Adaptive trims and LPG calibration

[JD79]

Hi guys,

Still plugging away at getting my LPG kit calibrated properly, it's running well enough if a little under-powered.
The voltran auto-calibration utility seems pretty good as long as you drive it enough to fill all the fields in.

The question I had was whether or not I should be resetting the long fuel trims prior to calibration.

The trip comupter thinks I should be getting around 20mpg, but it's between 16 and 17. Appreciate that the trip computer won't be correct on LPG, it'd be nice to get a little closer to what it's reading though!

Any input appreciated, cheers!

[Gilbertd]

That sounds about spot on. The trip computer should read high when running on gas compared to actual mpg. The computer calculates fuel consumption from the duration of the petrol injector pulses (it knows what the fuel pressure is so an injector open time of x milliseconds will allow y millilitres of petrol to be used and from there calculates the mpg). When running on gas the pulses are still thee but the injectors aren't opening so no petrol is being used but it doesn't know that. The important thing is that the mpg readout remains the same whether running on petrol or gas as this means that your LPG multipliers are correct and the petrol ECU isn't having to make any alterations to the fuel trims. It will never show your actual fuel consumption on gas but if a car does 20mpg on petrol it is likely that it will do around 16-17 on gas.

[JD79]

Thanks for the reply Gilbertd., appreciate the info.

I Didn't manage to get the car much over 65mph on the last calibration, and doing quite a lot of motorway miles so wanted to try and get another calibration run in with a wider range.
The fuel trims are sitting at about -17 at the mo, just wasn't sure if I should be doing the calibration with fuel trims at this level or if I should be resetting them prior to anymore jiggery-pokery!

[Gilbertd]

If the trims are at -17 when on gas, your multipliers are too big and the petrol ECU is having to decrease the injection times to compensate. If they are at -17 when on petrol, then you've got something wrong that is causing it to have to compensate for a rich mixture. Try running on petrol for a while and see if they reset to 0 ish. Doing a calibration while the short term trims are out isn't a good idea but if it is the long terms, it is only doing what it has learnt.

[JD79]

That's a job for this weekend then, I'll reset the trims and drive it around on the liquid gold for a bit and see what the trims do.
I replaced one the of the lambda's previously, noticed the other one was slow reading and changing so I've got another to fit as well this weekend.

Any idea what sort of distance I should be driving to allow the long term trims to settle?

[Gilbertd]

It shouldn't take long. You can see what the short term trims are doing and the long term ones will follow those. So if the shorts average 0 then the longs will too. Don't get too hung up on instantaneous variations on the short term trims, that's normal, it's the average you are looking at.

[Tubbs]

Kind of as gilbert says, at -17% so over rich STFT you will probarbly have an overall - figure on the LTFT.
Absolutely perfect is kind of between -.8 to 1.8 at idle STFT. Out of idle you will be looking at somewhere between -1.6 and 2.8. Dont get too hung up on odd figures above that.
You will never acheive 0. So dont be too bothered. Just remeber - is rich and a positive figure is lean.
However some cars run positive trimmers. And some run inverted trimmers. You have to work out which. So if you up the map effectively adding more gas, the petrol system will try to take away. So - STFT. If you up the map and the STFT stays where it is or goes positive in that situation you have inverted trimmers.

EDIT... Just noticed that its a range rover. So positive trims.