One for LPG geeks

Post Fri Nov 02, 2012 4:16 pm

The car runs fine on petrol, switch to LPG and it runs for 15 -20 seconds (longer at idle) then loses power and dies, switch back to petrol and its fine, switch straight back to gas and same 15-20 seconds and dead again.
Simple leonardo closed loop system, filter seems fine, and just put a tenner in the tank. No flashing warning on the switch either.

What to check next, I have found Jon_Bmw's old USB cable to plug into the ecu but have no sofware and the cable just has three wires to push onto individual pins on the diagnostic plug

Any help, much appreciated

Post Fri Nov 02, 2012 9:52 pm

Sounds like it's just running on the liquefied gas in the pipe from the tank to the front of the car. Possibly something amiss with the tank solenoid, only letting LPG through slowly?
Get yourself in a WELL ventilated area, not near any drains, and away from any other people, disconnect the pipe between the filter/solenoid under the bonnet and the vaporiser, and apply power to the (usually) blue wire on the most easily accessible solenoid coil.
You should get a strong stream of liquid propane.

Post Fri Nov 02, 2012 10:02 pm

Thanks Brian, I take it by applying the power this will activate all of the solenoids? Can I apply power to the one on the vapourizer?

Post Fri Nov 02, 2012 10:09 pm

All the solenoids are wired together, so anywhere will do.
Possibly a bit less dramatic if you pull the large pipe that runs between the vaporiser and the stepper motor off at the stepper motor end first, and power up the solenoids. This should result in a constant stream of low pressure gas vapour coming from the pipe, just like turning on a gas cooker hob without lighting it.

Post Fri Nov 02, 2012 10:11 pm

Will try in the morning, thanks for your help

Post Sat Nov 03, 2012 12:50 pm

Did what you said Brian, live onto solenoids produces steady hissing from pipe to stepper motor. The pipe going into vapourizer goes mental when disconnected and solenoids opened.
Got the car to drive very well on LPG for about 4 miles by removing the perm live fuse then reconnecting, however I switched back to petrol then back to LPG and the same fault reappeared, roughly 10 seconds of driving and it dies.

Disconnecting the stepper motor has no effect on idle or revving the engine whilst stationary, should it?

Also there is evidence of burning on one of the resistors and the component next to it on the ecu circuit board.

Post Sun Nov 04, 2012 12:03 am

Everything appears to be as it should be with the two tests you've done, so it looks like theory one is going down a dead end!
The 'burning' on the resistor looks more like discoloured board lacquer to me. Probably the same on the diode(?) beside it.
Wonder if it switches the petrol injectors back on for some reason? An engine will die almost immediately if it tries to run on both fuels at the same time because of the extremely rich mixture. Try disabling the petrol side of things by pulling out fuse #11.
The other possibility I can think of is that the stepper motor is closing completely, either because it's faulty (not unusual) or the controller is acting up. Try connecting the low pressure hose from the vaporiser directly to the mixer, ( i.e. take the stepper motor out of the line) and see if that makes any difference.

Post Sun Nov 04, 2012 12:13 am

Will do those tests tomorrow and report back. Thanks Brian.

BTW what does the stepper motor control? Is it just idle and should you be able to push the plunger in by hand?

Thanks again

Post Sun Nov 04, 2012 9:43 am

Stepper motor controls the amount of gas going into the engine at all revs. The design of all stepper valves may not be the same, but the ones I've had apart have the plunger on a coarse spiral, so that it turns as it retracts. It'll go in by hand id you allow it to twist.
I'm fairly sure the Leonardo software is available for free on line, and there are also circuits for the level shifter interface box and the rest of the laptop cable freely available.
Can you get your hands on an adjustable tap thing ("power valve") that is used in place of the stepper motor on first generation LPG installations that have no electronics at all? One of these is handy to have to be able to set the vaporiser adjustments up correctly at first installation, and for testing purposes in situations like yours.

Post Sun Nov 04, 2012 4:49 pm

Have the sofware now installed on the laptop, now sorting the cable, the interface cable that came with the car has four wires but one ( yellow is broken ).
Off to london for a couple of weeks tomorrow so I will resume the search when I get back.

BTW there is a valve as you described fitted already just after the stepper motor, it hasn't been touched in my ownership and is fully open.

Thanks for your help (and patience) Brian

Post Sat Nov 10, 2012 7:21 pm

Ok drove to london, on way back gave the car a bit of a thrash and about thirty miles in I decided to give the lpg another go and it drove faultlessly for the rest of the journey (40 miles).

Could this be an issue with the oxygen sensor and by me thrashing the car and getting it nice and hot the sensor began to work properly?

Post Sat Nov 10, 2012 7:25 pm

Is the lambda heater relay fitted? Is the vapourisor actually getting hot? I've missed what car/engine this si fitted to.

I'm not exactly close but am happy to help you set this up if you come to me, I have laptop, lead, Leo software etc.

Post Sat Nov 10, 2012 7:44 pm

Its a 318is, jon_bmw's old one, 3 wire type lambda added to exhaust by him. Doubt there is a heater relay in the system. Vapourisor gets very hot.

Thanks for the offer, but its finding the time to get down there at the moment. I have software, just neeed to sort my knackered lead.

Did you buy some gutter trim off me? I'm sure we have spoken before.

Post Sat Nov 10, 2012 8:03 pm

Yes, we have met, and yes buying gutter trim. You may know, three wire lambas are often earthed through the exhaust and this is very bad for a 1v (possible 5v if custom usage) signal! The other two wires are for the heater.

To be on the safe side I'd say a non heated lambda won't be hot enough and working until you're driving along, if it's even connected well enough to transmit the signal.

When stepper motors get tired they can struggle to release from rest position. Undo the two screws on the stepper, clean and very lightly copper grease the end of the piston.

Post Sat Nov 10, 2012 8:26 pm

Stepper motor already cleaned but will lube it, I will check the wiring to the sensor, is it worth putting a braided earth from exhaust in that area?

Post Sat Nov 10, 2012 9:44 pm

johnl320 wrote:Stepper motor already cleaned but will lube it, I will check the wiring to the sensor, is it worth putting a braided earth from exhaust in that area?

I've no personal experience, but from what I read about them for my install it's a case of get a 4 wire (signal, earth, heater +, heater -) or forever be messing about.

It's quite hard to test your lambda's output in situ (don't test continuity of the sensor only the heater circuit) because it's not connected to the motronic because you wouldn't expect it to switch to & fro when on petrol anyway.

Does your car have the stock lambda plug under the battery tray?

On your post above, the leo will only flash lights if the tick box settings tell it to.

Post Sat Nov 10, 2012 10:05 pm

e301988325i wrote:
When stepper motors get tired they can struggle to release from rest position.

This! If it unsticks at changeover then the car runs normally. If it doesn't, then it dies.
One wire and three wire lambda sensors exist, and are used by many manufacturers, so they can work OK. A rusty exhaust not making electrical contact was a common cause of the engine fault light intermittently lighting on Vauxhalls a few years ago, though.

Post Sat Nov 10, 2012 11:05 pm

Thanks for your help guys, will be going back to london monday morning so will see what it does then, will also sort this interface cable

Post Sun Nov 11, 2012 7:23 am

Brianmoooore wrote:
e301988325i wrote:
When stepper motors get tired they can struggle to release from rest position.
This! If it unsticks at changeover then the car runs normally. If it doesn't, then it dies.
One wire and three wire lambda sensors exist, and are used by many manufacturers, so they can work OK. A rusty exhaust not making electrical contact was a common cause of the engine fault light intermittently lighting on Vauxhalls a few years ago, though.

I put o-rings under the screws of the stepper motor to give the cover a bit of 'spring'. Fair point on the lambda's, they're just harder to work with, especially when it's so easy to plug a lambda with a BMW plug straight into the loom.