Oscillating idle

[pistrix]

Jack,

the difference between your problem and mine is that my LPG system is NEW.

btw, how do you re-map the fuel trims? I thought these are burned in the ECU and can not be changed but only reset by unplugging the battery for some time.

In a way I am happy that my car is not the only one having this problem. This gives me a bit more hope that we will find the cure. I am looking forward to hear from you after you change the regulator.

Best,
Jan

[capt jack]

Originally Posted by pistrix

Jack,

the difference between your problem and mine is that my LPG system is NEW.

btw, how do you re-map the fuel trims? I thought these are burned in the ECU and can not be changed but only reset by unplugging the battery for some time.

In a way I am happy that my car is not the only one having this problem. This gives me a bit more hope that we will find the cure. I am looking forward to hear from you after you change the regulator.

Best,
Jan

Hi Jan

If your conversion is new then I reckon you need to take it back to the installer. He will have the software necessary to re-map the LPG ECU, which he should be able to do as you drive the car. He should be able to connect a laptop computer to the LPG ECU and adjust the fuel trims as you drive according to the engine load.

LPG is a great way of saving money, so I hope that you get it fixed quickly!

Cheers

Jack

[pistrix]

Thanks Jack,

actually, they did this re-mapping of LPG several times but nothing changed at all with regard to idle oscillations. I thought you meant re-mapping of the petrol fuel lines...

I also hope to get this fixed but I am becoming quite desperate...

I will do some experiments tomorrow morning and post the findings here...

Cheers,
Jan

[capt jack]

The idle oscillates because the ECU is getting incorrect information about the mixture, and keeps trying to adjust.

I don't know your LPG system, but I'm guessing that somewhere it will have a vacuum connection to the throttle or inlet manifold.

If there is a vacuum leak then the ECU will be trying to compensate.

Try a careful and thorough check of all the fuel hoses and any vacuum lines to see if there are any leaks. Also, check that the LPG filter is OK.

It's possible too that the LPG system has a MAP sensor, which will have both a vacuum and pressure port - have a check there for any leaks too.

If the engine runs OK on petrol then that would suggest that either something is wrong with the LPG set up, or something is wrong with the ignition system. Petrol is far more forgiving of ignition faults than is LPG, and sometimes problems with the ignition will show up with LPG but be OK with petrol. You've already replaced the major ignition components, so my money would be on a small vacuum leak in the LPG fuel system, or insufficient LPG pressure.

Cheers

Jack

[pistrix]

I have Prins system that has no vacuum connections... I anyway checked all the vacuum lines and they seem to be ok. I also rechecked the throttle body, MAF and IAC valve...

Your words about ignition made me think... The only Ignition component I haven't replaced is the Ignition Coil. It looks fine, I measured the resistances of the primary and secondary windings and both are exactly as they should be (0.5 and 8000 ohms). However, if I put my ear very near to the coil, I can hear a buzzing sparking sound. I then checked the coil in complete darkness and I saw that there were some tiny sparks going from the coil to the metal frame. I guess this is not normal and it might cause my problems... What do you think? In general, is there any correlation between the cold engine and bad ignition coil? Could it be that when the LPG is cold, it is harder to ignite it and the weak sparks due to bad/weaker ignition coil cause the problem?

Thanks,
Jan

[capt jack]

Sparking in the dark is generally a sign that all is not well with an ignition component.

To be honest my understanding of ignition coils is more based on the old cylindrical type used in older cars, but I'd guess that as these older type coils would exhibit fault symptoms at different temperatures and different loads, then their more modern successors can too. Electrical resistance does change with temperature - that's how temperature sensors work, and as coils are all about resistance then I guess it's reasonable to deduce that there can be failures at one temperature that aren't apparent at a different temperature.

Basically, you'll get poor running from LPG if the spark is less than perfect, so if the coil is faulty you will definitely get a reduction in the spark. The ECU may then read the unburnt fuel as running too rich, and so try to lean out the fuel mix - which would indeed give you oscillations in idle as the ECU tried to compensate for what it thinks is too rich and then too lean a mixture.

If this is happening then I think that it would show up on LPG before it did on petrol.

I'm not 100% sure of my logic, but I do think it would be well worth carefully cleaning up all the connections to the coil, and if you can, try a different coil.

Cheers

Jack

[pistrix]

Thanks Jack for your explanation. I guess I'll just have to buy a new ignition coil.

The logic about resistance and temperature is probably not so relevant. The resistance of the wire used in transformers change too little to affect the quality. Of course, increased resistance may hinder the performance of the coil but I don't think temperature has a significant role here (to be sure, I measured resistances at 5C and 20C and they were the same). I think the problem with my 17 years old coil is the insulation around the windings. Probably it got porous with years and this causes the sparking from the high voltage secondary winding to the metal frame. This sparking for sure takes some energy that would otherwise be delivered to the spark plugs...

What I would like to understand is whether cold gas in cold engine needs a stronger spark than warm gas in warm engine or than petrol in cold or warm engine. If this holds than a "weaker" ignition coil might cause problems in cold conditions. Your explanation about how weak ignition can cause oscillations is very smart - I think it is very reasonable. It gives me hope that spending money for a new ignition coil won't be for nothing...

[capt jack]

Originally Posted by pistrix

What I would like to understand is whether cold gas in cold engine needs a stronger spark than warm gas in warm engine or than petrol in cold or warm engine. If this holds than a "weaker" ignition coil might cause problems in cold conditions. Your explanation about how weak ignition can cause oscillations is very smart - I think it is very reasonable. It gives me hope that spending money for a new ignition coil won't be for nothing...

The LPG shouldn't come in until the engine is sufficiently warmed up. This is because the LPG regulator uses the heat from the car's water system to vapourise the LPG. If the temperature of the gas is such that it's not fully vapourised and being delivered at the right pressure, then you'll get running problems on LPG.

Just a thought - what spark plugs are you using? I know that the only plugs my car will run on are the standard single-electrode copper-core plugs. If I try to use anything else - the 'multi-electrode' type for example, then the engine will not run very well on LPG.

I use either Denso or NGK plugs. I've found that Volvo, Champion and Bosch plugs simply don't last for very long. One set of Bosch Super+ plugs I fitted lasted less than 3000 miles.

As we've said previously, LPG exposes ignition weaknesses far faster than petrol does.

Cheers

Jack

[pistrix]

I use NGK BKR6ES as per your suggestion

I just ordered a new Ignition Coil. I will get it next week and then we'll see.

Cheers,
Jan

[volvobaggen]

Damn, I got the same problems as you guys now. Engine starts and runs very nice on petrol, idle is at about 750 rpm. Engine switches over to lpg without hickups, but when I stop before a roundabout 300 metres later I have had engine stallls and oscillating idle from about 800 down to 300 rpm. When engine is warm and nice I have not noticed these problems.

If I switch to petrol just before I let the engine idle while it is not fully warmed up, there are no problems, idles down at 700´ish.

I cleaned the iac and throttle body today, and I think the car runs smoother at idle. Thats the only improvement I had, engine still stalls when its not fully warmed up.

The first time I noticed the problem was on a very cold day 2 weeks ago, so I believe it somehow related to outside temperature...


Edit: Next step will be checking spark plug gap tomorrow and gap them a bit down. Maybe mixture is borderline lean in these stalling situations and ignition system is having problems.