[Stefan96]

Hi everyone!

With the 20 year mark approaching most of the p2 Volvos and no real option to replace the underfloor tanks when they reach their expiration date, and the rest of the CNG system not being sold either, semi-planned obsolescence seems to strike again.
Since my 2006 Bi-Fuel V70 will reach its 20 year expiration date next spring when the next inspection is due, and me not wanting to scrap an otherwise functional car or spend 20 to 40 thousand SEK on a tank costing me my trunk space, some solution was in order.
So I decided to remove the CNG system and reclassify the car as normal "petrol only".
The process itself is not too complicated, but removing the system takes some time.
Apparently one can also have Volvo plug the system & get a certificate, but then you're just waiting for one of the electrical components to break and have a check engine light ruin the next inspection.

So for the sake of documentation and in case someone also wants to do this, here's what I found out.

Before that, since it's the year 2025, I unfortunately I have to make a small disclaimer. I am in no way responsible for any harm you cause to yourself, your car or others by believing and acting upon anything you read here. Working on anything always involves some kind of risk, especially if its a large chunk of metal that can reach high speeds or if it contains something flammable, and if you're the kind of person that needs to read such a disclaimer I recommend you stop reading now anyway...

Before you go about removing the system I also recommend you double check what I am about to describe. It really only takes a multimeter and some wires to clip in between wiring harness and sensor (and technically an oscilloscope for one thing, but that is apparently irrelevant anyway).
I also ran the tanks completely dry before I depressurised the system. If you're stuck with a half-full tank because one of the components decided to die, I recommend you have a professional CNG technician depressurise the system for you.

Okay, now to the actually interesting part.

Let's go about describing the CNG system first:
The car has 3 under-floor tanks in total; One main tank underneath the trunk and two secondary tanks under the rear seats. The main tank is outfitted with a pressure and (I assume) temperature sensor that is on the non-plumbing side of the tank (left side in my V70). Each tank also has a solenoid valve sitting in the head. The tanks are all connected in series with the left secondary tank connected to the right and that being connected to the main tank. The main tank is connected to the filler receptacle as well as the rest of the system.
The rest of the system contains of a filter located just downstream of the main tank, the pressure regulator in the engine bay somewhat between airbox and engine on the (looking into the engine bay) right side, technically the cars left side. The pressure regulator also has a pressure sensor. After that only the gas distributor is left, that sits in the front somewhat below the intake manifold and plumbs right into it. The distributor has a solenoid valve (DFCO valve as far as I know) and a control actuator.

So what needs to be done in order for the car to show no check engine light?
It seems like the ECU doesn't like it if the pressure & temperature sensor on the main tank, the pressure sensor on the regulator and the valve and actuator on the distributor are disconnected. The solenoids in the tanks seem to not be monitored (and it honestly surprises me more that the one in the distributor is...).

Before I describe what needs to be done to make the car believe the components are still there, let's define a standard about the connectors, so we're all talking the same language. When I talk about pin numbers or anything on a connector, I look into the connector ON THE WIRING HARNESS (not the component) WITH THE RETAINING MECHANISM (clip) ON TOP. PIN NUMBERS ARE LEFT TO RIGHT (and row after row in case there are multiple).


Let's go from the tanks forward:

Main tank pressure & temperature sensor:
The sensors are analog and supplied with 5 volts. My empty tank read about 0.7 V on the pressure sensor and around 2.6ish when half full. The (I assume) temperature sensor read around 2.2 to 2.6 V when the outside temperature was between 15 to 20 deg C.
A voltage divider built from 2 resistors each seems to be enough to fool the car into believing the tank is always empty and at normal temperature.
The pins on the connector are as follows:
Pin 1 (top left): 5 V
Pin 2 (top right): pressure sensor signal (~0.7 V)
Pin 3 (bottom left): temperature sensor signal (~2.2 V)
Pin 4 (bottom right): GND
I used 1500 & 200 ohm resistors for the voltage divider on the pressure sensor line and 2200 & 2000 ohm resistors for the temperature sensor.
I ordered a matching connector so the solution is plug & play, but I should have just re-crimped the wiring harness to a different connector. The plug is commonly found on oxygen sensors with nobody ever giving out the name. You can find it on aliexpress for cheap (around 125 sek for 5 connectors including shipping) if you search long enough or on a certain site (that designates the connectors with CE and a number) and tries to rip you off & charges 87 dollars for a single, non-genuine connector, probably ordered wholesale from the same aliexpress supplier).

Pressure regulator sensor:
The sensor is also analog and supplied with 5 volts. With the car running in petrol mode the sensor read about 2.5 V as well.
Same story here; voltage divider from two resistors, in this case extra easy because they can be the same value.
The pins on the connector are as follows:
Pin 1 (left): not used
Pin 2: GND
Pin 3: signal (~2.5 V)
Pin 4 (right): 5 V
I used two 1000 ohm resistors.
After a lot of search I found the connector online at places like mouser etc. But they're around 60 sek per connector housing, without the actual pins. To be honest I already forgot what it's called. I used the generic, water-tight, 4-pin automotive connectors that Biltema sells instead and re-crimped the wiring harness. Very easy and I recommend you do the same, especially since that type is very common and also used elsewhere by Volvo themselves. I also used it for the gas distributor and SHOULD have used it on the main tank sensors.

Gas distributor actuator:
This one is a bit more weird, but thankfully easy. The actuator is supplied with the battery voltage (therefore around 14 V when the car is running, aka "12 V" system).
One of the pins seems to be the control signal, which is a generic PWM signal (logic level of around 12 V) with a duty cycle of ~83.33% (5 ms high) and frequency of around 166.66 Hz (period = 6 ms).
Thankfully that seems to not be important and can be left disconnected. I need to say that it is weird though: The damn thing somehow understands if you pull only the control signal line so that it stops sending it (No, it does not just need a pull-down or pull-up to stay enabled. Believe me, I tired a lot of stuff...). If someone wants to find out, get an oscilloscope and a logic analyser and go to town, but it doesn't matter for removing the system.
The other line on the actuator is probably some sort of feedback, that seems to be analog. I again replaced that with a voltage divider, but maybe it's not even necessary at all.
The pins on the connector are as follows:
Pin 1 (left): 12 V (~14.2 V while running)
Pin 2: GND
Pin 3: PWM with ~12 V logic level
Pin 4 (right); analog signal (~11.15 V while running)
I used 320 & 1200 ohm resistors for the voltage divider on the (what appears to be) feedback line.
I replaced the connector with the same 4-pin automotive connector from Biltema.

Gas distributor solenoid valve:
It's weird to me that the ECU even seems to check that thing, but when I pulled just that plug on its own, the check engine light was on on the next start, so apparently it does.
Anyway, the solenoid seems to be "normally active", so when the car is idling in petrol mode there's "12 V" (again, more like 14 V under normal conditions) on it.
The valve itself has a resistance of around 25 ohms (if the cheap multimeter is to be trusted).
I simply put a 390 ohm resistor on it, since I didn't want the resistor to get too hot or burn.
The plug used by Volvo is the same as the 2-pin variant of the generic water-tight automotive connector sold by Biltema, so one only needs to buy the (electrically) female side - neat, less work!

In case someone is not sure how a voltage divider is build: One resistor goes between ground and the "signal" line and one between supply and "signal" line. The higher the resistor value on ground in relation to the one on the supply, the higher the voltage will be. If you are unsure, there's plenty of better explanations on the internet - and also voltage divider calculators with schematics.
I always build the voltage divider onto the (electrically) female connector so that I can just plug it onto the wiring harness. I also sealed off that connector with a lot of heatshrink and adhesive/sealant (don't use hot glue, since the temperature in the engine bay or underneath the car on a hot day can be quite high).


Okay, now onto the mechanical side.

Tanks:
To remove the tanks one needs to remove the plastic covers first. The covers from the secondary tanks come off relatively easy, but the main tank cover needs some convincing and you also need to remove the exhaust hanger right behind it, as well as the exhaust hanger right at the end of the pipe to push the exhaust out of the way.
Depressurise the system first and then undo the plumbing. All the plumbing (to my surprise, since it's gas) is normal right handed threads. So lefty loosey, righty tighty!
Most of the clamps holding the tanks did not budge in my case. On the main tank one bolt snapped, and on the other one I rounded the head off. From the secondary tanks only the right tank clamps came loose, the left side was the same as the main tank. If that happens to you, have fun getting a hacksaw blade in there, it's a pain in the ass. The secondary tank clamps are more accessible, so you can at least use the hacksaw as is (not just the blade - and I didn't really want to use an angle grinder right next to gas tanks...). Also make sure to support the tanks so they don't fall as soon as you remove the clamp. I just used a jack to slowly lower them out of the car.

Filter:
The filter is easily accessible once the main tank is out and should unbolt without a problem.

Filler Receptacle:
The same goes for the filler. There's four bolts if you open the filler door. With a bit of luck the bolts can be cracked loose and pulling on them while turning is enough to keep the nuts on the backside in place. Otherwise you need to get a wrench in there, but that is also possible.

Pressure Regulator:
Remove the airbox. The bracket that the regulator is mounted to is bolted to the car underneath/behind that. Before you undo those bolts, remove the gas lines (hardline coming from the back and central hose going from the regulator to the distributor).
Additionally, the regulator is also connected to the cooling loop (the two hoses on each side of the regulator), which I assume is to pre-warm the gas. The fitting diameter is 10 mm, so I just pulled off the feeder and return line from the engine block and put some 10 mm hose in between to close the loop. The lower fitting on the block is on the back of the engine (right side looking into the engine bay, towards the firewall) and the upper one is on the right side of the block (again same orientation, looking into the engine bay) and the fitting is exiting the block towards the firewall. Coolant will come flowing out of the lower one if you unplug the hose, but you don't need to drain the system. I just pulled the hose off, put my finger on the fitting and then quickly put the new hose on, only spilling a very small amount of coolant. It helps to remove the brace connected to the upper engine mount & sides of the strut towers to have better access.
Besides that, the regulator also has a vacuum line that is connected to the front of the block. I simply used a short piece of tube (5 mm inner diameter if I recall correctly), two clamps and a plug to seal the system.

Distributor:
This one is very annoying to get to so I removed the fan, which is fairly quick and easy. Then you just need to locate the two bolts on the bracket holding the distributor in place.
Besides that, the injection lines that plumb into the intake also need to be removed. I removed the plastic cover on the top-front of the engine/intake to have a bit more access. It seems like the little "UFOs" that sit on the end of each injection line on the intake are just clipped onto their fittings, but I found it impossible to remove them without breaking some of them, especially since I had lost some of my patience at that point.
The brand new holes in your intake left by that also need to be plugged. The fittings on the injectors have a self-sealing conical thread, that I could not 100 percent identify. It seems like 1/8" x 27 NPT, but I am not very sure about that, could be some weird M10x1 conical thread as well. I also managed to lose two of the fittings in the engine bay because I did not want to pull the intake off.
If you can get proper plugs in the right thread somewhere, do so by all means. I personally got some M10x1 brake line fittings (very close in thread size and cylindrical), cut an M6 thread into the inside, sealed that up with an M6 bolt & thread sealant and then used some more threat sealant on the outside and threaded them into the intake, which seems to work fine, despite it being a litte bit of a ghetto engineered solution. Don't judge me, it was Sunday and I need the car. Alternatively, if you don't lose some of the original the fittings you can also just solder them closed and put them back on.
You can do all that without removing the intake, but the fitting for cylinder 3 requires a bit of flexibility in your arms, wrists and fingers, since it sits on the underside of the intake.

Lines:
Some of the lines come out easily as is, others I simply cut because it was too much work to remove them without damaging them or something else. I don't really need to describe this process in my opinion, you just need a saw or good bolt cutters.

That's it, whole lotta talking! I hope I was able to at least help one person with this.
The parts you need for this are basically pocket change since most things I already had lying around, so I didn't bother listing how much I spent. I'd guess just over 600 sek in total if you buy everything except tools of course. The inspection cost me 2198 sek and of course the yearly vehicle tax will increase a bit now.
I'm sorry I didn't take any pictures, but I had to do the conversion by myself, lying on my back, with the car on jack stands. Not a lot of room for activities there... Someone give me a garage with a lift, then I promise I will take pictures for the other steps ;)
Looking forward: Eventually I hope to put a normal, full sized tank in the car (which I already have here), as well as getting the normal fuel gauge to work. Until then it's the small tank and the digital gauge only. For now I'm happy that it was possible without too much effort and the Volvo engineers hadn't fully turned cars into smartphones on wheels that start crying at the slightest inconvenience just yet...

Also, please repost and translate this (to swedish or any other language, I can only do german and english...), maybe in the correct place as well so more people see it!

Stefan