Thread: OMVL fuel map

You don't just "load a different map" for an engine change, you do a thorough road test with the laptop plugged in and an assistant to report on what's happening on screen. Or a dyno if a big hill doesn't feature in your road test route. Last week I had a customer in whose map had crashed due to an ECU glitch, it took me around 1/2 an hour to recreate the programming from scratch, test drive the vehicle with the owner in the passenger seat with the lappy, and send him on his way. If I had been able to access my old tuning lappy I would have been able to load a saved map from that and fine tune it. Fiddling with a map is not terribly difficult, and you always try to keep a copy of the better maps just in case you stuff it up.

Note that this is not specific to OMVL, both the Zavoli and King software that I use are very similar clones from the same ECU manufacturer.

a road test sounds like a plan,,

and another explanation if possible,,

The LPG has a fuel map,
and the car's ECM has a fuel map,
if the LPG software mirrors the car ECM,
why does the LPG software need its own map?

Thanks bee utey

The LPG map is a translation map, not an absolute map like the petrol one. There is something like a 10 x 12 array of numbers for translating petrol injection times to lpg injection times based on the rpm range you're in. That is because the characteristics of a LPG injector are very different to the OEM petrol injectors. The LPG ECU doesn't connect to the wide array of sensors the petrol ECU has, it just reads the injector times and RPM. The petrol ECU remains in charge of engine sensors at all times.

Each value in the array can be individually changed (or changed in blocks) by any amount so that ideally the petrol ECU does not need to adjust its fuelling to match the expected outcome. In closed loop fuelling a petrol ECU adjusts to the O2 sensor output and the variation from the base map is the fuel trim. When you're running on LPG the LPG ECU sees the rpm and the petrol injector times. Applying the map values to the petrol times gives the LPG injector times. A well adjusted map means the petrol ECU will not see variation in fuelling outside of its comfort zone, ie no faults are logged. 10% fuel trim generally passes without comment on must vehicles, ie doesn't light the MIL.

A base map is provided by the LPG ECU manufacturer to roughly show the shape of the response needed. Different injectors have different response times and will need slightly different maps. The amount by which the LPG pressure drops under load will be a factor the map needs to account for, a bigger converter will drop pressure less but if the pressure remains above the minimum required the map can be adjusted to suit any converter. Some LPG programmes have the ability to automatically adjust fuelling for different pressure drops, not all systems have this feature.

On WOT running (open loop) your LPG map needs to be manually checked against the oxygen sensor reading to ensure adequately rich running. Without an oxygen sensor connection you can either watch the fuel trims or even use your seat-of-the-pants dyno to get adequate fuelling. My pet mountain allows me to safely do a couple of 0 to 80 km/h runs at WOT while my assistant watches the screen. Major lean outs mean I add a bit at a time to the map area concerned until it goes as it should. Some but not all LPG injection systems have a connection to the OBD2 port and can fine tune the LPG map to suit by reading the fuel trims. But this does not take the place of setting up a good map in the first place.

What causes the need to redo the LPG map? Changing the engine size obviously, as the LPG injector response times and the converter pressure will be different at a given throttle level. Your petrol map may work adequately but the LPG map may be quite inaccurate at correctly translating the increased petrol times. Also wear in the converter, ageing injectors, and the varying mix of LPG that you are purchasing. Then there's corrupted data from whatever cosmic reason you might imagine, possibly interference from poor shielding or bad connections.

Finally, you will have real trouble running on LPG if there is a major change to the petrol mixtures due to faulty inputs to the petrol ECU, the changed petrol injector times will have a flow on effect on LPG as the translation map will see different inputs. An example of a major change would be a failed MAF resulting in an emergency-stagger-home petrol map. The LPG ECU does not have a stand alone map that it can run on.