Getting LPG not to run lean

Hi, I've been scratching my head over this for a while now, no matter what I do I cannot get my system to do anything other than run lean. I will also say it's just above full lean at idle and then lean for every other driving condition.

My set-up in the old Series 3 88" is as follows:

Lovato Tank, switch/gauge, and filter
OMVL R90 vaporiser
Blos carb
Lambda sensors in each down pipe connected to a Air/Fuel ratio gauge
3.9 Disco 1 engine with injection manifold and plenum, but injectors disconnected.

So far I've:
Checked for air leaks and am happy there are none
Replaced the sparkplugs for standard NGK ones
Replaced the dizzy cap
Replaced the rotor arm
Replaced the plug leads
Re-timed the engine to about 22 deg advance, vacuum disconnected
Put a recon kit in the OMVL vaporiser
Replaced the LPG filter
Fitted the BLOS carb in place of the original twin SU set-up with mixer plates

Everything is set to maximum fuel delivery yet the thing runs lean, there is no misfire and the engine feels very strong when you drive it. I've not dismissed the fact that the two eBay special Lambda sensors could be at fault, or do not work correctly with LPG.

Is there anything else in the system that could be reducing the fuel flow to cause this? Or am I missing something here?

As a simple test, I put my propane blow lamp into the air intake to feed extra LPG into the engine on top of what was being delivered by the standard system. This isn't a little blow lamp, its a separate torch and big 19kg orange bottle, I had the thing pretty much full on to get the Air/Fuel ratio gauge to read somewhere near correct at idle. That's a lot of LPG going in, the engine rpm increased a little, but not much.

How are you defining 'lean' ?

LPG engines run with almost no deposits and bleached plugs, and if your Lambdas are responding correctly, that's as far as you can go. If you can control the fuel/air mixture in response to the Lambdas in real time, that would help as you'd have closed loop control.

Peter

Hi Peter, remember Lambda sensors only toggle/switch when an ECU is controlling the fuel, when used on their own they are very analogue in their operation. Mine are both showing around 0.2V at idle (very lean), I am used to seeing around 0.4 to 0.6 for a good mixture, above 0.8 is rich.

Are the oxygen sensors correctly matched and calibrated to the AFR gauge ?

I believe so, though I have asked myself this question, the AFR gauge uses standard narrow band sensors with a 0V to 1V output. I've tested this set-up by blowing on the sensor and it goes from lean to rich as I would expect and have seen before, this I've also double checked on my volt meter.

So what make gauge is it ? How many wires on the sensors ?

I don't know the make of the gauge, came from ebay, the sensors are three wire, two for the heater and one signal. I've tested that the heater is working, and also that the signal is good using my DMM. I've used the Gauge a few times before with no issue, but the Lambda sensors are new and I'm beginning to wonder if they are not somehow reading properly with the LPG system, though it should only be an O2 count.

I think my best plan of attack is to buy an exhaust gas analyzer and try that.

You should be able to adjust the LPG kit to run rich.

What air filters are you running?

On my 101, which had a high compression 3.5 on twin su's, there wasn't enough mixer depression to give me a good mixture.
I had to cut a restrictor plate, with about a 2" hole, and put it in the air box.

The standard 101 airfilter and pipe work was so free flowing, there wasn't enough depression to pull the gas through, so I had to create a little bit of restriction to get enough depression for the LPG evaporator to work.

Try putting a bit of a restrictor in your intake - see if you can get the mixture to richen up.

Sounds contrary, but unless you have a really hot tune engine, big intakes don't really help. Oh, and if you have a mechanical evaporator, make sure your intake doesn't point I to the incoming air, or you will run lean at speed.

Murtle said:

I don't know the make of the gauge, came from ebay, the sensors are three wire, two for the heater and one signal. I've tested that the heater is working, and also that the signal is good using my DMM. I've used the Gauge a few times before with no issue, but the Lambda sensors are new and I'm beginning to wonder if they are not somehow reading properly with the LPG system, though it should only be an O2 count.

I think my best plan of attack is to buy an exhaust gas analyzer and try that.

Click to expand...

With 3 wire sensors - which are probably standard Zirconia ones your gauge is only going to be a basic narrow band.To get the best out of your LPG you need 5 wire UEGO's running a wide band of operation,like these; LM-2 Wideband O2 Digital Air/fuel Ratio Meter (Probably a bit OTT and expensive,but will be accurate back to the required 12-1 ratio needed for max power on LPG - if my memory is right...)
I think the reason you cant get your system to work is simply because the sensors don't match what the gauge was set up to work with.

Sometimes it's best to ignore gauges, and just listen to the engine.

Get the car idling, and play with the idle mixture only.

You should be able to swing the idle mixture all the way from lean misfiring, through fast idle lean, "splashy" exhaust sounding rich mixture through to spluttery rich misfire.

This is at idle - no revs.

If you can't get this sort of response from the idle jetting of the evaporator, then you have some other problem.

I've not come across these Blos devices, so can't comment.

One other thought - all these mechanical LPG kits don't run too well with open intakes, or free-flow air filters. They need a little restriction to provide enough of a vacuum signal to the evaporator.

Captainbeaky, I know exactly what your saying as I had this issue once with an old Lovato gas vaporiser, unless about half of the intake was covered the engine would stall and not idle. I replaced the vaporiser with an OMVL R90 because of this and it cured that problem. This was back when I was running twin SU's and restrictive mixer plates. I no longer have mixer plates, I have a Blos carb instead that works much like an SU and a very free flowing air filter. The lean problem existed before I fitted this new intake system so it's not to blame, if anything its improved things a lot.

eightinavee, The narrow band sensors that I have are accurate enough for using as a tuning aid, I do understand they will read slightly lean for an LPG set-up but not full off the scale lean. As for the gauge, I've also tested the lambda sensors with my DMM and it's showing a very similar result.

Captainbeaky, Everything is set at full max fuel currently, if I screw the mixture screw in on the Blos carb (to weaken the mixture) the engine slows, if I screw the mixer screw in on the OMVL R90 vaporiser the engine slows, but only after many turns inwards untill the gas is almost off. I believe this is because the Blos carb is adjusting it's self as I reduce the gas flow.

I've bought an exhaust gas analyser now and am just waiting for this to arrive, I'm beginning to think as mentioned it could be the new Lambda sensors not working correctly on LPG. It's strange though as this set-up has worked for me in the past with believable results and instant results when adjustment the gas mix on the vaporisor.

nb sensors with gauge are waste of space unless hooked up to oe engine ecu.(then can read fuel trims via obd11 port and sort lpg tuning)

tune by ear and feel..check plugs..

or buy wb afr meter and sensor..so recommend!