Volvo V70 CNG bifuel 2004., engine shutting down randomly

[mickey21]

Hello everyone


My faithful Volvo V70 Bifuel has started making a strange problem so I decided to share it here to try to get some opinions before I start throwing money randomly into parts which may not be the culprit.


The problem is that the car randomly shuts down the engine without any warnings or announcements during regular drive and it only happens when driving on CNG. It just cuts down on power and refuses to respond to the accelerator pedal. If it has the momentum and speed and if I immediately press the button and switch to petrol, it continues to work normally
but will not switch back to CNG with the next press of the button. If I stop the car and turn on the engine once again and try switching immediately to CNG it cuts out the same.
However, If I stop and turn the engine off and take the key out of the ignition and I wait for a minute or two, it will start to work normally until the next occasion, maybe after a day or two.

What I have noticed in addition:

- frequency of this issue started to increase with ambient temperatures rising now and warmer days coming.
- it seems that this happens more while driving on lower revs, sedated pace in the city.
- I have noticed smell of sulphur/rotten eggs from the exhaust when I register this issue, even if car starts running once again on CNG.


Taking into consideration from experience I have from before when I was driving an LPG powered car that car's engine might run on petrol with some unresolved engine problems while it will not run on CNG or LPG if everything is not perfect, these came to my mind as the culprit:




- air filter (not changed for the last 30000km but it does not look too bad)
- spark plugs have not been changed for 70000km, they are regular type Denso
- clogged catalytic converter
- ignition coils - however I recently replaced 2 of them and engine is idling fine
- air leak, however I checked the intake rubber hose and the filter housing and noticed no holes or gaps.
- Fuel pressure sensor going bad
- some part of the CNG system, like stepper motor?


The glove test for PCV is good.

Throttle body has been replaced 5000 kilometers ago.

Necam gas regulator has been refurbished recently and has done some 30000 km since.

There are no gas leaks that I can detect under the bonnet or under the car.

First O2 sensor before the catalytic converter has been replaced recently although I remember I registered this issue once before I got it changed.

There are no engine error codes reported or even pending ones scanned with VIDA diagnostic software, neither with my Torque bluetooth module.


Does anyone have any ideas or suggestions?



Many thanks.

[dikidera]

This is actually a common problem for CNG. There is a solution, however my memory is poor and I cannot remember it.

It has to do with a bad solder connection on something in the engine bay. I am sure there are people on here that know about it.

The previous owner of my 2005 CNG Bifuel S60 had the same problem. Highways and it dies without warning. He fixed it by doing this re-soldering.

[mickey21]

Thank you,that is something I did not think of and a great suggestion.

It is well worth investigating.

[ele288]

Looks like the symptoms of failing stepper motor. You can remove the stepper, remove the the plastic lid and redo 4 solder points. I did mine few years back and is good till now.

[mickey21]

Originally Posted by ele288

Looks like the symptoms of failing stepper motor. You can remove the stepper, remove the the plastic lid and redo 4 solder points. I did mine few years back and is good till now.

Thanks, ele288, that is something which came to my mind today.

Car is having trouble switching to petrol automatically when the CNG gets depleted so I have to do it manually.

I think it is highly likely that it is the stepper motor.

[mickey21]

I believe I should give an update on the situation.

I have been given the price quote for an overhaul of the complete gas distributor with the stepper motor by a workshop in Netherlands owned by a former Koltec mechanic who is specialized in refurbishing their products. The offer included overhaul of the distributor and relocation of the stepper motor electronics away from the engine block which was a good idea. The offer was fair but to my surprise when I started working on the engine to take the part out I discovered that gas regulator which was repaired 2 years ago leaks gas. Taking into consideration that repairing both of these parts would cost almost what local shop would charge me for a dedicated system, I considered it unreasonable and opted for installation of: AC Stag Q max Bacic ecu for a 6 cylinder engine, new injectors and Tomasetto AT12 pressure reducer. I had a manometer installed on the reducer as well so I have a CNG led level fuel indicator in the cabin. What is also a good thing that Volvo's gas level sensor from the cylinder is connected as well, so the old fuel gauge on the instrument cluster also works.


For now, system is working great: less fuel consumption, more gas used from the cylinder before it is depleted (more range), seamless switching between petrol and gas. I do notice a slower response to the throttle when running on CNG but not a lack of power once it goes up to the revs and I have error codes for low fuel pressure and lean mixture at cylinder 1. However, the system was just installed last week and I have been told to run the car for a while until ecu maps are rebuilt and data collected so they can connect diagnostics later on and see what needs to be adjusted. Probably gas nozzles would need to be rebored a bit, that is what they told me.

So my factory CNG system gave up finally after 400000 km, hopefully this new one will serve for as long with less maintenance needed which will be more available and cheaper.

@dikidera and ele288, thank you very much for initially sending me on the right track with your suggestions and helping me resolve the problem with my car at the end.