Hi I have a 3.3L v6 1999 voyager which I bought in the spring. The lpg conversion isn't the best and it runs quite lumpily and has been getting worse. It also trips back to petrol when accelerating hard. The fuel consumption is also pretty terrible, a lot worse than what the computer calculates, to the point I thought it was leaking gas. I took it to a specialist who told me it wasn't leaking just misfiring on LPG and running badly. He suggested changing the manifolds and spark plugs.

I have bought new gaskets, spark plugs and leads and will change it all over. Has anyone else had experience of this and know if what I am about to do will make a difference. Or are there any suggestions to what else I might do whilst I replace the plugs leads and gaskets?
Thanks!

 

In the past I've had a petrol Jag. running on propane, and it was quite successful -- it even ran better on LPG than it did on petrol, prob. due to a clogged injector. On the motorway the cost effective fuel consumption was 44mpgUK on a 3.2 petrol XJ6.

There are two types of LPG conversion:--
!) The so-called "gas-ring" type. In his type, the vaporized gas after the water-heated vaporizer has done its job is simply vented into the area before the throttle body. So the air going into the TB. is well-laced with gas instead of a petrol mist like with a carburator. Very simple, nothing partic. to go wrong. However, you have to get the mixture ratio right at idle with a gas adjuster, and then at much higher revs and load with a different adjuster.
2) The second more complex type uses gas injected directly into the manifold, one injector per cylinder. The injectors are solenoid, so they are active, and there will be a wire to each one from the control unit. The advantge of this type is that any backfire can't damage anything, (like the mass airflow sensor device) and the control of gas is a little more precise, but inevitably it costs more, and if you have to drill a water-heated manifold to fit it -- problems! This type is easy to spot as there are lots of gadgetry on top of the engine for the injection.
There was a third type from Ford that injected neat liquid propane gas into the cylinders, but that one never got off the ground. But it did get rid of the vaporizer and its expense.

If I were you, I'd take your problem to one of those outfits that do conversions, it's likely they will know what's wrong immediately. They don't usually charge a lot for advice and adjustments, on account of them being enthusiasts -- like most of Leeds taxidrivers who use a LOT of propane....

BTW, the higher the compression ratio the better with propane. The ideal is 12:1, but nothing nowadays has one as high as that. The Jag was 10.5:1 -- you'll not find anything modern higher than that because of no-lead petrol now. The 1970 Boss Mustang engine was the last with 12:1 I think.
BTW#2 -- many running on propane favor platinum tipped spark plugs, but my jag didn't have these and worked ok. Also, I couldn't find any difference in fuel consumption between petrol and gas, both being around 15mpgUK for town use when litres were converted to gallons. When you go for emissions test, a propane engine has almost no carbon monoxide at all -- my tester couldn't believe it.

Leedsman.

 

Thanks for the info Leedsman. Mine must be the more complex type as there is a lot of trickery and what looks like individual injectors.

I got the advice about the misfire and told to change the manifold and sparkplugs/leads by an lpg specialist. He said my system wasn't pretty but it works! Fingers crossed the new parts sort it, the lumpy running and fuel consumption are a little hard to live with!

I should have say the fuel consumption on petrol isn't good either so that that does point to general faults that would effect both lpg and petrol.

 

Fuel consumption for a 3.3ltr. Voyager? I'd guess13 to 15mpgUK around town, 20 to 22 on the motorway. These cars are heavy brutes and can be thirsty. Having said that a petrol RangeRover driven by Jeremy Clarkson around London -- 9 (yes, nine) mpgUK.

You may have discovered the reason someone converted this motor of yours to gas LPG in the first place...

Leedsman.

 

Oh yes definitely expecting it to be thirsty! but it's a bit thirstier than I expected and the calculated consumption is miles off the computer calculation. Around town I get around 10-12 mpg(UK) and on run 14-17mpg(UK). The best I got was 18mpg(UK) (60mph for a 160miles) and the computer was reading 25mpg(US) so about 30 mpg(UK).

The previous owner claimed you could get 240miles from a 40L tank of lpg on a run, about 27mpg which sounds a bit optimistic, but I am hoping to see the low to mid 20s.