Disco 3.9 V8 cutting out on onverrun

[sidewaysste]

I've got a 1995 Discovery 3.9 V8 Auto with a singlepoint OMVL R 90e vapouriser, 2 emulators and a (now defunct) Leonardo gas ECU to control a (now disconnected) stepper motor in the gas feed pipe to the ring attached to the inlet throttle body.

I disconnected the stepper motor and replaced it with a thumb screw type valve as the stepper motor packed up. Also both O2 sensors are buggered, not least cause some tit of a previous owner welded one in :s

I'm no LPG expert but I am a mechanic who fits standalone ECUs so I'm not completely clueless

Problem 1) on the overrun it cuts out. It only does this on LPG so I figure that rules out the ignition system and the fuel injection system.

it will idle fine when not moving or very low speed work but when slowing down for a junction or throttle off (when going down hill for example) the revs drop to idle (as you would expect with an auto) but then they keep on dropping to the point the engine cuts out. It feels as if it's running out of fuel rather than the ignition is cutting.

I've tried adjusting the idle bypass and mixture screws but to no avail.

Now recently I've completely sealed the ignition system for when I go off roading so it doesn't die at the sight of water. The coil is in a sealed box, the dizzy has every hole silicon sealed up and the cap is also sealed on. The dizzy is vented to allow the vac advance to work correctly.

I've also fitted a snorkel recently but in true idiot style I made both these changes at the same time, which makes diagnosis somewhat more protracted.

I did wonder about whether I'd got the air gap on the dizzy reluctor incorrect but it shows no other ignition issues whatsoever.

As I write this, I can't help but wonder if it's something to do with the fitment of the snorkel because as I understand it a vaporiser is in effect a demand valve similar to a scuba diver's DV, therefore if the engine is force fed air or denied air at idle it will throw the mixture way out of the 8 - 16 :1 runnable afr ( are the ratios different for LPG from petrol btw?)

Could the snorkel be creating a ram effect thereby pressurising the inlet tract and cutting off the gas supply?

Could the snorkel be causing a restriction, causing a high vacuum and flooding the inlect tract with gas and causing an overrich scenario?

Problem 2, the LPG tanks foul when offroading as they're slung under the chassis next to the sills. A friend has an LPG tank that could replace the fuel tank but then I don't know what I'd do about fitting a smaller petrol tank for starting and emergencies elsewhere - any ideas?

Problem 3, may not be an actual problem as such but the fuel economy is in the 9-11 mpg range. I had just assumed that's because it's a 2 tonne house brick powered by 1960's technology...

[Gilbertd]

The snorkel is almost certainly what is causing it to cut out on the overrun due to, as you rightly guessed, pressurising the inlet and shutting off the gas. If the top is facing forwards to give a ram air effect, try turning it sideways so the airflow goes past it rather than being forced into it.

Small petrol tanks are available for the Classic Rangie that hold about 4 gallons so you can fit a gas tank in where the petrol tank would normally live. You may be able to fit one to the Disco or get one made up that will do the job. Alternatively, if boot space isn't too important, you could fit a cylinder tank in the boot or even see if you can find a vertical toriodal tank and fit it at one side of the boot. That's what I've got on my Classic LSE and, with a carpet cover over it, it looks like I'm carrying two spare wheels as the spare stands up on the other side. The boot is a bit narrower but still quite usable although the tank will only hold 60 litres.

Economy sounds either about right or a bit thirsty depending on how you drive it. I get between 12 and 16mpg from a 4.2 LSE which is effectively the same engine but in a car that is even bigger and heavier. I would suggest getting the Leo working again. As long as you can replace one of the lambda sensors to supply a signal, a replacement stepper isn't expensive.

[sidewaysste]

Thank you for the quick reply. I've turned the snorkel head round so it's facing backward. I'll test drive it later. I don't really want to lose boot space, I often carry 6 passengers so the dickie seats are useful. Would I be right in assuming the osr under arch area ona disco is similar to that of a rangie?
I'm not sure how the Leonardo is wired in, I've been unable to find any wiring digrams for it - or any of the LPG electrics for that matter.
I don't know which Lambda sensor it takes signal from and I'm assuming that as they've been left in the exhaust stream both are probably buggered anyway. Where would I purchase a new stepper motor from?

[sidewaysste]

Errant thought A...

If I were to connect the atmosperic side of the vapouriser to the airbox, I should be able to create a localised normal atmospheric pressure -

i.e the problem is arising because there are effectively two base atmospheric pressures, that acting on the back of the vapouriser and that inside the intake system. these two differing "base" pressures are causing a positive pressure to act on the working side of the vapouriser and shut off all gas flow.

Yes? No?

[Gilbertd]

Leo installation manual is here http://www.aeb.it/products/feedback-sys ... ttachment1 (English from page 20 onwards). Why do you assume the lambda sensors are shot? Does it run OK on petrol or don't you use it often enough to know? The Leo only requires one to take a feed from.

Plumbing the open side of the air intake is the standard way of doing it on a forced induction engine although whether you could get it to work on an R90E I've no idea.

Tinley Tech will sell you a new stepper, here http://www.tinleytech.co.uk/acatalog/El ... nents.html (about 2/3rds the way down the page).

Not sure how similar the under wing area on a Disco is to a Rangie but it's a convenient place to put a tank of some sort. Unless you can find one made specifically for the job, you'd need to use an inline external petrol pump rather than the in tank one and would lose the fuel gauge sender too. They do pop up on eBay at odd times too.

[sidewaysste]

It runs very lumpily on petrol upto ~3000 rpm then clears - I'm assuming that it goes open loop at this point. also one of the sensors has been welded in, as in the sensor has been welded to the bung. If the heat of welding didn't kill it, I should imagine the current would have.
It's running an RPi switching power ignition amp that does appear to switch to a more retarded profile when on petrol ( base timing is 20deg) also I get no pinking.

[Gilbertd]

The easiest way to find out if either of the lambdas are working would be to test them. The standard Disco V8 ones are 3 wire jobs with the signal on one of the black wires (the other is earth). If the engine is running rich, you'll get about 1 volt output, if lean 0 volts. If the mixture is correct, it will wobble about the middle. As long as one of them is working, you can use it to drive the Leo.

So you're the person that's got the working RPi amp are you? Most of them have died long ago and they aren't really needed if the ignition system is in good nick. But they do give the advantage of allowing you to run more advance on gas as LPG has an octane rating of around 113 RON. Which probably explains base timing of 20 degrees btdc as the book says 5 degrees for a 9.35:1 cat equipped car. I assume the amp retards the ignition by 15 degrees when on petrol.

[sidewaysste]

Well turning the snorkel around appears to have 90% cured it. I guess it must have been ram air issues. Yes the power amp still works, what alternative is there bar a fully mappable ECU? If I've got a high RON I want to make best use of it, I don't like chucking efficiency and therefore £££ down the exhaust pipe... I'll eat my boots if an O2 sensor that's been welded in still works, but I'll try testing them, thankyou for the advice

[Gilbertd]

AEB, the same people that made the Leo, do a range of timing advance processors. Probably more reliable than the RPi one and a lot cheaper.

http://www.aeb.it/products/timing-advance-processors

[mat_fenwick]

I did read something on a knowledgeable sounding website suggesting that although greater ignition advance on LPG was beneficial at light and medium throttle openings, but less required at full throttle.

There is definitely a wing mounted LPG tank available for Discoveries, my mate had one and I've also seen one fitted in a magazine article. Probably the same as the RR tank but not certain.

I used to get between 12 and 16mpg from a 3.9 Disco auto on a single point system, so yours sounds a little on the thirsty side. FWIW the injection system I've got now returns very similar figures - 13 to 18mpg. Economy is massively affected by hills/corners I've found, can't get better than 14 around here (Wales).

EDIT - if a sizeable proportion of your driving is off road, that might explain it.