Hi guys, really stuck with this.

08 BF 6SPD airod mixer system 28L/100 city 21/L100 freeway, consistent over the last 20 tanks or so.

When I first got it was around 17L/100 city and 13L/100 freeway, no backfires or issues at all ever since owning it.

PULP consumption is around 14L/100 city and freeway 11L/100.

Changed both o2 sensors, changed stepper motor no change in consumption, engine runs well on both fuels, idles a bit rough on LPG compared to Pulp.

It's dearer to run on lpg than pulp. Any ideas appreciated or the lpg will be getting the chop.

If you can get someone with a scan tool to look at it, it would be better than asking for someones guess. The scan tool will show long and short term fuel trims and will be able to be seen how the ecu is compensating using the info provided by particular sensors.

But if i was to just guess for you, i would say coolant temp sensor or dirty of faulty MAF sensor.
I would probably lean more toward the coolant temp as i know your LPG an Petrol system both would take a reading from it, not sure if they both take readings from the MAF.

Thanks Eafalcon 5.

I do have a scantool & have no issues showing up. Wouldnt a dud coolant sensor throw a code?

What sort of AFR numbers should I be looking for at idle?

If the coolant temp sensor was bad wouldn't PULP consumption be terrible also?

Thanks for your help

Your scan tool will be useless trying to scan stepper motor pintle steps or anything related to the gas side of things, you need ford software in your scan tool for ford straight gas systems. But you should have seen strait away your 02 short term fuel trims in the positive direction meaning, the pcm is adding fuel because it thinks the engine is running lean. Check your pcv valve on the cam cover, if the guts have fallen out it will let more air bypass the valve and trick your upstream 02 sensor signal voltage, it will be low and the stepper motor will retract more making it rich thinking it is lean. It will effect the gas side a lot more than the petrol side with pulse width on the injectors. The pulp consumption is not that bad so I would not think a temp sensor or open thermostsat problem there, if it was, If it ran too long under 60deg it would log a code, and your temp gauge would be low. If you have a decent scan tool you should be able to see your 02 signal voltage going between .1 and .8 of a volt at idle and cruise and when it does this it is called stoichiometric, with an AFR of 14.7 to 1. and your short term and long term fuel trims should be no more than 10%.

The inlet manifold gasket does go brittle over time, and can leak.

Thanks so much Rod for your detailed response!

I changed the PCV is a metal one, so that way I wouldn't have issues with LPG if it did ever backfire.
I will check 02 sensor signal voltage & report back. Engine always seem to warm up fast & needle usually sits around between 1/3 to half so I would conclude sensor & thermostat ok. If I had a bad intake gasket would I get any other symptoms? Thanks for everyone's assistance:)

It would effect the fuel trims but it would have a rough idle and a miss, you have got a nice clean air filter element in the air box. My straight gas bf around town 14 to 15 litres per hundred klm highway 12 to 13 litres per one hundred klm.

Update:

Ran a scan and its throwing a P1131 & P0302, so I can only assume the new oxygen sensor is faulty.

I also will swap the coil pack around to see if the p0302 goes away. Checked intake manifold gasket from underneath car & it appears ok.

Seems to smell rich on LPG or Pulp, when near the exhaust outlet.

Will keep you updated !

Mate your not using one of those cheap code readers off ebay are you, if you have access to a scan tool you can look up all the freeze frames and check all the live data. That P1131 is the O2 sensing a lean condition, remember I said the sensor could be tricked lean, it may have a wiring problem keeping the signal voltage low. The P0302 is a misfire on no 2, all the data would have been stored in the freeze frame, how many times when and so on, that would trick the 02 but the cylinder would have to be missing a lot, and the mill light would have come on. You said the engine is running good, and I am sure you would be able to detect if one cyl was not working. Remember the pcm cant tell if it is a plug or injector blockage, it is only measuring the speed difference between cylinder events of rotation of the crank. I would not think the new o2 is crook but anything is possible. Get the car hot on petrol, connect up your scan tool look up the live data on the 02 and with the engine running at 2000rpm watch and see if the signal voltage will cycle between .1 and .9 of a volt, if it does switch to gas and see what it does.

The only way you can check the gasket properly is spray/run some soapy water or leaktek on the gasket preferably whilst its idling on LPG. If you see some bubbling, and/or the idle picks up for a second, you've found your culprit. Spraying Aerostart works too. I'd spend a bit more time on No2 intake runner. Be aware too if nothing comes up, to check the underside as well. I doubt your p0302 will be fixed by swapping coils, so hold off on that.

You would think if it was an air leak on number 2 he would notice that distinct miss and rough idle and high fuel consumption on petrol as well as gas. It could very well be the runner, I still think the 02 is being tricked lean.

Thing is though we don't have the car to look at, and the OP doesn't have the experience and trained eye in knowing what to exactly look for.
The petrol side being injected I would think a manifold leak wouldn't affect it anywhere near as much as it would on LPG, the LPG mixture is already created back at the mixer ring. Well, around idle, anyway. The LPG mixture would be leaned out if there were extra air fed in before entering the cylinder, the petrol side wouldn't be overly leaned out I'd think, as the leak is still before the injector, it's simply getting unfiltered air along with filtered air.
When I was running an impco converter/mixer setup, I made an anti backfire flap on the rubber intake elbow - slightly open it on LPG, the engine would stop, on petrol, it didn't affect idle at all.

As you know, LPG is more sensitive to ignition, so I'd think a lean mixture wouldn't ignite as consistently, hence the worse idle than petrol. So I'd say changing coils at this stage unnecessary work - it's easier to properly check the gasket, and rule it out first before moving on.

And if you were to change coils, may as well pull all 6 spark plugs - a leaned out plug should be a noticeable difference to the others.
Agree with your reasoning, the O2 sensor is simply trying to correct a lean mixture - correct me if I'm wrong on what I've come up with.

Didn't get notified of any further replies sorry fellas.

Finally an update!!

I do have a slight miss if run on lpg when first started for around 30secs-1min only if idling, if accelerating even to the floor runs on all 6 no problem.

I mucked around with the lpg idle screw to smooth out the idle, no perceived difference.

So I now start on petrol & run on it for about 1 minute before flipping over.

This somehow has had the effect of reducing lpg consumption to about 23/100 100% city & 12.5/100 100% freeway.

I also changed the coil pack & oxygen sensor again but it is still throwing the codes, but does not throw them unless running on LPG(ran on premium for a few weeks strait)

Will read values next time I hook up to scantool to verify voltages.

Thanks for everyone's input !