I drive 1991 Volvo 940 with LPG (existing in the car since new). It reads on the "pump" (is it called the LPG pump?) "specially made for Volvo by Vialle". The system has been modified as I think there are remnants (electronics with some relays) that did not have any use now, cut wires in the engine compartment, then some very strange system for providing good idling rpm on LPG, etc. But the car run PeRfEcT on LPG - literally perfect, making it 8-9 EUR / 100 km in town, for years.

Then today, at a traffic light, the rpm stayed high - 1.200 rpm, and did not want to go down. I hate to be in a car with higher rpm, and my first thought was that there was either a mechanical issue with the accelerator cable, although it never happens on 940s and should not happen on this one as it is in good order, or something in connection with that strange idling control system on the main LPG pipe (there is a valve linked to a vacuum controller, but that controller is disconnected from any vacuum system, no pipes present or other parts to which it could be connected, and its lever was fixed in one position with a piece of wire; if it would be moved tiny amount from its good position, idling will become very bad, but it was well fixed and working like that for years). another option was some problem with any of the working vacuum / air pipes - but that would then affect both fuels somehow, I think.

Because it was a very busy street, of a highway profile, without possibility to stop immediately and have a look under the hood, I continued to drive (it is few km of such road, and disconnection would mean loss of too much time and I was in a hurry, so I continued). At the next traffic light it was the same higher rpm. In normal driving, everything was fine. I was thinking whether it would be wise to change to petrol or not, but I did change the fuel while standing still on that traffic light (it is a computer controlled LPG, not G3, and the car has electronic petrol injection, so change of fuel is possible at any moment by touching a sensor button).

The fuel change went well, I reversed it back to LPG, initially it was ok, normal rpm, but after pressing accelerator pedal, the rpm stayed high again, I did another change, moved forward and made another 2 - 3 changes of fuel while driving, when on the last change from petrol to LPG, motor died, I immediately changed to petrol, everything works, LPG change again, doesn't work.

I decided to go immediately to a place where they work with LPG, because it was in a vicinity of where I was, thinking maybe when it is this fresh, they could detect something. (LPG tank is full, filled 30 km before this happened with 67 lit of LPG fuel. I have to say that I was surprised that it took 67 lit, when earlier it would usually accept around 10 lit less when all the LPG will be used, and I was always filling it up in that way, enabling the car to drive on petrol a bit).

When I filled it that last time, it started with more than one familiar sound from the back, when after turning the engine on, LPG engages automatically, this time it was 2 times the noise, but the car went on smoothly and I made those 30km after that filling. The problems, in fact started on that day before the filling, when I was expecting the LPG to be fully spent - I was going uphill (20m short steep uphill ramp), the engine went off while the car was not horizontal, which was strange and not expected - so I thought could it be that the engine went off because of very low LPG level - but then - isn't it different than with petrol - I don't know - but I did not bother myself about that, as it started normally after that ''incident'' (and with Volvos 700 and 900 (and 200) series any shaking let alone dying while driving is a big incident as they are extraordinarily reliable); now I think was it an announcement of this problem?

Later on, the car behaved like usual when LPG really went out soon that day - it looses power, but still possible to drive it low speed on LPG, this is when I always put it on petrol until filling LPG.

The point of this part of the story was that I had a thought - could it be that those 2 times LPG engagement sound immediately after filling the LPG was because it somehow was overfilled (as I know it reads on the pump that it must be filled with 80% of the tank volume, and I always thought my tank is 60 lit - but maybe it is 80); then putting it in context of today, yesterday it was much colder when that filling was made, much below 20 degrees, and today it was extremely hot outside, and inside the car and on that street definitely more than 30 degrees when it died.

So question here is: what determines when the lpg tank is full and can the overfill happen and what happens if it is overfilled? Or, can such problems be put in connection with the fact that the tank is simply full, and something not good in the back of the car? That "back sound" when LPG starts does not seem to be that strong as usual, but maybe this is my paranoia. There is that sound now after changing to LPG, and the car dies immediately.

Anyway, I stopped by the mechanic, not knowing that the real "LPG professor" is on holidays, and person who offered to help me immediately with some diagnostics, in fact knows about LPG not more than myself. But it was good to try something, and he did some very unskilled checkups of the 2 valves i.e. solenoids in the engine compartment, and also the one in the back, on the tank. he concluded that the one in the tank is working, but for the 2 in the motor compartment he was not sure if they worked or not i.e. he replaced both (there are 2 solenoids, different in size, one before the "LPG pump" and another on the pump) with at least 10 other similar valves, leaving it always as a possibility that none of them was good, as all of them were found among some used parts. So he concluded that he cannot help me. He said he never saw such a system with so many wires and unknown elements (it is not exactly that complicated or messy, and wires are clean and good looking, connectors also.

So I went home, and encouraged with such poor performance of a professional, and all that in a workshop which is specialised for LPG cars, I checked both front valves at home in 5 min by connecting the solenoids directly to 12v, and both valves are working well, the noise of moving of the valves is very clear, and I can felt it on my finger if I hold the solenoids. I even unscrewed the one valve on the pump to see how it all looks inside, and was happy to see everything clean and good looking. When I started the engine on petrol, and had my neighbor touch the LPG button to change to LPG, I did not hear or felt anything on either of the valves, so I concluded that both valves do not get 12V to open when the conversion of fuel is made from petrol to LPG, and the engine dies because there is no LPG coming to the pump.

I removed the inside panel to see what was once shown to me to be the LPG computer, to check if the connectors are fine. There are 6 connectors; "engine'', ''general", "diagnosis", "switch'', "actuator 2" and ''actuator 1". When ignition is on and sensor (connected to the "switch" connector point on the computer) touched, I can hear the moving of the relay switch inside the computer box, the light on the sensor changes accordingly, so everything looks like working fine. Computer memorises which fuel was on when engine was last time used. The two connectors that read ''engine" and ''general'' - if any of them is removed, motor cannot be started, there is no light on the sensor, "diagnosis" and "actuator 2" are free connectors, and connection or disconnection of the cable on "actuator 1" has no effect on the present situation - the engine now starts immediately on petrol, doesn't work on LPG, with that "actuator 1" connector or without it. What is that "actuator" wire for?

There are also 3 separate "hand made'' and added later, not by Volvo factory, fused wires in the engine compartment, I checked all the 3 fuses, they are fine, one of them when removed, will not let the light on the sensor to be lit, computer forgets if the last time it was on LPG if that one is removed (but the engine will start and run on petrol, without the red petrol led if that fuse is disconnected), so all that behaves like it should (I had once the problem of LPG intermittently working, and not working, when at that time, the LPG light would be off - this reminds me of few visits to LPG "specialists" at that time, who could not even detect the existence of that fuse, because it was taped with some other wires... and at the end I found it, cleaned the contacts, and the car made another almost 100.000 km without a single problem, until today.

So, I like the car and don't want to discard it (in spite of 400 EUR tax every 3 months...), and I would like to determine where the problem is: but where to go (and not spend hundreds of EUR for "specialist" or specialist work), is there a place that can utilize that "diagnosis" connector as I assume it is there to be connected to some diagnostics equipment, or use common sense and real knowledge of automotive technics to find out the cause for this?

I think that maybe with those changes of fuel while I was driving I might have damaged the computer if the switching was attempted very fast, without letting the car's electronics in that computer to really adjust on the change, on the other hand, I tried afterwards to switch it manually with a bit higher rpm when running on petrol (car standing still), when after changing to LPG engine goes off, immediate touching of the sensor to convert to petrol will make it alive, and so on few times, with no problems, it will always react as ususal, immediate change of light from red to green and vice versa etc etc.

I live in Den Haag - can somebody advise what would be the best approach to resolving this malfunction of the LPG? Thanks to those who had nerves to read all this...