Hello and Discovery 2 V8 misfire when idling and pulling away

[sponcom]

Hello All

Yesterday I bought my first Land Rover - a 4.0 V8 Disco 2 with Prins LPG conversion. It has a couple of niggles like the rear sunroof and a sticky door lock and other common issues but otherwise very pleased!

The main issue is that when idling it is running a bit rough/misfiring on petrol and LPG and when pulling away is misfiring. Once it gets going and the RPM increases it appears to run fine and when idling it ticks over at just over 700 RPM with no hunting. It also seems that it only does this once the car has warmed a little, it appears to run evenly on a cold start when the RPM/Fuelling is increased.

There are no warning lights on so there wont be any codes that I can read with my OBD tools. Does this sound like a common problem that has been experieced before or does anyone have any suggestions where to start looking or what to try to perhaps eliminate the cause before I go and start purchasing replacement plugs, coils, leads, lambda sensors etc. I'd like to try eliminate causes if I can as it's been an expensive month so far!

Many thanks, Paul

[jiginc]

So the problem starts when the system goes to closed loop (when the engine gets warm) Try disconnecting the MAF and see if that makes any difference also you could try unplugging the lambdas.

I had a similar problem and now run with the lambdas permanently disconnected. I find it makes no difference to performance or MPG.

I have also replaced the expensive plugs with standard ones and that gave a much better performance on LPG

[boon]

i had the same problem same engine with multi point system anyway £800 pound later turned out it had 2 different size nozzles on the injector rails 2.5ml on 1 rail and 3.0ml on the other so the engine was constantly trying to compensate (advice is make sure you get someone who knows what they are doing with lpg) hope this helps.

[FridgeFreezer]

MAF is not part of the "closed loop", it's required to run open-loop as well. Lambda sensors are the feedback source for closed-loop.

Don't run fancy plugs in a V8, they are just as likely to make it run worse than better.

[sponcom]

Thanks for the replies. I connected up my ODB reader today and found a fault that the heater on bank 1 sensor 1 was open or shorted. This did not throw an engine management light on though. If I disconnect the lambdas I assume it will run off the default values in the ECU and therefore should be better. It certainly seems to be fine when first started. It also misfires until it gets going which I assume is somewhere above 1500 RPM and then seems OK.

I'll disconnect the lambdas in the morning - I tried this evening but couldn't get the sockets off the tabs where it's mounted. It was getting dark though!

Thanks for the tips, I was looking at LPG plugs but I read conflicting reports on whether they were needed or not. I assume the people that did the 2.5K Prins conversion knew what they were doing so I'll stick with the ones in there!

Cheers

Paul

[sponcom]

As an aside, sorry for my delays in replying to you. I have to have my posts (understandably) approved by a moderator before they are made public until I've made another couple of contributions.

Paul

[chrisbowler]

If you have a diagnostic tool then you should be able to see engine misfires - by cylinder - this will help you to identify which cyl is misfiring - usually get a misfire count and it normally points to the problem cylinder - then it is either coil pasck, lead or spark plug - if not it will tell you which cylinder is under performing - vlaves, rings and compressions

Chris Bowler

[Ally V8]

Just to add a bit more,no check engine lamp does not mean no fault code.Land Rovers are well known for not giving up info to just any old code reader,and often giving wrong info.Testbook or similar is the best way to see whats going on.

The most common reasons for faults like this are a low reading maf.(Should read 22-25KG/HR at idle,90+ at 3000rpm engine hot all loads off) Long term fuel trims being off as a result of poorly set up/maintained LPG system.Or just duff plugs / leads.Genuine leads and NGK BPR6ES plugs work very well.The recommended platinum plugs have a very wide gap and require a very high voltage to fire them,esp on LPG.(Very easy to see/prove on a scope)The high voltage is tough on the leads,esp when damp.They often burn out at either end,usually taking the top od the coil tower out at the same time.....

[sponcom]

Hi All

A bit of feedback and recap. The Disco was running pretty bad on idle and hesitated puling away. It was indeed down to ignition and the O2 sensors are OK. They made little difference whether they were unplugged or not. I started by reducing the gap on the existing plugs and it ran considerably better but far from perfect. I then replaced them with a new set of PFR6N-11 and left the gap unchanged. It ran pretty well and had no hesitation but idle was still a little uneven and signifcantly worse on LPG so much so I would change back to petrol when at traffic lights. I read a lot about plugs and gaps on gas, much conflicting but I ended up changing the spark plugs for BKR7EIX. I set the gaps to 1.0mm (down from 1.1mm of the PFR6N-11). Note these are not the BKR7EIX-LPG plugs. It now drives very well and idles pretty much perfectly.

I suspect that the BKR being a colder plug made it work better on LPG and the reduction in GAP of both lugs was helping. I suspect that new coils and leads wouldn;t go amiss but those coils are damned hard to get to when you've LPG fitted and the magnacore leads are bl**dy expensive as well!

Thanks for your pointers chaps.