Oscillating idle

[Pete_c20]

Hi Jan,
Which regulator/vapourizer is on your Prins system?
One thing that has caused an uneven idle with my S40 was that the spark plugs loosen off in the head over time by themselves. Not sure why.

[pistrix]

It's Prins for max 135kW.

btw. I replaced lambda sensor a few days ago (I got a new and original one at US ebay for 60€ ) and remapped the LPG system. I wonder how the old lambda sensor was still able to work - It was deformed and rusty, literally falling apart. However, the new lambda sensor didn't completely cure my oscillations. For a few minutes after switching to LPG when the engine is cold, the idle still feels unstable if I depress the gas pedal. It oscillates a few times but settles. It is much much better than before I cleaned/replaced all the sensors.

I can live with the system as it is now but I am still not happy. But, I have no idea what to try next. The only thing that comes to my mind is to try and play with mapping myself. Or to contact Prins and ask for help...

I wonder how the idle should behave when the engine is cold and running on petrol!?! With my 850, the idle is not completely fixed at 850rpm if I depress the pedal. It usually falls a bit below and then settles. I wonder if this is normal. This may indicate a general problem that only gets more obvious when running on LPG.

[pistrix]

Hi,

I am still trying to solve my idle oscillation issue and maybe this time I (we) am getting somewhere...

After I cleaned and replaced (almost) everything I could, we hypothesized that maybe the injectors are not the correct size (5 x Keihin orange - they are correct by specifications for my 125kW engine). Since we didn't have smaller injectors to try, we instead decreased the pressure from 2.1 to 1.85 bar and to my surprise, the oscillations are almost gone. The idle still oscillates sometimes but this is more an exception than a rule.

So I wonder if smaller injectors would cure the annoying oscillation problems with my car... Do you have any thoughts on this?

Cheers,
Jan

[classicswede]

Changing the gas pressure or changing the injectors will have the same effect.

By reducing the gas flow you are effectively buffering the changes in injector duration.

If Prins could be mapped that than just adjusting offset and K line then increasing the amount of fuel at injection times below the time for idle would help with the issue.

[volvobaggen]

So winter is over around here and spring is in the air.

LPG switch-over happens after the same distance as before, even if it is warmer in the air. The oscillations are initially after changeover not as bad now, and when the engine reaches fully operational temperature, the oscillations are gone. Spark plugs are the same.

So my conclusion is that my lpg system doesnt like the winter.

[Clifford Pope]

This may not be directly relevant to your car, but I have recently found that the 240 is extraordinarily sensitive to tiny leaks in the crankcase ventilation system.
Mine started to have a very uneven idle on LPG, and I did a lot of messing around with the mixture, apart from rechecking all the usual suspects like plugs, leads, coil, connections etc. Then it started to do it on petrol too.
I suspected the Idle Air Valve, and then the throttle position switch.
After taking off the throttle body, cleaning, checking the TPS it was much better, and the idle was absolutely steady on LPG and petrol.

Then it suddenly reverted, and became so bad that it wouldn't idle at all on LPG.
I took the whole crankcase ventilation pipework off again, wondering if there could be a blockage, and discovered that the pipe from the oil separation box to the flame trap was severely perished. It appeared to quite recently have broken down chemically, and had swollen so much that it was extremely loose on the separator box. The problem was cured with a new pipe.

All my previous attempts at diagnosis by removing and cleaning components had simply disturbed the pipe - sometimes it sealed, sometimes it didn't.