[chetanhanda]

I have not worked or had experience with CNG systems but atleast I know that playing with signal to ECU is not the right way to supress MIL.

Originally Posted by rajeevpatel

Now we want that the MIL light should not glow when running on CNG. What can we do. We have tried to insert resistor of different values on the output of o2 sensor to reduce the output voltage to ecu but it's not working. Is there any way to solve this. Can we make some arrangement such that no change is made when it runs on petrol but when it runs on CNG the reading given to ECU is altered so that it doesn't glow the MIL light.

The voltage of the o2 sensor should not be altered, there is a reason why the MIL is coming on, by artificially altering the o2 voltage the problem will not go away.The reading must be brought down by increasing the fuel and not by masking the voltage.You will be simply causing engine knock or engine damage with those lean fuel ratios..

Originally Posted by rajeevpatel

While driving with scantool connected it was observed that the short term fuel trim remains high while running on CNG and long term fuel trim is low compared to the same when run on petrol.

How high is it ? Im assuming your STFT has maxed out and the LTFT is more than 20% which is triggering your MIL.

Originally Posted by rajeevpatel

So how to reduce the short term fuel trim which is making th MIL ON on CNG ?

The STFT is being increased by the ECU based on your voltage which is screaming out that it is leaning out.
Either reset the ECU and let the ECU add the STFT to the LTFT on its own and see if STFT becomes stable. BUT this is obviously not working as it has already gone above acceptable range of LTFT.

By the way can you check your IPW how long is it on for ? or if your OBD2 software shows IDC , how much IDC % your injectors are on for ? are they maxed out ?
If it was a petrol engine ..u could have increased fuel by increasing injector size, fuel pressure with bigger fuel pump, used a piggback AF controller etc to alter fuel delivery with help of a wideband etc ...similar may be applicable in case of CNG.. ? (maybe similar)
Basically try increasing your fuel/CNG supply..
(Try getting hold of a wideband AFR meter)

Is there a way your CNG system understands/compatible/takes feedback from 02 sensor/obd2 port/ECU ? if NOT then Im confused how the system is designed to work on a existing MPFI engine.. ?