Replacing the bifuel CNG system with aftermarket LPG?

[dikidera]

So my 2005 S60 bi-fuel CNG volvo has it's CNG tank expired(21 years old). I talked with many techs and they want to install their own LPG(lpg because cng price skyrocketed + small tank) ecu and own button. I don't agree with this because :

1) It's ugly. They most often drill the dashboard to install an ugly button.
2) Horrible wiring skills and every car I've seen with LPG system was a mess of cables.
3) The car already has a button for this + CNG level meter on the dash. They told me they can't get that to work, so I will never be able to use the dash to tell how much LPG I have left. + the car automatically switches to CNG as soon as it warms up.

Do you guys think we can replace the CNG tank with all the components necessary for LPG, but fool our ECU into thinking it's still using CNG and let it control the LPG injection process and avoid aftermarket buttons and LPG controllers.

One of the potential many issues I foresee is the different quantity of liquid gas injected vs factory CNG as well as differences in how many sensors there are + their different readings(e.g a CNG system is much more high pressure vs LPG)

[classicswede]

I have upgraded a few of the factory fit LPG cars to modern LPG systems.

To a fair extent they are correct in what the installers have been telling you.

As you are converting from CNG to LPG you will also need to replace the filler point. The filler can be fitted in by the petrol filler behind the flap as the cng one way

Switch can be fitted to one of the switch blanks

The factory fit electronics are not capable of controlling a sequential injection system as the factory fit system does not have injectors but a distributor much like the old mechanical Bosch K jetronic systems. As the factory fit cuts the petrol injectors you must have it in petrol mode for a modern system to function as it measures petrol injection pulse width and uses the signal as a trigger. Not saying it is impossible to use the factory LPG gauge but it would not be simple

There is a lot more wiring as the modern systems are much more sophisticated than what was used on the factory fits back then. Does not need to be messy.

[dikidera]

Thanks, this was very informative. So basically the reason modern LPG systems "work" is because they intercept the injector pulses and copy them. And these injector pulses are the result of the roughly 100 fuel maps in our ECUs.

But how then does the distributor work? It sprays the same fuel every time in all ports? (even if one cylinder is in the exhaust stroke?)

So after reading a bit more...its simply a bad system through and through. And even if I could switch to a distributor LPG, I would not get the mileage I want and will always be plagued by random faults. Basically as per your sticky, it is not a good idea to switch back and forth between CNG/Petrol as this could lead to poor fuel trims in a split moment?

And if I switch to aftermarket LPG system, the petrol button becomes redundant, and basically apart from RPM and Speed(km/h) I can never know my LPG fuel level on my dash AND have to have a separate button elsewhere. The CNG fuel gauge on my dashboard was at least very informative, has a lot of divisions to tell exactly how much fuel I have left, while most modern LPG systems have about 3 LEDs.

The whole conclusion here is to drive as-is or remove the system entirely.

[classicswede]

That is kind of how they work. They read the injector duration and and adjust that duration via mapped tables in the ECU. The petrol ECU still can adjust the mixture via its trims from Lambda readings.


The distributor works like the Bosch K jet as I said but instead of a air metering flap opening valves it is done with an electric stepper motor

It should be possible to make the factory switch only operate the dash if the gauge really ment that much to you but it could be very consuming doing so

[CNGBiFuel]

I'm afraid, Classic has it right.

You can use the factory fit CNG fuel-gauge for sure. It takes a 0-5V input. Better than me will say which LPG equiv. sensor does that, it's pretty standard. Your installer will duplicate the output of the CNG pressure sensor. I've flipped two cars from CNG to LPG and vice versa. In each case you'll retain the factory-fit gauge.

You'll likely end with two LPG gauges, one as now (former CNG) and one wherever the new switch gets plonked. These systems don't have to be installed in ways to look unsightly. Sadly, usually they are.

My changeover switch is on the transmission tunnel. It looks like it was meant to be there. The trouble being, this part of the install took a day in itself. That's a labour charge a non-DIY installation likely wouldn't be prepared to find. Using the CNG filler/ opening / bracketry will potentially make a very tidy factory-fit 'look', but there's a good day's work again. This against firing a drill thru' a panel in 10 mins.

Classic will be better informed, can't see why your installer can't keep the principal stainless pipe runs. Mate the LPG fittings 'top & tail', to it. It'll stay factory in 'look'. Careful removal of the CNG receptacle for its LPG match. More work. Bigger bill.

Make sure you keep your Necam parts. Stepper and reducer. Very sought after.

[dikidera]

If I have to be honest, I'm down to do it. But I am no licensed technician to work with pressurized containers like LPG and CNG(or oxygen) and I could very well be blown to smithereens if I did something wrong. CNG is stored at much much higher pressures than LPG, but it's lighter than air and would escape the vicinity faster.
LPG is stored in less pressures but is heavier than air and lingers longer if not ventilated well.
Moreover, both the CNG and LPG tanks weigh a ****ton and if I had to secure it to the underside of the car by myself, or even drop down the old one without any support, I wouldn't be able to do it myself. I can already imagine myself not securing the tank properly and having it fall off the car while driving or hitting some bump...the mere thought of this scares me.

I believe hardware and software-wise, I could probably do the switch. I've been meaning to switch the original gauges either way, the S60 CNG gauge would become the digital one, and the petrol fuel will become like in standard cars and how it is in the other pure petrol S60s.

I have one more question regarding CNG. I have the dreaded error code ECM-F00D the version where it says Signal too high(or was it low). I don't know if my sensor on the distributor has died but this doesn't seem to have affected much. The sensor is damn near impossible to find and replace.
Do you guys know what this sensor does and what I can do to fix it? The error code is permanent, it comes back after being cleared by VIDA.