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.