Does LPG affect O2 sensors differently?

[capt jack]

Regulars on here will have seen quite a few threads recently on poor idling on LPG, engine management lights and fault codes - all with converted engines.

The common themes are that the cars run fine on petrol, but not so well on gas, and that the problems are intermittent.

My own V70 has ben playing up for a few months now. It's been hunting at idle, running rich, running lean, triggering fault codes etc etc - always on LPG.

Pistrix and Volvobaggen have reported similar issues, albeit with new conversions rather than my 280,000 miler!

My problems have been improved (but not resolved) by fitting new injector solenoids and a new gas regulator, together with a series of re-maps.

Others on here have reported various repairs - new temperature sensors, re-maps, etc etc, again which have given some improvement but not a complete cure.

I'm now begining to wonder if it's possible that O2 sensors work the same with LPG as they do with petrol?

My car now runs absolutely fine on petrol, and for most of the time on LPG. But sometimes it throws a fault code - always 2-2-1. I can go for hundreds of miles without a light, or the light can pop on literally every few hundred yards. There is no pattern.

According to the fault code table I have this means 'Lambda Operation; also Heated oxygen sensor (mixture too rich under part throttle)
/ long term fuel mixture too weak in part load stage'

In other words, the Lambda sensor might be faulty, or the mixture might be too rich, or it might be too lean!

I ruled out a faulty sensor, because it's OK on petrol. But I've had the LPG re-mapped and as far as we (the LPG guy and me) can see it's absolutely spot-on. But with the fault code being so random, I'm begining to wonder if this is the first sign of the Lambda sensor failing? Trouble is it's not a cheap option just to fit another, so on a 16-year-old car with almost 300,000 miles up, I'm reluctant to start swapping bits just to see what happens.

Could it be that when Lambda sensors start to fail, problems are more apparent on LPG than on petrol? If that is the case then I'd feel more inclined to renew the sensor.

The thing is, this might also explain some of the other problems reported on here. Pistrix and Volvobaggen have new conversions, but the problems are the same as mine. But if Lambda sensors are more sensitive to LPG than they are to petrol, it might just explain things regardless of the age of the conversion.

Any ideas anyone?

Cheers

Jack

PS: I ran the local main dealer yesterday, but they said they couldn't help at all because they haven't got the right diagnostic software any more!

PPS: I am tempted just to ignore the fault light, but I've found that if I do then the car starts to run a bit ragged on LPG, then not all, then the petrol performance starts to suffer, presumably as the petrol ECU tries to puzzle out what's happening. The only solution then is to disconnect the battery and wait for 10 minutes!

[classicswede]

99% of the time Lambda sensors fail in one of two ways.

1) Sooting,

the sensor becomes clogged up with soot/carbon and ends up showing a lean reading constantly and the ECU will keep richening the fuel mixture to try and compensate. The first sign is increased consumption of fuel and can be checked via a scan tool. In OBD11 cars it can be either the pre or post cat probe causing the problem.

2) circuit failure,

a break in the lambda circuit can often go un noticed. The reason for the is is that the cars ECU will read a default correct mixture reading of 0.45 volt (with a 0-1v probe) this can easy to detect via a scan tool or any reader that connects direct to the sensor as you will have a static reading.

Cars running on LPG are less prone to failure type 1 as there should be a lot less carbon in the exhaust.

All the cars mentioned are 10valve with pre OBD11 engine managment and the early systems can be a problem when running on LPG.

Ignition problems can add to the idle issue as can incorrect fuel mixture. I have a 10 valve in with me with the exact same problem and I'm going to try a couple of things to sort it out. I have a few idea's and will let you know what I come up with as I'm sure it will be of interest to all that are suffering from the problem.

[capt jack]

Thanks for that Dai.

It would be good to get to the bottom of this.

I've had to give up running on gas - when LPG puts the EML on the car starts running rough on petrol, and I can't afford to take a chance on it letting me down altogether.

I don't think that my LPG system is fundamentally at fault, and I'm not sure there's any problem with the petrol system or ignition. My feeling is that given the mileage that the car has done, there's sufficient wear and tear within the two systems to create an intermittent malfunction.

The LPG injectors and regulator are all new any way.

I reckon that if I were to replace the coil, the plugs, the leads, the rotor, the dissy cap, the petrol ECU, the LPG ECU, the MAP and inlet air temperature sensors, the O2 sensors, the RPM and camshaft position sensors, the knock sensor, the ambient air temperature sensor, the coolant temp sensor, the gas pressure sensor and the gas temp sensor then I might hit on the solution. Of course it could be mechanical in the engine - valves, piston rings etc, or it could be a vacuum leak (I've re-made every vac connection in the engine bay). But to do all these things would cost way more than the car is worth, and might still not resolve the problem!

And I need the car every day for work, or the kids don't eat, so I can't really afford the time to be chasing elusive elctronic faults.

LPG has been fantastic for me - almost 250,000 miles on gas in 8 years has saved me a small fortune. But maybe it's time to admit that the party's over.

Cheers

Jack

[mikealder]

Originally Posted by capt jack

Of course it could be mechanical in the engine - valves, piston rings etc,

A simple compression check would show up the state of poor valve seats/ piston rings etc - Mike

[capt jack]

Originally Posted by mikealder

A simple compression check would show up the state of poor valve seats/ piston rings etc - Mike

Good point Mike, thanks.

The engine is a bit more 'clattery' now than when it was younger, and burns a bit of oil, but all in all, there's no major rumbles and no sign of blue smoke at all, and nothing to suggest head gasket problems.

But it has done 280,000 miles, so I'll forgive it the odd growl and grumble!

Cheers

Jack

[pistrix]

Hi Dai,

I am still hoping to find the reason for these idle oscillations. Did you maybe found anything with the car you mentioned?

Thank you.
Jan

PS: My Volvo is 20V (1995)

Originally Posted by classicswede

I have a 10 valve in with me with the exact same problem and I'm going to try a couple of things to sort it out. I have a few idea's and will let you know what I come up with as I'm sure it will be of interest to all that are suffering from the problem.

[capt jack]

Hi Jan

I think that Dai came to the conclusion that the Lambda sensor on that 10v was dying, and so he'd planned to replace it. I think he found that he was able to measure different output voltages from the Lambda when switching between the two fuels.

Why a failing Lambda sensor would see a problem on LPG but not on petrol is a bit of a mystery.

I wonder if it's something to do with the fact that LPG operating temperatures are higher than petrol, and that this in some way affects the way a Lambda sensor works, but that this is only significant when the sensor is beginning to fail. I guess the other possibility is that the products of LPG combustion are different to the products of petrol combustion, and that these chemical differences in some way negatively influence a failing Lambda sensor. I guess as we're talking about very small voltage changes then maybe it doesn't take much to upset the system.

I've done several Google searches on this, but can't find anything that helps.

Cheers

Jack

[pistrix]

Thanks Jack,

if I was only sure that changing the lambda sensor would help... So, have the oscillating idle problems disappear when Dai replaced the lambda sensor?

I suspected that lambda sensor might cause the problems but everybody were telling me that if it works on petrol it should also on lpg. Since the lambda sensor is one of the most important sensors in the feedback loop of the control system, it is a strong candidate that could cause oscillations. Through time, deposits build on the sensor which may cause delays in the response. And delays in the feedback loop are nr.1 factor for a closed loop system to oscillate.
But the question about why only on LPG and not on petrol remains. My logic that could explain this is that since LPG burning is much more sensitive than petrol burning, the gain of the ECU, controlling the fuel mixture based on the lambda sensor, is too high for LPG. Together with a time delay from lambda readings, this could cause oscillations. I am sure that if we could decrease the gain of the ECU, the oscillations would disappear. Unfortunately we cannot do this but we can only try to make the rest of the closed loop system as "pure" as possible.
I believe that the answer to the remaining question about why the oscillations occur only when the engine is cold or when the temperature change, lie in the ECU strategy of dealing with temperature changes...

Cheers,
Jan

[capt jack]

Hi Jan

I don't know if Dai has cured his wobbling 10v with a new sensor yet, but I'm sure he'll post the results on here in due course.

Like you, I can't really understand why a lambda sensor would be fine on petrol, but not fine on LPG, if we can assume that the LPG is in fact working OK.

But I've been reading up on this and there may indeed be a reasonable explanation. You have to think about how a Lambda sensor works, and what would cause it to signal a fault.

Basically, the sensor is measuring the amount of oxygen in the exhaust gases. It does this by means of monitoring a continuous chemical reaction in it's core. This reaction produces a small voltage, and if the amount of oxgen available is within pre-set limits then the voltage produced is within range that the car's ECU measures and recognises is OK. If the amount of available oxygen for the chemistry in the sensor is either too high or too low then the voltage produced will go out of the acceptable range and the ECU will try to correct things by changing the amount of fuel. It does this by changing the rate at which the injectors open and close. If this doesn't correct the oxygen content then the engine warning light will come on.

The ECU controlling the LPG injectors can receive instructions from the car's own ECU to alter the LPG/air mixture.

In theory, the type of fuel being used shouldn't make any difference because with the correct fuel / air mixture, there should be no difference in the oxygen in the exhaust gases between petrol and LPG.

So the light will come on with LPG but not petrol if the sensor detects the wrong level of oxygen in the exhaust and is unable to get the car ECU to tell the LPG ECU to change the mixture.

This can happen if:

There is a loss of communication between the two ECUs - a simple elctrical malfunction
There is insufficient LPG available to add to the mixture (LPG pressure too low, injectors blocked)
There is too much air getting in to the inlet manifold (vacuum leak)
The LPG is not igniting properly so unburnt fuel is replacing oxygen in the exhaust(timing wrong, spark plugs wrong, ignition wrong).

The process can be cheated if the LPG system is so set up as to trick the oxygen sensor into thinking that the oxygen level is always correct.

Everything needs to be working within specification in order for the system to work properly. But something not quite right with one parameter may be masked by another being out of spec too. For example, the engine may be running hot, which could normally mean that the mixture is lean. But if the LPG ECU can compensate for this by slightly increasing the fuel supply then you'll never know, until it reaches a stage where the LPG ECU can't add any more fuel. Thus it looks as though you've a lean fuel problem, when in fact the engine is running hot. The overheating problem may be down to inefficient cooling or an incorrect grade of spark plug!

On a car with full OBD11 engine management you can see all the critical sensor values in real time, but on the 10v cars this isn't possible, which makes diagnosing problem more difficult. The car ECU can see that something is not to specification, but it can't tell you exactly what or by how much.

I'm now finding that my car will be OK on petrol and on LPG with Denso K20PRU plugs, but will throw a lean fault code on LPG if I use NGK BKR6E plugs. On the face of it the two plugs should be interchangeable. But when you investigate more closely the NGK plug is a 'cooler' plug than the Denso.

Add to this the fact that the car ECU is 16 years old and done 280,000 miles, the LPG ECU is 8 years old and done 225,000 miles, and they are connected together by a wiring harness that's wrapped around an engine bay that can be hot, cold, damp and dry all at the same time. You can quickly see the potential for problems when the sensors are measuring millivolts! How many lap-top computers would carry on working in these conditions!

Add to this even some slight mechanical wear on valves, piston rings, fuel injectors etc etc and again, the potential for problems grows and grows. Even variation in fuel quality is going to make a difference. And LPG quality does vary.

As our cars get older, and the mileages climb I guess that we've just got to learn to keep tweaking things a bit to keep up.

Jack

[classicswede]

There are a number of things to do and check with the 10valve idle problems. If you are having problems with your 10 valve please contact me directly and provide make and Model of the system you have fitted.