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Oscillating idle

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Old Nov 20th, 2012, 10:55   #21
capt jack
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Originally Posted by volvobaggen View Post
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 700īish.

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.
Yes. That's exactly the symptoms with mine too. I'm changing the LPG regulator on Friday, so we'll see if that cures it.

How old is your conversion, and what make is it?

Cheers

Jack
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Old Nov 20th, 2012, 14:04   #22
volvobaggen
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my system is from elpigaz, itīs a polish make I believe. It was installed in April this year, so itīs quite new.
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Old Nov 21st, 2012, 10:40   #23
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I did some searching about our problem and found some quite relevant discussions:

Post 12 and 13 (I guess this does not directly apply for our cars since they have hydraulic tappets):
http://www.subaruforester.org/vbulle...ing-lpg-35882/

Various causes but more or less "trivial":
http://www.aussiemagna.com/forums/ar...p/t-77298.html
http://forums.mercedesclub.org.uk/ar...p/t-10875.html
http://forums.whirlpool.net.au/archive/1849550

You might go through these discussions and maybe you will get a hint... I got a few:

- there might be some valve clearance problems when the engine is cold and the gas leaks... I don't know how to check this.
- some not-metered air comes to the intake manifold... But from where? False vacuum solenoid valve?
- could it be some vacuum leak... But why only when engine is cold?
- the air filter is dirty... Again, why only when engine is cold?
- maybe someone could help on this forum: http://www.lpgforum.co.uk/


The temperatures are getting lower and lower...
Jan
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Old Nov 21st, 2012, 15:12   #24
volvobaggen
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The first time I had the stall episode on lpg, I got no engine mangament light or error codes. I drove for another 450 km without problems.

The car was not used for 2 weeks, the engine managment light came on for the first time when after I drove 500m and left the car to idle while I checked the tire pressure. I could not hear any rough idle or oscillations, but when I climbed back in the car, the light was on.

I drove the car with check engine light for 300 km without noticing any symptoms from the car. I didnīt manage to read codes because my OBD2 doesnīt work with the stupid fenix 5.2 system. So I just cleared the code by disconnecting the battery to see if the car threw any new codes, but it has been quiet now. But idle issues persists.

I suspect either lazy oxygen sensor clogged with carbon giving mixture issues and ignition faults. I have not had the time to check the plugs yet, but I will install the slightly hotter Denso K20PR-U with 0.6mm gap.

Thanks for your links pixtrix, but the first subaru link about valve clearances and valve seat recession doesnīt apply to our cars, our valve seats are hard and not made of butter like those of the subaru.

I believe in a fault in the category "more or less trivial", plenty of good information in those links.
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Old Nov 23rd, 2012, 13:40   #25
capt jack
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Well my new regulator is fitted and working, and so far so good.

I think that the old one was indeed simply worn out - it had done 220,000 LPG miles, so I reckon I can forgive it being a bit tired!

When it came to setting everything up again after the regulator had been fitted some judicious remapping was needed, but certainly having driven the car home 40-odd miles, it is much much better, and certainly no oscillating.

The probelm seemed to be caused by the LPG regulator delivering gas irregularly. This would cause the car's ECU to think that either the mixture was too rich or to lean, so it would try to adjust the engine speed. But in doing so the mixture would then change, so making the ECU think that it needed to adjst the engine speed again - and so it went on, up and down!

When the system was re-configured, very close attention was paid to getting the injector speed of the LPG injectors correctly matched to the corresponding injector speed of the petrol injectors, and also making sure that the gas pressure was correct. The whole process took the best part of 2 hours.

As I say, the root cause of the problem would seem to be the fact that the system in my car was just old and worn out. However, having seen how carefully the controls software was set up again after the new regulator was fitted, I can't help but wonder if the other two problem cars reported on here simply need some time spending with an experienced technician.

Cheers

Jack
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Old Nov 23rd, 2012, 18:54   #26
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Congratulations Jack.

I still believe the guys that fitted my Prins system are indeed experienced. However, it worries me that, as I was told, they can only adjust two parameters on a Prins system - mapping (ratio) of gas vs. petrol at idle and when driving. If your suggestions are true also for my case than I believe that these two parameters are not enough to solve my issue.

I thought Prins was one of the best lpg systems!?!

I hope you won't completely forget about this thread.

Cheers,
Jan
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Old Nov 23rd, 2012, 21:15   #27
capt jack
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When the technician reset everything for me today, he was able to use the LPG software firstly to look at the petrol ECU settings for injector timing, and then use this information to set the injector timings for LPG.

We then went for a drive with the laptop connected, and he was able to adjust the fuel trims at different engine speeds and loads.

When we got back to the workshop we switched between gas and petrol, and there were clearly problems in doing so. When switching back from gas to petrol the idle would fall away. There was a diagnostic function in the software, and this identified that the injector nozzles were too big. This meant that the new regulator was delivering too much gas. We changed the injectors for a smaller bore and tried again. There was a definite improvement, and it was possible to adjust the LPG fuel trims to give a better changeover to petrol. However, the diagnostic function again indicated that the nozzles were on the high side of correct, so we swapped them again for even smaller bore ones. This time everything worked out fine.

It could be that with your new installations, the regulator and injectors have loosened up a bit and just need re-checking.

The reasoning goes like this:

The petrol ECU runs the car fine on petrol. On LPG the O2 sensors detect that the mixture is rich and adjust the petrol trims accordingly. However these trims are wrong for petrol so the cars runs poorly when next used on petrol, until the petrol ECU recalculates the fuel trims. These will be correct on petrol of course, but wrong for when the car is next used on LPG. So in effect, because the LPG is too rich, the petrol ECU keeps getting the wrong information and is always trying to compensate.

The answer is to make sure that the LPG fuelling is within the proper limits. If not then when you start the car cold on petrol, or when you switch to petrol from LPG, the idle will drop for a while, and then recover. On LPG you will get an oscillating idle.

It's possible to test and see if this is indeed the problem:

1. Start the car on petrol and let it switch to LPG automatically. The idle on LPG may well be uneven. Switch manually to petrol and the RPM may dip at first, then recover. Run the car on LPG for a few miles, the switch off. When you next start on petrol, the RPM at idle may be lower than usual - 750 or less, but the idle will be steady.

2. Start the car on petrol and let it warm up fully, then manually switch to LPG. It should idle OK on LPG. Run the car for a journey of several miles, and then for the last five minutes of the journey, manually switch back to petrol. The petrol ECU should have chance to recalculate the petrol trim. When you pull up, allow the car to idle on petrol. It should be fine with no dip in RPM.

Next time you start from cold on petrol, the RPM should initially rise a little above 1000, but quickly settle to around 900. The RPM should not dip.

What you're doing is trying to show that if the petrol ECU is allowed to recalculate the trims for petrol immediately after it's been running on LPG then the problem goes away.

If however the petrol ECU doesn't get a chance to recalulate the petrol trims afetr running on LPG, then it gets confused next time you start the engine on petrol, and this continues when the car switches automatically to LPG, which then causes the oscillating idle on LPG.

This trick of finishing every trip on petrol provides a short term solution and allows you to use the car on LPG some of the time. However, it's not good in the long run, and will probably cause the engine warning light to come on. The fault code will show a long term fuel trim problem. If you clear the code the warning light will go off, and the car will run fine on petrol. If you go back to LPG then the fault code will come back eventually.

The real answer is to get the system set up by someone who knows what they are doing!

If you're prepared to have a try yourself then this is the sort of kit you'd need, plus of course a lap-top computer. http://www.ebay.co.uk/itm/Prins-VSi-...item53ed9a890d

The other thing to do may be to get in touch with Prins direct.

This is my own understanding of the system, I'm not an LPG technician so please understand that I might be bit wrong on some details! I do hope it helps a bit though.

Let us know how you get on.

Cheers

Jack
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Old Nov 24th, 2012, 11:49   #28
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Thanks Jack for your effort.

I understand your point of view and the method how to show that the oscillating idle is due to LPG setup. However, how can this explain that the idle only oscillates when the engine is cold after the engine had enough time to cool down (~8 hours when outside temperature is ~10C). In summer or when the engine is warm, the engine runs perfectly with both petrol and lpg, at idle or when driving, when the engine is "cold" or warm... It actually runs perfectly without absolutely any difference between petrol and LPG. If your reasoning would hold for my case, than I would have problems regardless of the temperature. Am I missing something here?

I anyway tried this morning what happens when the idle oscillates on LPG and I switch back to petrol. There is no rise or drop of RPM. The RPMs simply stop to oscillate. If I switch back to LPG, no drop or rise of RPM as well. It immediately starts to oscillate around the petrol RPMs. Sometimes the oscillations dampen but most of the time they remain until the engine is warmer. When the engine is warm and I switch from LPG to petrol or the other way, you can hardly see any change in RPM.

Also, when I started the engine this morning (after I drove on LPG yesterday an ECU didn't have time to recalculate fuel trims for petrol), the RPMs initially raised a little above 1000 and then fell to about 900 like you suggested that it would be in case when ECU had correct trims for petrol.

What I also tried once is that I reset the ECU first thing in the morning but the problem was still there...

I also noticed that sometimes the oscillations are stronger and sometimes (very very rarely) there are no oscillations at all or only a couple. However, I can not figure out any correlations...

Cheers,
Jan
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Old Nov 24th, 2012, 15:26   #29
capt jack
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Hi Jan

Good point about your problem only being noticeable when the engine is cold.

Which makes me think....

The oscillating idle is caused because the engine management system is first opening the throttle and then closing it.

It's doing this because at idle when cold, the mix of LPG and air is wrong, and can't be compensated for within the normal parameters.

But when the engine is fully warmed up, everything is fine on LPG. The probelm is not the same as I had, because the petrol system is unaffected on your car.

So, the question is, what is different about the system when it is idling relatively cold on LPG compared with when it is idling hot?

For the LPG to be properly vapourised, there needs to be water from the cooling system flowing through the LPG regulator at a high enough temperature. Before the engine is fully warmed up then this may not happen properly. A new coolant thermostat might be the cure for this. Also, getting the LPG system re-set so that it switches to gas at a higher temperature might help.

When the engine is idling in cold conditions, air should be drawn into the air intake via the trunking which passes over the exhaust. This means that the air is pre-warmed. This in turn is controlled by a thermostatic flap in the air filter. Therefore, could this thermostatic flap be faulty?

There are three temperature sensors feeding information about the ambient and engine temperatures to the ECU. There is the coolant sensor in the thermostat housing, the ambient sensor under the front bumper, and the air intake temperature sensor in the duct that runs from the air filter housing to the throttle body. One of these might be faulty. Maybe not enough to trigger a fault code yet, but not working properly nevertheless.

Finally, the LPG fuel rail must have a gas pressure sensor and a temperature sensor. The temperature sensor must feed back to the LPG ECU, and the pressure sensor must somehow be referenced to the inlet manifold pressure, so there must be a pressure sensing / switching device which also then feeds back to the LPG ECU.

If one of these components is faulty then it will cause a problem. If none are faulty then the cause of the problem must be that the LPG system cannot supply gas at the right pressure and flow rate to allow the engine management system to believe that all is well. This comes back to the LPG ECU not being correctly set up.

The thing is that you could spend a lot of money changing sensors to see if it makes any difference. But what you could do for free perhaps is to make sure that the electrical connections to all the sensors are clean and good. I wonder if it would help disconnecting each one in turn with the engine idling on LPG, to see if this has any effect on the oscillations?

What do you think?

Jack

One more thought - how easy would it be to strip down and clean the LPG injectors? I wonder if one is partially blocked?

Last edited by capt jack; Nov 24th, 2012 at 15:30.
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Old Nov 24th, 2012, 17:15   #30
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I have also wondered what would be the symptomes if there is some contamination in the vaporizer or in the injectors.

I donīt know when your systems switch to lpg, mine does it almost at the exact place down the road after about 1-1.5 kilometers. I canīt see that the temperature gauge in the dashboard has moved much at all from cold. The dry vaporised gas cannot be very varm, and maybe there is some contamination of heavy oil in or after the vaporiser that needs to be heated for the system to work?

I opened a thread at lpgforum.co.uk, if you guys want to see, here is the link.
http://www.lpgforum.co.uk/viewtopic.php?f=28&t=11847

The first modular 10v engines could have a problem with running lpg on cold engine, but mine is sadly not one of them (1996)
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