LPG running lean - how will I know?

[RollingThunder]

I'm about to return to the Volvo fold after a break of a few years. Sold my old 850R and regretted it since it was collected

Anyway, I'm looking at a '98 V70R with a certified LPG conversion - it all looks so good in pics (and having spoken to the seller I'm reassured by the service history) that I'll probably buy it at the weekend. Now I've read a number of threads here that refer to cars running normally on petrol but lean on LPG - how do you know that the LPG system is running lean? In my experience running lean (OK very lean) results in melted pistons, so I'm quite concerned!

Thanks as ever, RT

[bongodave]

Hi.

I've also got a 98 V70 running on LPG. The car runs with the exact same power whilst accelerating and switching between LPG and Petrol. If it's running lean on LPG you will notice a difference.

Another, more accurate way, would be to get a full LPG service where they hook it up to the correct bit of kit and check all these things. That costs around £120 for first service tho but many stations half the cost after that.

[capt jack]

If it's running lean on LPG you should get the Lambda light come on at some point.

The O2 sensor will still be looking for the O2 content of the exhaust gases to be within the correct range. If the O2 content goes out of range (because the car is running lean on LPG) then the O2 sensor will tell the ECU which will then try to enrich the petrol mixture. Obviously the O2 content will still be wrong so the ECU will reach a point where rather than try to compensate for the incorrect O2 it will ping the lambda light. You'll probably also find that the car will idle and run poorly on LPG, but be OK on petrol.

So, when you first start the car make sure that the lambda light comes on a switch on, but goes off and stays off at start up. Of course it's possible that a dodgy seller could wire up the lambda light to the oil light or something, so if you've access to a code reader you could also check for any stored codes in the petrol ECU.

Before buying the car go for a nice long test drive, ideally from cold. the LPG should switch in after maybe 2 or 3 miles, and you should be able to switch between fuels manually with virtually no noticeable difference. With the engine idling on LPG switch back to petrol - if there's a noticeable surge or dip in revs then something may be amiss. With it idling on petrol, just blip the throttle enough to get it to switch to LPG. It should return smoothly to idle - if it hunts up and down a bit, or the idle on LPG is much faster/slower than on petrol then again, something may be wrong.

Access to the right software and interface for the LPG set-up would make it all very easy to check, but even without that you should be able to work out if anything is not right.

Good luck

Cheers

Jack

[RollingThunder]

Brilliant explanation Capt Jack, many thanks. A few pointers for me to look at on Saturday

Cheers, RT

[capt jack]

A modern LPG conversion properly done should be nigh on next to un-noticeable. If the pipework is untidy, the electrics tied up with electricians tape and the filler point hanging off the car then walk away.

The installation could and should be so neat that it looks like it was done in the factory.

Driving-wise you should expect the same - the fuels should swap over without any hesitation, and the engine should run equally smoothly on both.

You might, just might feel that the car has a little less oomph on LPG, especially with air-con on, but that's all. Rough idling, misfires, backfires, gassy smells are simply not acceptable

I had an LPG-converted V70 for 9 years and 250,000 miles and it reached a point where I could just feel when things were going off tune a bit! The key with LPG is firstly get it put in right in the first place, make sure you change the plugs frequently (every 10,000 miles - using single pin copper-core plugs - the cheapest!), and then that you get the LPG filter changed every 15,000 miles.

Good luck with the car - hope it's a cracker. Oh and by the way, you can check that it is on the LPG register: http://www.drivelpg.co.uk/

If it's not don't buy it.

Cheers

Jack

[bongodave]

I am soglad you highlighted that about the plugs. Folk spend 70 quid plus on stupid ones which problast 3 times longer, but the copper ones are a tenner! Even changed every 10,000 it works out less than half the price.

[foggyjames]

The trouble with relying on the lambda light is that on a Motronic car (-98), at least, the narrowband lambda sensor (i.e. am I above or below 14.7:1 AFR?) is ignored under elevated load conditions (aiming for circa 12:1), which is when you're likely to do the damage...rather than at idle or on cruise.

I would want to run a wideband lambda sensor on any LPG converted car, for reassurance that the mixture is healthy. I just don't trust installers to get it right, and the consequences of getting in wrong are holes in pistons.

cheers

James

[capt jack]

I'm no LPG expert, but I think I disagree with James a bit about a non-OEM O2 sensor.

The stoichiometric mix for LPG is different from that that for petrol. For LPG it's 15.8 :1, for petrol 14.7:1.

Remember that the throttle pedal on a fuel injected engine works by controlling the amount of air taken in, not the flow of fuel.

The O2 sensor works by conducting a continuous catalytic reaction in the exhaust gases, and a voltage is generated by the reaction. This is measured by the monitoring the voltage generated by the catalytic reaction in around the O2 sensor. The voltage generated is related to the amount of oxygen present. So if the O2 sensor transmits a voltage change to the ECU then the ECU assumes that there's either too much oxygen (ie: lean) or too little (ie: rich) mixing with the fuel. There is a degree to which the fuel mixture is adjusted by the ECU to maintain the correct oxygen level in the exhaust stream.

It does this by adjusting the amount of air, not the amount of fuel. The fuel is constant. That is why the air measurement sensors - MAPs and MAFs etc are so critical

If the ECU reaches the limit of the air intake adjustment without correcting the oxygen level it will ping the lambda light and the code will be recorded.

The LPG installer sets up the LPG system to provide the same amount of fuel at any given time as would the petrol system, allowing for the difference in the stoichiometrics, so that all the car's ECU has to think about is managing the air intake. If the LPG supply isn't matched to what the ECU thinks that the petrol supply should be then the O2 sensor will ping the lambda light because it can't adjust the air intake enough to compensate for the perceived lack/oversupply of fuel.

So the system relies not only on the correct amount of air and fuel being delivered, but also on the O2 sensor working properly.

Also critical are the plugs - which have to be exactly right to ensure the fuel is burnt properly.

So the best LPG set-up for the engine is one which most closely matches the petrol set-up.

On that basis the best O2 sensor surely has to be the right one for the car running on petrol, as the LPG set-up is referenced to that. In that way, if the LPG mixture ain't right the car will quickly let you know by running badly at idle and under load, and by triggering the EML and a DTC.

Interesting debate, and I'd be happy to be proved wrong if I am!

Cheers

Jack

[foggyjames]

The ECU absolutely does change the amount of fuel - that is its primary output. Some ECUs have absolutely no control over any sort of air delivery device (aside from for cold-starting). Yes, new electronic throttle cars can vary airflow independently of throttle input, but the main purpose of the ETM is to shut the throttle to allow for effective traction and stability control. Historically, the driver controlled airflow by opening the throttle and the computer responded with an appropriate amount of additional fuel. The AMM and MAP sensors are there to measure the air flow and pressure (respectively) resulting from throttle inputs, and allow the ECU to provide a corresponding amount of fuel. As a side-note, some ECUs monitor only throttle position - neither air flow, nor pressure. Adding more airflow as a lambda correction would raise the idle speed - it's fuel which is trimmed.

More to the point, you absolutely do not always want to be at the stoichiometric mixture. That is fine at idle and on cruise, but engines need to run a richer mix (typically circa 12:1 AFR) on-load. With old narrowband lambda based systems (i.e. a stoichiometric switch), the sensor becomes useless when you put your foot down. The ECU literally ignores it above a certain load point. A wideband can tell exactly where you are, anywhere in the likely AFR range, and allows the ECU to accurately control fuelling everywhere in the load range - hence how later turbocharged Volvos are so much better protected against shoddy remapping. As far as I'm aware, anything ME7-onwards in the Volvo range has a wideband, but I could be wrong on that point. Motronic-equipped cars certainly have narrowbands.

In theory, the "multiplication ratio" between petrol injector pulsewidth and LPG injector pulsewidth ought to be a constant (and some cheap LPG ECUs have a single multiplier value, rather than a map)....but no injector has a flat line for fuel delivery characteristics, so a map of load vs RPM is required for best delivery. One LPG installer told me he sets the system at idle, "then it must be right under all conditions". I thought to myself "no wonder LPG has such a bad name for destroying engines..."!

As a side-note, one LPG installer suggested that it's best to ignore the different stoichiometric ratio for LPG, as the important factor is what the engine is set up to burn. I raised the subject, as I was proposing to use a wideband with an emulated output to report the typical 0-1v curve for the petrol ECU (as if at 14.7:1), but to do it at 15.8:1...hence avoiding mixture fault codes from the petrol ECU. I don't think I agree with what he was saying, but it was an interesting thought.

cheers

James

[capt jack]

Thanks James - I take your point. Having run a V70 LPG conversion very happily for over 250,000 miles it always intrigued me as to how it was possible to get the car to run on a fuel that it wasn't designed for without doing damage.

My car had the Fenix engine management system - don't know how much that would affect your thinking. The O2 sensor died and was replaced with a new Volvo one at around 140,000 miles, and the car would throw lights and codes at around the 250,000 mile mark, but a new LPG vapouriser and injector nozzles resolved that.

Apart from that the only real LPG-related attention mine ever had was new plugs every 10k, and a new LPG filter every 15k.

So in an LPG set up where the fuel pressure is fixed, presumably the way in which more fuel is supplied in response to the driver's right foot is simply by opening the injector solenoids at a faster rate? Each time they open they'll presumably pass a fixed amount of gas?

Great insight - thanks

Jack