What EVAP control does
Fuel in the tank constantly gives off vapor. On a road car, that vapor is stored in a charcoal canister instead of venting to the air. The canister then has to be emptied, or "purged," by pulling the vapor into the intake so the engine burns it. A purge solenoid between the canister and the intake controls how much flows.
The Elite's Evaporative Emissions Control function drives that solenoid with a pulse width modulated (PWM) duty signal. More duty means more flow. The catch is that purge vapor is extra fuel the ECU didn't plan for, so the job is to let it in at a rate the fueling can handle.
Wiring the solenoid
Assign the EVAP Output to a spare output, usually a DPO. Most purge solenoids take switched 12V on one side, and the ECU grounds the other side to open it.
Active State sets which way the duty works:
| Active State | 0% duty | 100% duty |
|---|---|---|
| Low (default) | Solenoid closed | Solenoid open |
| High | Solenoid open | Solenoid closed |
Leave it on Low unless your solenoid is known to work the other way. A quick test in ESP with the engine off, stepping duty up and listening for the solenoid, will confirm.
Frequency
Purge solenoids are pulsed slowly. The typical range is 2 to 20 Hz, and the default is 5 Hz. A low frequency lets the valve fully open and close on each pulse, which gives you fine control between shut and a small amount of flow. If your solenoid's spec sheet lists a frequency, use that.
The duty tables
There are two base duty tables, and which one is in use depends on whether closed loop fueling is running.
| Table | Used when | Typical values |
|---|---|---|
| Duty Cycle (O2 Active) | O2 Control is enabled and the ECU is in closed loop | Higher, because closed loop can trim out the extra vapor |
| Duty Cycle (O2 Inactive) | Closed loop fueling isn't active | Lower, because nothing is correcting for the vapor |
The logic is simple. When O2 control is watching the mixture, it can take fuel away from the injectors to make room for the vapor. When it isn't, every bit of vapor makes the engine richer, so you purge less.
Corrections
- Coolant Temp Correction: the usual practice is to purge very little until the engine and catalytic converters are at operating temperature. That means large negative numbers at cold temperatures. It can be mapped in up to two dimensions. See EVAP coolant temp correction.
- Generic Correction: lets you pick any condition to scale duty. Limiting purge by throttle position is a common use.
- Fuel Tank Pressure Correction: only works with a fuel tank pressure sensor fitted to report vapor line pressure. It ships as all zeros and supports up to 2D mapping. See fuel tank pressure correction.
How we approach it
We finish the fuel tune with purge disabled first. Then we bring purge in gradually, starting with low duties, and watch closed loop trims and wideband readings. If trims swing hard every time the solenoid opens, the duty is too high for that condition. The goal is a canister that empties over normal driving without the driver ever noticing.
Troubleshooting
| Symptom | Usually |
|---|---|
| Rich spikes or rough idle when warm | O2 Inactive duty too high, or purge allowed at idle |
| Rough running when cold | Coolant temp correction not pulling enough duty |
| Solenoid always open | Active State wrong for the solenoid |
| Fuel smell, canister saturated | Duty too low overall, or output not assigned |
Can I just leave EVAP disconnected?
On a race car, often yes. On a registered road car, emissions rules may require a working system, so check your local requirements before removing it.
