Oscillating idle

[pistrix]

Jack,

answers to your questions:

I replaced the thermostat.

The temperature when lpg is switched on was first 45C, then 50C and now 55C. It doesn't help, just decreases the time of oscillations. After all, this is not a cure.

I just checked the thermostatic flap and it is working properly.

I replaced the coolant sensor.

I cleaned MAF sensor that also calculates the inlet air temperature.

The ambient sensor seems fine - the temperature readings at the dashboard are correct.

There are no pressure nor temperature sensors at lpg rail; only injectors

Prins has no pressure sensing at intake manifold or at least I can not see anything.

I checked and cleaned all connections to sensors. I also tried to disconnect the MAF sensor and the engine stopped immediately.

I don't know how to check injectors. And I don't want to, since the system is under warranty.

My only hope now is the Ignition Coil that I will get next week. It's a weak hope but still...

Unfortunately, I am loosing my patience... If the Ignition Coil won't help, I don't know what else I can do. If I could only know whether the problem is with the engine or lpg system.

Thanks for the link volvobaggen.

Jan

[pistrix]

Hi,

still thinking and dreaming of my cold oscillations...

this morning, the engine ran absolutely without any oscillations. The only thing that was notably different today than most of the other days is that it was very wet and humid. How could this influence the idle? The only thing that I could think of is MAF sensor...

Any ideas?

Also, after Jack explained his logics about how to debug the possible problem with wrong LPG settings, I tried to switch between LPG and petrol several times when the engine was cold and warm. What I noticed is that when I switch from LPG to petrol, there is a short dip of RPMs which in a couple of seconds stabilize at the same level as with LPG. When I switch back to LPG, the RPMs rise for a couple of seconds and then settle back at normal. What can I infer from this? Is LPG to lean or too rich?


Cheers,
Jan

[capt jack]

Not sure about the humidity affecting things, but I do think that possibly your LPG is running too rich?

The logic is as follows:

Car is running on LPG, so the petrol ECU reads the situation as 'normal'. You switch to petrol and because the LPG mixture was too rich, the petrol ECU suddenly thinks that there isn't enough fuel, so the RPM dip a bit. Then the ECU adapts to the new petrol fuel mix and speeds up slightly.

When you are running on petrol, the petrol ECU thinks all is well. Then you change to LPG. For a few seconds, there is more fuel, so the engine speeds up a bit, but then the petrol ECU realises that the mixture is now richer, and slows the RPM down again.

This logic makes sense (I think!) if it is the case that even when running on LPG, the petrol ECU still manages the engine by reading the oxygen sensors, and can make adjustments to the engine speed. This logic doesn't make sense if the petrol ECU no longer manages the engine when it's running on LPG!

I think though that with an LPG conversion, the LPG ECU is the 'slave' of the petrol ECU, and the petrol ECU still controls all the engine's sensors and trims, with the exception of the petrol injectors.

The LPG ECU only controls the LPG pressure and the LPG injectors, but not things like the signal from the oxygen sensors etc.

Don't know if all that is absolutely correct, but I think it could explain your RPM dips and surges, and might explain the oscillation if the LPG is always running too rich.

Cheers

Jack

PS: The petrol EC takes information from the oxygen sensor about the contents of the exhaust gases, and compares it with data for an optimum fuel mixture. If the atmospheric humidity is such that the exhaust gases effectively become 'normal' in accordance with what the petrol ECU would expect to see, then I guess yes, extreme humidity could fool the petrol ECU into thinking things are OK, and so no oscillation. I don't know if this makes sense in terms of the chemistry and physics of the process! Jack

[pistrix]

Thanks Jack,

actually, it is as you said, the petrol ECU is all the time managing the engine while the LPG ECU is a slave simply copying the ECU output.

are you saying that when you switch between LPG and petrol in your car, the RPMs do not go neither up nor down?

When I rev-up the engine on LPG from idle (either warm or cold engine) the engine makes a hesitating sound with "puffs" from the exhaust. What would this mean?

Cheers,
Jan

[capt jack]

Hi Jan

As I understand it, when you switch between LPG and petrol at idle there may be a very slight change in the RPM, but it really should be barely noticeable.

Last week when I first had my new regulator fitted the changeover between the two fuels at idle was perfect - you could not tell from one fuel to the next.

I ran the car over the weekend and everything was fine.

Since then however I'm now really struggling to get the car to run on LPG however, and so it's going back again on Monday to be checked over!

Jack

[capt jack]

Well, back once more to Autogas 2000 - and this time Bingo! We found a good old-fashioned mechanical explanation as to why I've been struggling to get the car to run on LPG.

One of the brand new LPG injector solenoids was knackered! This clearly would have caused mis-fueling, with all the ramifications that would bring in terms of poor running and idling, and the engine management light.

Which just goes to show that you should never assume that just because something is new there can't be anything wrong with it!

To say I'm pleased is an understatement. I was on the verge of giving up on the old bus and opting to take a company car. I'm sure a shiny new motor would have been lovely, but it would have made a serious dent in my pay packet as I'd have lost my car allowance and been stung for more income tax.

300,000 miles here we come!

Jack

[pistrix]

Great news Jack.

I've also found a major issue on my engine but I will see tomorrow morning if it is actually connected with my oscillating issue.

Anyway, I installed a new ignition coil but it didn't help at all. When I changed the coil I also checked the spark plugs and reduced the gap from 0.8mm to 0.7mm. I noticed that the spark plugs were completely black so I concluded that the engine must be running rich. Since I cleaned the MAF sensor and the throttle body, replaced the coolant sensor and checked the O2 sensor, I had no idea why rich... Than I recalled that when Jack suggested to check the thermostatic flap, I had to heat it with the hairdryer for quite some time to open the flap a little bit. At that time I thought this is normal because I thought that actually it is the hot exhaust air that is feed back to the air filter (how stupid of me!). Then I realized it is only the heated air from around the exhaust and that it will never be so hot to open the flap...

So, I googled a bit and it didn't take me a long time to realize that my poor Volvo was breathing air through that tiny tube for years!!! I remember years ago when my car was getting sluggish and I thought it was just because it is getting old...

So I reset the ECU, removed the air box thermostat and put a piece of 15mm tube instead to keep the flap open for cold air and went for a test drive on petrol... I was instantly amazed. It felt like I was driving another car!!! - the performance improved significantly! Then I tried to switch to LPG but the settings were obviously completely off; the idle was oscillating even when the engine was hot, acceleration was not smooth at all,... so I decided to keep driving on petrol and ask the Prins service to recalibrate the LPG ECU. They recalibrated the ECU today and now the car is running perfectly on LPG. BUT, I have to wait and see how it will run tomorrow morning. What is still bugging me is why there were oscillations only when the engine was cold... I will report here.

Anyway, I am sure many Volvos are driving around without fresh air. To me the design of that thermostatic flap is definitely a flaw and should be replaced for free by Volvo! The default position of the flap should be opposite so that if the thermostat fails, the flap channels the fresh air into the engine and not the hot air through that tiny tube!

Jan

[capt jack]

Sounds like you're making good progress Jan!

Been thinking about your oscillating idle. Whether running on LPG or petrol your petrol ECU is constantly receiving information from the O2 sensor, and if too little oxygen is sensed - ie: the engine is running rich, then the ECU will try to reduce the amount of fuel - which will slow down the idle.

When the idle gets too slow the RPM sensor will tell the ECU to speed the engine idle up!

BUT alongside all this, your car has a MAF sensor from which the ECU calculates the air mass, and will try to make changes to the fuel mixture in response to changes in the air mass. The MAF sensor has a heated wire. The wire is cooled by the incoming air, which changes the resistance of the heated wire, which in turn is seen by the ECU, which then tries to change the fuel mix. The temperature of the incoming air is therefore critical. Or more to the point, changes to the temperature of the incoming air are critical, because it's changes to the intake air temperature that will change the resistance of the MAF heated wire.

So, in order to work efficiently, all the various sensors have to detect conditions that are within the range of parameters that the petrol ECU can adjust to. If one of the sensors is faulty then the ECU will get confused and won't be able to respond properly, and so will cause problems like an oscillating idle. It will do this because it's constantly 'cycling' between reading - first one way then the other.

Also, if one of the key parameters - like air temperature, or air mass, or fuel pressure etc - are outside of the range that the ECU can work with, then you'll get similar problems.

In your case it sounds as though the air mass was wrong. This tricked the ECU into making the wrong calculations and adjustments. Hopefully now that you've identifed that the air mass intake was restricted and resolved this, the engine can operate within the ECU's proper parameters.

From your description of the plugs, I'd say that the fuel mixture was definitely too rich - which means that there was not enough air - which fits with what you found with your air intake.

The thing about engine management ECUs is not what's actually going on, but what the ECU thinks is going on.If everything is working within the pre-set parameters then all is fine, but if one factor goes outside of the pre-set paramters then the ECU is quickly fooled into thinking that something is wrong. The trouble is, it isn't clever enough to work out the problem correctly every time. Therefore if the ECU detects a rich mixture it might try to decrease the amount of fuel, slowing the idle speed. But the real problem is a lack of intake air mass. The only answer it has to this is to open the throttle, causing the engine idle speed to increase - and then you are in to an oscillating idle. Because air is more dense when it's cold, the MAF sensor works OK when the ambient air is warm, but goes out of it's comfort zone when the air is cold.

In other words, when everything is working properly, the system can cope with 'normal' variations. But if just one key factor is outside of 'normal' then the system will try to compensate - but it can only do so within limits, and can only modify certain values. If it has to modify a value outside of the limits then it will cause another problem to occur, which the system will then to to compensate for, and in doing so will cause another problem - and so it goes on and on!

Phew! Now my brain hurts! But I do believe that something like this has been causing your oscillating idle!

Cheers

Jack

[volvobaggen]

Just as a follow up on my idle problems: They are no longer just occurring while warming up the engine. Yesterday I was doing a long run while it was about -22 Celsius outside, and after 80 km, with engine fully warmed up I stopped to stretch my legs. Check engine light came back on again while I was outside lol.

Oscillations are no longer happening all the time idling, sometimes the engine is idling perfectly on LPG.

At one point I had perfect idle while stopping and I touched the gas-pedal and revved at bit to see If I could trigger issues and sure, it started surging when I let it idle again.

I´m starting thinking I should try to replace the IAC-unit, it never moved when I cleaned it, and I have read it is supposed to move when you tilt the IAC unit from side to side. It is always idling perfect on petrol, so this might not work.
I´m gonna build the flash code reader this time also.

To cold to work on the outside now, so you might have to wait a bit for more feedback from me, happy to hear your tractor is now running fine capt jack.

[pistrix]

Hi.

So, my oscillations are almost gone but not entirely. Now the system is set to switch to LPG at 45C and when it switches, the idle is frozen without any oscillations at all. After a while (5 minutes) when the engine gets a little warmer but not fully warm, the idle starts to oscillate but the oscillations are not so strong as before and can be lived with. They dampen after about 5 oscillations but when I press and release the gas pedal they reoccur. This oscillating state then lasts for another 5 minutes and then everything is perfect again. I noticed that sometimes the idle also oscillates a little when the engine is fully warm...

I tried to switch between LPG and gas several times during the warm up:

If I switch from gas to LPG when the engine is not fully warm and the idle is sensitive for oscillations, the RPMs dip a little and recover. When I switch from LPG to gas, the RPMs rise a little and settle.

When the engine is fully warm it happens the opposite: it rises when I switch from gas to LPG and falls when I switch from LPG to gas.

I will try tomorrow to switch between LPG and gas just after the LPG turns on and see whether it will be like when the engine is fully warmed up.

I am not sure when LPG is too rich or too lean (in the cases above) but it is obvious that the situation changes during the warm up of the engine. I am still figuring out how to explain this and how to try to solve it - I will know more tomorrow when I'll try switching when the engine will be cold and the idle does not oscillate...

Also, one funny thing, when the idle oscillates and I switch on the AC, the oscillations stop for a few seconds and then they reoccur.

Cheers,
Jan