Help Understanding Live Data From Code Reader

[Steve P]

Anybody out there know where I can get an explanation of what part of data means from my fault code reader? I have the memoscan U581. Tried the manufacturers website but no joy yet.

 

When reading live data I want to understand the read out from the 02 Sensor. Reason is that I am trying to understand some funny readings when running on LPG.

 

On petrol the 02 voltage is about 0.7 volts and there is a % reading which reads about 0%. Obviously these readings change as you drive about.

 

I thought that % reading was emissions - CO perhaps.

 

However when I switch to LPG the readings aren't as sensible. The LPG system is an old one and there are some adjustments - it's not fully automatic.

 

Anyway on LPG the voltage tends to be closer to 0 and the % goes very high - 25% or more. I tried adjusting the LPG system but not a lot seemed to be happening so I thought I'd better check exactly what these readings are. Previously I have set the system up using anAir / Fuel Meter plugged into te Lambda probe and this seemed to work quite well (or I thought it did!!)

 

If the O2 voltage had gone up and the % readings had gone up I might have been less confused. However I am ASSUMING that a rich mixture gives a higher voltage and a higher CO ?

 

So I suppose I have two questions really:-

1. What is the % reading on a u581 memoscan live data (the parameter was something like 02S11)

2. How does a lambda probe work - does a rich mixture give a higher voltage?

 

Thanks for any help.

 

Steve

[sepulchrave]

0.7v is too high and indicates a rich mixture, conversely 0.0v is low indicating a lean condition (which you'd expect on LPG). You should be running around 0.45v at Stoich.

 

Ignore the % reading, it's not really relevant since it's simply a rough indicator of how much free Oxygen remains in the exhaust gases (lean being a higher %).

[Steve P]

Thanks - I'll try again tomorrow - I have managed to find some more info on the web as well.

 

Should I not be able to achieve about 0.45 volts on LPG as well - because even though some of the emissions are reduced (because LPG is cleaner) I should still be aiming for Stoic with minimum free 02?

 

Thinking about it - the AFR meter only uses voltage - it's just that it's a lot easier to plug the code reader in rather than root about under the bonnet splicing wires to connect the AFR in.

 

Cheers

Steve.

[Smilge]

Here's a good post on the Lambda probe Steve

 

http://www.fordgalaxy.org.uk/ford/index.php?showtopic=12318

[Steve P]

Well I've read all the info and connect the various gauges up again and am still struggling.

 

On Petrol.

The AFR meter spliced into the lambda probe wiring swings back and forth across the optimal range with the engine at idle. GOOD!

 

With the u581 connected the live data for 02S11 reads about 0.45 volts and the % is 0. GOOD!

 

Switch to LPG.

The AFR meter spliced into the lambda probe wiring swings back and forth across the optimal range with the engine at idle. GOOD!

 

With the u581 connected the live data for 02S11 reads about 0.15 volts and the % is -34%. Pending codes show 02S11 too lean (can't remember actual words). OH DEAR!

 

So I try adjusting the LPG idle mixture. I can see it changing a bit on the AFR meter - enough to work out that clockwise is richer and anticlockwise is weaker. It's very hard to set it accurately using a gauge that swings nearly 180 degress! But I know I'm about right because half a turn one way and the engine idle drops and half a turn the other way it drops, and I have it set roughly between the two extremes. The idle sounds smooth

 

However with it set like this I'm still getting about 0.1 volts out of the U581.

 

Whilst typing this in I've remembered something one of the installers said with one of my earlier LPG motors - something suggesting that the lambda signal to the Engine ECU is interrupted or altered when the LPG ECU takes contol.

 

If the car were really running with a lambda voltage of about 0.1 would you expect it to be rough?

 

You might wonder why I'm wittering about this but I'm still trying to get to the bottom of why our gal is so damn thirsty. Perhaps the loose MAF plug I posted about elsewhere won't have helped - but that only showed up recently as slightly lumpy idle.

 

Any comments ?

 

Thanks

Steve

 

PS - I'll obviously have to go and find an LPG forum.

[sepulchrave]

I am confused as to why you think this may be a problem since when the engine is running on LPG the mixture is NOT under the control of the ECU.

 

Since the ECU cannot use injector pulse duration to vary the mixture dynamically, how do you expect closed loop lambda mode to make a blind bit of difference?

 

When you're on LPG all the ECU is doing is controlling the ignition advance.

 

In fact engines running on LPG need far more ignition advance than when running on petrol, often a quality LPG install will include an ignition advance module (interupter) to over-ride the retarded petrol setting.

 

LPG is considerably less potent than petrol so you generally use a lot more of it, therefore MPG will always drop by around 15%, often more on cruder non-sequential injection installations.

 

Power and torque also drops and to maintain the same progress you did on petrol means you tend to use even more fuel.

 

LPG is great for lots of motorway miles but worse around town and on hilly 'B' roads.

 

LPG is VERY sensitive to poor ignition systems, coil-packs, leads and plugs must all be in tip-top condition.

 

The MAF makes a BIG difference to economy, make sure it's properly sorted.

 

Since I have absolutely no idea what kind of system you have fitted and the information you're providing is sparse at best then these are just general guidelines really.

 

If you'd care to elaborate I'll try and be more specific with my help.