DISCO 2 V8i (LPG) RPM HUNTING

Have been experiencing missfiring/hunting on my Disco 2 V8i 2001 when running on LPG around the 2000 ~2500 RPM, above and below this it is smooth and when changing over to Petrol is also smooth?
I have checked and replaced the LPG Filters to no avail.
I am about to renew all the HT leads and spark plugs, however, who has hands small enough to enable accessbetween bulkhead and inlet manifold.
It looks like I'll have to remove all the LPG solenoid assemblies.
Has anyone else suffered the same problem with HT lead renewal?

Did my leads an plugs last week.......... no fun, but possible without takin off the center manifold assy. If the gas stuff is in the way, u need to demantle it, then remove the secondary air pipe from its spot above the coils. then u can pry off the old plug leads from the coils using a long pre-bent screwdriver. just enuff room to get a finger or three down to replace the new leads then. Buy some good leads - Magnatec etc - cos u only want to do the job once!! Total job took a couple of hours

If yer tanks are on the outside it will hesitate a bit. Cos it's been cold.
Below zero mine really struggles on LPG (underslung tanks)

pandj said:

Did my leads an plugs last week.......... no fun, but possible without takin off the center manifold assy. If the gas stuff is in the way, u need to demantle it, then remove the secondary air pipe from its spot above the coils. then u can pry off the old plug leads from the coils using a long pre-bent screwdriver. just enuff room to get a finger or three down to replace the new leads then. Buy some good leads - Magnatec etc - cos u only want to do the job once!! Total job took a couple of hours

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Cheers
Thanks for the reply, will attempt this method, however will also have to remove the JW piping to the vapouriser which runs accross this space.
As a matter of interest what plug gaps did you use? The set I have just fitted are 0.6mm. as recommended by suppier for LPG use. However, since fitting them, I now find the engine "hunting" over a larger rev range? Hopefully changing the HT leads will rectify this problem. The leads to be fitted are triple supersilicone (JANMOR) USA manufactures (again recommended by supplier).
Again thanks for the feedback.

lpg systems need to have perfect leads and plugs as it does not take much to make them run rough. your problem sounds like the temperature is too cold for your system to work properly especially if it runs ok on petrol.you will probably hear it backfiring or "spitting" and this is due to the gas freezing before it can be burned.are you sure the heater in the vapouriser is working properly or you have not run low on engine coolant as the warm water usually flows through the gas system.if you allow it to backfire sometimes you can destroy your airflow meter so keep your eye on it as i have known a backfire on gas to blow the airbox apart!

my 96 3.9V8i LPG used to hunt on gas aswel when i got it, couldnt figure out the problem. I noticed if i put my foot on the accelerator just a tiny bit it would stop. In the end i ended up tightening the accelorator cable just a tiny bit and the problem stopped!

One of my mates had a RR V8 with LPG and had similar problems fault was slight crack on an air intake hose going to the inlet manifold perhaps you got a slight leak

Plug gaps are 1.0mm and firing fine according to test book check up today. Generally find backfire probs on LPG only come with the old single point setups........ newer multipoints are fairly trouble free in my experience (famous last words, no doubt....) I have underslung tanks and have never had probs in cold weather with gas freezing up.

colinl said:

lpg systems need to have perfect leads and plugs as it does not take much to make them run rough. your problem sounds like the temperature is too cold for your system to work properly especially if it runs ok on petrol.you will probably hear it backfiring or "spitting" and this is due to the gas freezing before it can be burned.are you sure the heater in the vapouriser is working properly or you have not run low on engine coolant as the warm water usually flows through the gas system.if you allow it to backfire sometimes you can destroy your airflow meter so keep your eye on it as i have known a backfire on gas to blow the airbox apart!

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Thanks for the feedback on my problem, but do not think it has anything to do with the vapouriser or temperature pre-injection as this would be more apparent when there is a greater demand for the gas, whereas this is at mid rev range the "hunting" occurs.
Another member mentioned the "free play" in the throttle cable - this may seem strange but there is no "hunting" when accelerating only when trying to maintain constant speed? I have also checked that the Cruise Control servo linkage is not affecting the throttle response too.

mrnice said:

my 96 3.9V8i LPG used to hunt on gas aswel when i got it, couldnt figure out the problem. I noticed if i put my foot on the accelerator just a tiny bit it would stop. In the end i ended up tightening the accelorator cable just a tiny bit and the problem stopped!

Click to expand...

Thanks for the response, much appreciated.
I had previously checked this cable tension and had also thought that the Cruise Control servo linkage may have been causing interference with the main throttle response, but since changing to a new set of plugs with the gaps at 0.6mm. the "hunting" is now over o wider RPM range - so this makes me more convinced that it is electrical( HT leads and Plug gaps) that are causing the problem and not mechanical.

I have what sounds like exactly the same problem on my 1995 3.9 efi with lpg. I thought it was definitely an ignition problem but I have changed plugs, leads, dizzy cap, rotor arm and checked out the dizzy and vacuum advance and the problem is still there.

Local Land Rover specialist says it sounds like a gas problem. I am now focusing on the gas and plan to change the gas filter and change the vaporiser as I have been given a new one. Interesting point about leaking hoses though so I'll check them again first.

v8 disco biscuit said:

I have what sounds like exactly the same problem on my 1995 3.9 efi with lpg. I thought it was definitely an ignition problem but I have changed plugs, leads, dizzy cap, rotor arm and checked out the dizzy and vacuum advance and the problem is still there.

Local Land Rover specialist says it sounds like a gas problem. I am now focusing on the gas and plan to change the gas filter and change the vaporiser as I have been given a new one. Interesting point about leaking hoses though so I'll check them again first.

Click to expand...

Changed leads today(who designed the positioning of these coils and leads??) and still find engine running similar to before - possibly slight improvement, but stiil giving slight "hunt" when maintaining revs around the 2000~2500RPM range.
Will now have to investigate the "Gas" side now. I have a 'Bigas" vapouriser which has a dual outlet to the gas solenoid manifolds, these in turn have inline filters fitted, possibly they are partially blocked? Has anyone had any experience with these units blocking? I now have the feeling that possibly one of the inline filters is partially blocked, which could be causing the erratic/uneven running. The blocked filter could be causing the gas to expand after the filter( same principal as a Freon Expansion valve assembly).

Changed vapour filters to no avail, still "hunting", experimented with various plug gaps -again no difference.
Will have to bite the bullet and get some diagnostic testing carried out to see exactly what is happening within he system. the basic and more logical remedies have been tried.
Anyone with any further ideas?

Further investigation through various other web sites on LPG running problems has highlighted some problems with the MAP sensor which in turn can affect the air/gas balance via the ECU and cause RPM variation at certain loads.
Has anyone else experienced problems with the MAP sensor on V8 LPG systems?:doh: