Multipoint Sequential Fine Tuning
Fine tuning of the multipoint system is all about the Tinj petrol timing. This is the length of time in milli seconds that the vehicles ECU tells the petrol injectors to stay open for. By tapping into the petrol injector negative wires we are able to see this timing on the laptop.
It will help to remember that the vehicle doesn't know that it is running on LPG, it always thinks it is running on petrol.
This is how we can check that the mixture is correct when running on LPG. If the Tinj petrol timing is the same when the engine is running on LPG as it is when it is running on petrol the mixture must be correct.
Remember that the petrol ECU is highly advanced at achieving the best possible mixture under all load and driving conditions. All we have to do is match it. 'Auto Calibration' will normally get the timing very close but it is important to compare it in all load ranges. To do this go to 'Modify Carb' page (Main menu-vehicle configuration-Modify carb) Here you will see eight boxes showing 0. One of these boxes will be red, this is showing the load range of the engine. If you increase the engine revs you will see that the red square moves as the engine speed/load increases.
Start with the engine running on petrol, at the bottom of the page you will see the Tinj. petrol timing.
Make a mental note of this figure, for the purpose of this will will say it is 3.00 at idle.
Switch the system to LPG look again at the Tinj. petrol timing if it has increased, say 3.30 then the mixture is lean. This is because the petrol ecu has looked at the lambda probe and seen that there is not enough fuel, so it has told the petrol injectors to stay open longer. It does this by increasing the Tinj. petrol timing, there by, injecting more fuel into the cylinders.
So we must 'pair up' the LPG system, to do this, click into the box that is red and delete the 0. The timing has increased, showing us that the system is weak, so we need to put more fuel in at this load range. We do this by putting a positive figure in the red box. Say '5' then press enter, when you do this you will see that the Tinj petrol timing will drop, if it is not enough, inrease the figure, if it is to much decrease the figure.
If, however, the Tinj petrol timming is lower when we switch to gas then this means that the mixture is rich ( the petrol ecu has shortened the pulse to the petrol injector, there by putting less fuel into the cylinders). This means that a - (minus) figure is required. Say -5. If the Tinj petrol timing increases too much, reduce the minus to say, -3. If the the Tinj petrol timming is still to low then increse the figure-. Say -7.
Have an assistant drive the vehicle for you at steady speeds. Every time the red square moves into a new load range, look at the Tinj petrol timing whilst running on petrol switch to gas and compare the figure. You may not be able to drive the car in all of the load ranges, or squares, on the screen, this is normal.
The petrol ECU is capable of adjusting the Tinj petrol timing by 25% either way (+25 more fuel / -25% less fuel) ((this is called fuel trim)). So, as long as the Tinj petrol timing stays within 15-20% whilst running on gas then all is well!
The fuel trim is constantly adjusting to keep the mixture correct on petrol and will do the same on gas.
If at any point whilst driving the vehicle on LPG, the check engine light comes on, then the fuel trim has probably reached its maximum. (Ofcourse it could be a totally unrelated problem!)This means that the petrol ECU has tried to increase (or decrease) the fuel trim by 25% but the mixture has still not adjusted enough. If this happens the check engine light will come on to show the driver that there is a possible fault with the fuel system. A fault code reader will tell you at what speed, engine revs, etc, the fault occured. If this happens you must clear the fault and check the Tinj. petrol timings. Work out which load range is incorrect and adjust as necessary.