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[General Confusion]

Number One Son has a '98 4.0L XJ running on petrol with an LPG conversion. It's single point injection. He can get it to switch and run on LPG, but it will not tick over and will not rev above about 2K when under load. If you just sit outside the house with it in Park you can rev the nuts off it on gas.

Any idea's please?

[Hillbilly]

My XJ and ZJ both did that. Rough tickover and in fact, they cut out when you floored the kickdown. Most embarrassing. New set of ht leads sorted it out on both. Just to add, it ran ok on petrol even with the dodgy leads.

[Green Jeep]

Start with plugs, leads and cap as we know LPG needs a better spark than petrol, maybe coil is failing? Check for a cracked coil as they get hot and break down under load.

Also remove and clean all of the earth connections, check the connections to the lpg ecu and even open it up and check water hasn't got inside. Invest in some electric spray clean connection stuff?

Failing that ask and find a decent lpg specialist who will repair and not try and sell you a new system/waste your time & money, took me a while to find one and he had the system sorted and running within minutes where others had spent hours... charged £20 now runs better than on petrol.... which is prob as the cheap petrol pump on the conversion is not great?

[Brynjaminjones]

Could the injector possibly be knackered, therefore not allowing sufficient gas flow under load?


A couple of bad injectors in the sequential system on our ZJ were to blame for a bad misfire.

[General Confusion]

Could the injector possibly be knackered, therefore not allowing sufficient gas flow under load?


A couple of bad injectors in the sequential system on our ZJ were to blame for a bad misfire.

It's single point.

[Brynjaminjones]

Could the injector possibly be knackered, therefore not allowing sufficient gas flow under load?


A couple of bad injectors in the sequential system on our ZJ were to blame for a bad misfire.

It's single point.

Yup, which is why I figured that if the only injector was bad it could cause it to completely cut out

[LairdScooby]

A single point system isn't an injector though, it's commonly known as a mixer because basically it's a mixer ring that sits between the throttle body and air intake trunking.
So far everything you've said points to exactly what all the others have suggested, however if the new ignition parts don't cure it, there could be several other problems. As i really don't want to throw confusion into this or possibly suggest something that would upset the system so that it wouldn't run properly even after finding the fault, i'm not going to mention them now.

If the ignition bits don't sort it, can you try and get some more information on the system, maybe even a photo of the LPG ECU and how the vapouriser is plumbed into the cooling system - all this sounds a bit odd i agree but i've had my Jeep just under a year now and that has LPG on it, so far i've had to replace a few bits, tweak others and alter the installation to get rid of running faults. It now runs as good on LPG if not better than on petrol, even taking full throttle a few minutes after a cold start with no problems. Not something i do often as you should never thrash a cold engine but it was done a couple of times simply to prove the point.
I'm not an expert by any means but i've learnt a hell of a lot in the past year and can almost certainly help you get to the bottom of the problem and if not, point you towards someone that can. I'm also in the middle of converting one of my Rover 827s with the other one to be done soon after.

[Brynjaminjones]

A single point system isn't an injector though, it's commonly known as a mixer because basically it's a mixer ring that sits between the throttle body and air intake trunking.

Ahh, my bad. I knew mixer systems were available but thought the newer single point ones were injected.

Sent from my HTC One M8s using Tapatalk

[LairdScooby]

I have heard there are a couple of single point injection systems out there but the ones i have seen are for little engines like Metro size or smaller.
All the 4.0L Jeeps i've seen are either multipoint injection or single point mixer like mine :



The bright, shiny bit is the inside of the air intake trumpet - not sure how it got that shiny though! 

[Panic mechanic!]

when was the last time it was serviced? and had the filters cleaned?

[General Confusion]

when was the last time it was serviced? and had the filters cleaned?

About three months ago. He does have a crack in his airbox. I'm wondering if that might not be helping.

[LairdScooby]

I doubt the cracked airbox is causing it but how did the airbox get cracked? Was it because of a backfire through the manifold?
If so it's possible it also loosened the mixer on the throttle body which should be sealed to the throttle body with RTV sealant. Also what sort of system is it and have you got the software/leads and a laptop to check out the settings on the LPG?
Have you checked the vapour hoses from the vapouriser to the stepper-motor and mixer to make sure they're not loose, split, perished or whatever?

Now for a strange question - what's the coolant level like? If it is low there is a possibility the vapouriser isn't getting the full flow of coolant so could be icing up which will cause blockages, this will obviously be worse under load because more gas is going through until the vapour pipe freezes and any moisture in there causes ice which blocks the pipes.

[General Confusion]

I believe the airbox was caused by a backfire. We have the software and lead, just arrived. I'll get him to check everything that you have mentioned. Thanks for the advice.

[LairdScooby]

Sounds like it could be running rich, hence the backfire. Sometimes lean running can cause it as well (for example if he's run low on gas) but worth checking the stepper-motor operation with the software. Don't know what ECU he has but the AEB Leonardo 175 software has a display on the computer screen to show what the stepper-motor is doing. In this pic it is shown as "Actuator" :



You can see the difference running on LPG here :



Neither of those were anywhere near spot on but i did improve considerably on them, the LPG system on my Jeep was horrendously badly set up when i first got it, way too rich (and backfiring!) and prone to flooding, cutting out on the over-run and several other problems including an eye-wateringly high (and MoT failing!) 3.9% CO at idle! No wonder i only got 12-15mpg on gas!