What a bodge

[Gilbertd]

Got a phone call from a friend who had been to look at a potential new car. He currently has a Jaguar XJ6 with an elderly Tartarini LPG system on it and had found a much newer (2001) XJ8 already fitted with LPG kit at a very good price. When he looked at it there was no gas in the tank, the check engine lamp was lit and it was in limp home mode (wouldn't rev beyond 3,000 rpm). He isn't familiar with the V8 engine but having got home and still trying to decide whether to increase his offer, he looked at the advert again. He then asked me to look at the LPG installation as it is nothing like his and he couldn't quite believe a system that just squirts gas in by the air filter.

How about this then folks, a single point mixer on an engine like this?



I told him he'd either need to budget for regular new intake pipes after the existing ones are blown to little pieces or a few hundred quid for a proper front end!

[ryncharlton]

Not had any problems with my discovery 3.9 on a mixer system. assuming that engine is well maintained and the gas instalation is good , a mixer system should work ok on that engine.

Worth checking its a closed loop mixer though! , if it isnt catalytic converters wont be lasting long

[rossko]

Not had any problems with my discovery 3.9 on a mixer system. assuming that engine is well maintained and the gas instalation is good , a mixer system should work ok on that engine.

Don't reckon it will, the JagV8 management is rather later and more sophisticated than the 3.9's Lucas and won't be fooled for a moment, hence the engine management light. Unfortunately, if the management does have a hissy fit it also affects the ignition etc., again unlike the 3.9, so it really won't work well on gas either.

[Gilbertd]

and Rossko beat me to it! No disrespect to the Disco (I actually quite like the LR Disco) but it's very old tech in terms of engine management. Not only that but I hope you don't have a mixer installed a good 3 feet before the intakes! Despite a lot of generalisations that say a petrol injected engine needs an LPG injection system, more basic ones will work OK with a mixer. My Saab 900 has petrol injection and a mixer LPG system, but the mixer is stuck in the throttle housing not next to the air filter at the other end of the trunking.

[tommo2009]

A mixer system will work on pretty much any engine.
As long as the emulation is correct and the Lambdas are working correctly there is no reason why it shouldn't work and run Fine.
I agree that for this car an injection system may be better.
The mixer should be nearer the throttle body.
The Jags will only rev to 2500 if they are in park, they will only go the full rev range when in drive and being driven.
Some newer high tech injection systems will work better with a mixer system as the LPG injection system sometimes cannot cope with mutli pulsed injectors. To get around this Emulators are used, the same emulators found in a mixer system!!

Don't write off old and simple technology, Old diesels are far better than a modern diesel, especiallly if they go wrong.

[Bat]

Hi,
Looks like the Mondeo I bought, copper pipe glued into the plenum, that was the mixer!
Cheers,
Gavin

[rossko]

A mixer system will work on pretty much any engine.

No it won't.

Put a mixer on a direct-injection petrol engine and the petrol injectors will be trashed. Mind you, I suppose that isn't a good example as the mixer still "works".

Put a mixer on any engine with a reasonably sophisticated management system - like the XJ8 - and the management system will take offence. It's unavoidable, as the mixer system, no matter how 'good', will do things the management system doesn't expect.
That does not in itself stop the mixer working, if you don't mind the twinkling dashboard, but - offended management systems can and do go into limp-home panic mode. This can set ignition timing into default, activate rev limiters (as in the XJ8), limit throttle movement, even affect autobox gearchanges. In short, it don't work.

Put a mixer on an engine with variable-length induction (like most jap V6s) and sooner or later a backfire will occur, bending internal flaps or bursting plastic manifold runners. Wasted spark systems greatly increase the chances of this.
Many engines shut off sparks on the overrun - and the fuel supply of course. A mixer won't, so unburnt fuel/air mix gets in the exhaust and fries the cats up nicely.

There's loads of pitfalls, note that many would also apply to first-generation LPG injection systems with independant mapping. It's not so much the fuel-adding mechanism thats suspect here, it's the management of it, which needs to act in concert with the car's original management. Meaning a 'slave' type injection system.

Don't write off old and simple technology.

Not at all !! It's a horses for courses thing - you absolutely should use appropriate technology e.g. a mixer on a carb engine, and you can 'get away with it' on many simpler EFI engines. But not on most modern cars.

For this particular XJ8 horse, it's a dead one that won't be fixed by tinkering with mixer position. Multipoint slave type system essential.

cheers, Ross K

[tommo2009]

A mixer system will work on any petrol engine no matter what it is
A mixer will not cause anything to take offence as long as the emulation is right.
The vehicle does not know it is on gas, it may see a change in the fuelling but it still does not know it is on gas.
The only electrical connections with a modern LPG injection system are the injector wires, the same as a mixer with an external emulator

A modern system uses the petrol management system to control injection times too keep the AFR happy.

If the lambdas are happy and seeing the Correct AFR and the emulation is correct then the vehicle will run fine and with no lights.
I agree that mixer system can be prone to backfires but as long as the ignition system is good and the engine is in good conditon it should be just fine.

An injection system can give you the same troubles you described, put the car in limp mode etc.

Running an injection system on a direct injection engine will give the same result as a mixer system,burnt out petrol injectors, unless you put a modified version on that uses a small amount of petrol as well to keep the injector tips cool.

As for rev limtiers, was the car being driven or was it stood still in park or neutral. if it was the later then the vehicle will not rev above 2500.

[tommo2009]

by the way, I did not fit this system to the Jag, I would have fitted an injection system LOL

[rossko]

A mixer will not cause anything to take offence as long as the emulation is right.

This is simply dangerous advice so I have to respond. Did you see my comment about fried cats? You can't 'emulate' that away.
Luckily they're rare and so far as I know not made anymore, but a few japs shut off a few cylinders in the cruise, you can't emulate those away either.

I agree that mixer system can be prone to backfires but as long as the ignition system is good and the engine is in good conditon it should be just fine.

You really haven't played with variable-path induction systems, it won't work right with gas/air pre-mixed, I guess it unmixes in the dead-ends.

Running an injection system on a direct injection engine will give the same result as a mixer system,burnt out petrol injectors, unless you put a modified version on that uses a small amount of petrol as well to keep the injector tips cool.

That's right. That's an option not available with a mixer system, ergo you can't run such an engine on a mixer, at all, ever, without causing damage.

[Gilbertd]

As for rev limiters, was the car being driven or was it stood still in park or neutral. if it was the latter then the vehicle will not rev above 2500.

The car was driven and was found to be in limp home mode, revs limited and gearchanges at low revs. It just needs the codes resetting said the vendor. What he didn't realise is that my friend has a code reader and had reset them already, only for them to pop back up as soon as the engine was started.

Bottom line is it was a dog with a totally unsuitable, poorly installed LPG system. Just the sort of thing that gives LPG cars a bad reputation amongst the unenlightened.

[tommo2009]

many engines shut off fuel on the overrun not spark. so yes there will be unburnt fuel in the chambers. but that is the case with all mixer systems, not just "sophisticated" engines.

I have "played" with many types of variable induction manifolds, and some injection systems don't like them unless you can get the inlet nozzles very close to the petrol ones.

my point was that a mixer will work on any petrol engine as long as it draws air through and past the mixer to create a venturi effect.

They are not suitable for some engines agreed, plastic manifolds etc and I would not fit one to anything later than 2001 but they will still work.



You say the codes came straight back up, was that before it switched to gas, what were the codes

[rossko]

my point was that a mixer will work on any petrol engine as long as it draws air through and past the mixer to create a venturi effect.

We obviously have fallen foul of different definitions of 'work' - mine includes 'under normal control', 'efficiently', and 'without damaging anything'.

[Gilbertd]

many engines shut off fuel on the overrun not spark. so yes there will be unburnt fuel in the chambers. but that is the case with all mixer systems, not just "sophisticated" engines.

If they shut off the fuel, where does the unburnt fuel come from? You'll only get unburnt fuel being spat out the combustion chambers if you cut the spark. I suspect the sophisticated ones cut the fuel injectors as well as the sparks or there'd be a big bang out of the tailpipe when the sparks were turned back on.

my point was that a mixer will work on any petrol engine as long as it draws air through and past the mixer to create a venturi effect.

I'm sure it will at constant revs but cars aren't driven at constant revs. The engine management is designed to give optimum performance, and economy, under all conditions. That involves all sorts of clever alterations to numerous engine parameters.

You say the codes came straight back up, was that before it switched to gas, what were the codes

It had no gas in it so was running on petrol. At least one of the codes related to the cat though.

My experience with gas systems is fairly limited I admit, but have been playing with cars for nearly 40 years. The only gas systems I've any experience of are both mixer systems, my own fuel injected Saab 900 and a carb Nissan Prairie 2.8 that a friend used to own. On both these the mixer was fitted immediately before the throttle butterfly, stuffed in the throttle body on mine and bolted to the top of the carb on the Nissan. The operation of both of these is/was nice and simple and worked under all conditions. I still can't actually believe that a mixer at the far end of an intake tract is going to work efficiently but they obviously do. I mean, the mixer does exactly the same job, and works in exactly the same way, as a carb, and you put the carb on the manifold, not on the end of 3 feet of plastic trunking by the air filter.

[rossko]

... You'll only get unburnt fuel being spat out the combustion chambers if you cut the spark. I suspect the sophisticated ones cut the fuel injectors as well as the sparks or there'd be a big bang out of the tailpipe when the sparks were turned back on.

Yup, that's how it goes. We're not talking terribly sophisticated here - this is exactly what 10 year old Landrover V8 do (Motronic), right down to the bang when fitted with mixer ....
I expect there's an emission related reason not to use that strategy on more modern cars, but its just one of a range of hidden traps for mixer installs that will make them impractical. Can we settle on 'impractical' ??!!

[tommo2009]

The unburnt fuel in the chambers is from the gas system not the petrol system. Many modern cars turn the injectors off on the overrun, called the cutoff period. a mixer system can't do this so there is the possibilty of unburnt fuel going into the exhaust. this is the case with all mixer systems, so by what your saying, anything with a cat should not have a mixer system fitted!! ( anything from 1990 on really)
If unburnt fuel is going into the exaust though then there are other problem as the exhaust valves are only open on the exhaust stroke.

The problem you have is a complete plenum chamber full of an explosive mixture, which sometimes ignites causing a backfire in the inlet manifold.

The mixer does not work at constant revs, it uses the amount of air being drawn past the mixer to alter the amount of gas that gets drawn through.

The more you put your foot down the more gas is drawn through, if it only worked at constant revs then the system would never work.

all engine mangement systems use the AFR to alter parameters, it is called closed loop, so if what is going out the exhaust is correct then the engine will be fine.

I did say that the mixer should have been placed at the throttle body and it should also have been fitted with an anti blowback device (catflap type)

[tommo2009]

Forgot to mention, the spark does not usually get cut off, that is why they are called wasted spark systems.

[tommo2009]

There are plenty of shoguns out there on a mixer system, they have variable intakes, they do suffer with backfires.

jeep cherokees have a simple intake and suffer from backfires.

Land rover V8 have simple intakes ans suffer from backfires.

I believe there is a V6 mondeo on here using a mixer system

what is important is the setting up and the maintenace.

I would fit an injection system over a mixer system everyday of the week

This forum is about giving advice, had the Jag owner come onto this forum he would have been advised to fit an injection system, if he had then proceeding to fit a mixer system against advise, would we have been right in telling him it would not work?

[Bat]

Hi,
I'll throw my hat in and say there's mixer systems and there's mixer systems.
The ones with the stepper motor valve in the vapour hose are fine on efi systems with less sophisticated ECUs say around 1995 - 2000ish.
The stepper and control box will do the gas control and overrun cut off for you.
As with just about anything thing you've got to fit the right parts, properly, for a good result
Cheers,
Gavin

[Gilbertd]

Forgot to mention, the spark does not usually get cut off, that is why they are called wasted spark systems.

No, they are called wasted spark systems because the spark plug fires on every stroke and not just the compression stroke. This allows only two coils to be used on a 4 cylinder engine. It makes production costs cheaper but means that there is a wasted spark when the piston rises at the end of the exhaust stroke/beginning of the intake stroke. If you have any amount of valve timing overlap, at top dead centre on that stroke both inlet and exhaust valves are open at the same time. On petrol injection system this doesn't matter because the fuel hasn't been injected into the inlet manifold yet so there is only air in there. On a mixer LPG system there is an inlet manifold full of gas/air mixture ready to be ignited by that 'wasted' spark.

It isn't a new idea either. The Citroen 2CV goes back to the early '50's and they have a single coil firing both cylinders simultaneously on both strokes. This means there is no need for a distributor to complicate things.