading them.
Channels to record
| Area | Channels |
|---|---|
| Engine state | RPM, MAP, TPS, coolant and air temperature, battery voltage |
| Fuel | AFR or lambda, AFR target, closed-loop correction, VE, pulse width, injector duty |
| Ignition | Advance, knock count or level |
| Boost | Boost target, boost duty |
| Health | Sync loss count, engine status flags, oil pressure if fitted |
Channel names come from the firmware's INI, so they differ between Speeduino, rusEFI and MegaSquirt.
Getting a log you can trust
- Start with a warm engine unless you are tuning cold start.
- Change one thing at a time and log before and after.
- Repeat the same conditions: the same road, gear and throttle.
- Hold steady states for a few seconds. Cells you pass through in a blink don't give usable data.
- Note what happened and when, such as a stumble at a roundabout. A marker in the log saves searching later.
- Keep the tune that made the log. A log read against a different tune leads to the wrong correction.
What to look for
- Lean under load: AFR above target at high MAP or in boost. Stop and fix it before anything else.
- Steady closed-loop correction: a correction that stays at +5 % in an area means the VE table is short there.
- Knock: any knock count at full load or in boost. Note RPM, MAP, advance and air temperature where it happened.
- Injector duty near your ceiling at peak power.
- Boost overshoot, or MAP that never reaches the boost target.
- RPM spikes or drops, or a rising sync loss count: usually a trigger, wiring or noise problem.
- Battery voltage dips, which change injector dead time and coil dwell.
- Air temperature rising pull after pull, which raises knock risk.
Remember that the wideband reads a few hundred milliseconds behind the engine, and that AFR means nothing during deceleration fuel cut.
Using efi2000 for datalogs
Recording
Turn on continuous logging and efi2000 records every engine run as its own standard MLG file while the ECU is connected. You don't have to remember to press record.
Each log remembers which tune it ran on and marks every ECU write and burn, so you always know what changed between two pulls. Logs still open in MegaLogViewer and TunerStudio.
Reading
- Log viewer: play back a log, overlay channels and jump between markers. Import MLG logs recorded elsewhere too.
- Target tracking: see how closely AFR, boost or another channel followed the target logged with it, overall and at idle, cruise, full load and boost.
- Log coverage: overlay a log on a VE, spark or AFR table to see which cells you have data for and where the mixture was off.
- Recorded-log autotune: tune VE from a log you already have, away from the car.
Questions
- What file format do ECU logs use?
- TunerStudio records MLG, a binary format, and MegaLogViewer reads it. efi2000 records and reads standard MLG files.
- How do I know which tune a log was recorded on?
- In efi2000, every recorded log remembers the tune it started on and marks each write and burn.
- Why does my AFR look wrong right after a throttle change?
- The wideband reads a few hundred milliseconds behind the engine, and acceleration enrichment adds fuel briefly. Judge fuelling from steady sections of the log.
- Can I analyse logs without the car?
- Yes. Upload or record logs into a project, and the log viewer, target tracking, log coverage and recorded-log autotune all work with no ECU connected.