Range Rover Backfiring

neilw · Post by **neilw** » Sun Jan 25, 2015 11:01 pm

I have a Range Rover with a 4.2 with LPG, owned it for about 9 months.

Before I get onto my current problem it's probably worth me explaining some background.

This is not my first V8 on LPG, I had a Discovery 3.9 a few years ago, and based on that experience (i.e. it not starting after being left in a damp station car park), as soon as I got the RR I fitted NGK plugs, Magnacor leads, new Lucas coil, Dissy cap and rotor arm. All was good and as I run it on petrol and LPG I had an RPi Advance and Retard box fitted.

All was well for a couple of months, doing 75-100 miles a week.

It then started hunting badly. Initially on tick over then throughout the rev range. It would stall a few minutes after starting.

I took it to a local Land Rover specialist (as it was already booked in to get the LPG system serviced). He found a broken Lamda sensor, but could not get the time to get to the bottom of the hunting issue. Other ECUs, distributors and coils were tried, but as it was in his yard for a couple of months with the issue not getting resolved I ended up collecting it.

When I collected it I had a nightmare trip home, it stalled at every junction and the revs would die as as I took my foot off the accelerator.

Managed to get it booked into another independent Land Rover specialist, and a new Mallory Distributor and coil was fitted, timing was done, vacuum checked, plenum cleaned out etc - this has solved the hunting problem, but has given me a new issue.

If I take my foot of the accelerator it will now backfire in the exhaust. If I keep on accelerating then there's no problem, and the acceleration is better. But as soon as I take my foot off it backfires, looses power then surges back again. Does it on both petrol and LPG

This is getting frustrating. I've checked all the ECU diagnostics and I know the Lamda reading and Airflow meter readings are fine.

I'm now at the stage of working out what to do next.

The specialists have said it could be:

sticking valves (I did a compression test and all cylinders are giving similar reading)
stretched timing chain, may have jumped a tooth
worn cam shaft

As there's a lot of experience on the forum, I was wondering if anyone has had similar issues, or care to offer advice on what the next stage of investigation should be

thanks.

ChrisJC · Post by **ChrisJC** » Mon Jan 26, 2015 6:36 pm

As an experiment I would mark the distributor position, then try altering the timing to see what that does for you.

Chris.

kiwicar · Post by **kiwicar** » Tue Jan 27, 2015 5:01 pm

Hi
get a new cam chain the one you have has too much slack in it so the engine is firing the wrong cylinder on over run, you could try re-centring the rotor arm on the electrodes in the rotor cap but if you are starting on petrol and switching over then it will be out of spec on petrol. I think you are not going to get it to work without a new chain. From my experience you probably have exhaust valves seats that are burnt by now and are not sealing properly, if the cam is more than about 80K old then it could probably do with replacement, but will not be contributing to the back fire much unless the distributor gear on it is exceptionally worn. Better still take the LPG off and chuck it over the nearest hedge rebuild it and run it on petrol, you get about half the service life out of ignition and valve train components on LPG as you do on petrol so you never make any real savings and the car spends half it's life running like a dog.
Sorry personal thing but I hate the stuff.
Best regards
Mike

ChrisJC · Post by **ChrisJC** » Tue Jan 27, 2015 6:44 pm

kiwicar wrote:. Better still take the LPG off and chuck it over the nearest hedge rebuild it and run it on petrol, you get about half the service life out of ignition and valve train components on LPG as you do on petrol so you never make any real savings and the car spends half it's life running like a dog.

I have to disagree about that. I like LPG!

Chris.

kiwicar · Post by **kiwicar** » Tue Jan 27, 2015 9:16 pm

Hi Chris
thank which ever deity you affiliate yourself with that we are all different, some like LPG some don't.
After all if everyone liked it everyone would have it and the big car manufacturers would sell it as an OEM option on their cars and where would the fun be in having LPG.

best regards
Mike

neilw · Post by **neilw** » Wed Jan 28, 2015 9:44 am

thanks for the replies

The MOT is due next week, so I'll put it in for that and see what else needs doing before I do anything else, just in case there's more undiscovered horrors lurking.

I suppose worst case scenario, if there's a load of structural work to do, is to SORN it for a while.

I'd suspected the timing chain, the engine has done 170k, no history but I did contact a previous owner who had done a lot of work on it a couple of years ago, but I have no idea if the camshaft and timing chain have ever been replaced.

I did mange to pick up a a genuine new Land Rover 4.2 Cam shaft a while back in a collection of spares so that's a few quid saved.

So, time to learn some new skills then. Apologies in advance for any numpty questions I'll be asking once I get cracking on this.

DaveEFI · Post by **DaveEFI** » Wed Jan 28, 2015 11:44 am

I'd be surprised if it had managed 170k on the original camshaft and it was still ok. Might do on a vehicle which only cruises the motorway - but that's unlikely given the age.

neilw · Post by **neilw** » Wed Feb 04, 2015 5:38 pm

MOT passed, very happy about that.

So planning to sort the top end out, so heads off and to be sorted, new timing chain and camshaft will get done. Also gives me a chance to sort out the annoying idler pulley bearing and a load of other stuff.

neilw · Post by **neilw** » Tue Mar 03, 2015 9:19 pm

OK, heads now off. Timing cover came off this evening. Wouldn't mind some input on the slackness of the Timing Chain, how does it compare to others? I have no idea if this is too slack and the cause of my timing/backfire issue.

I'm assuming it should be tighter than this. It will be replaced when I put it all back together again.

ChrisJC · Post by **ChrisJC** » Tue Mar 03, 2015 9:34 pm

Clearly not new, but I wouldn't have said that was too bad. If you turn the camshaft back and forth, you can see there's only a few degrees variation.

Chris.

kiwicar · Post by **kiwicar** » Tue Mar 03, 2015 9:38 pm

Hi
yes that is slack even for a rover v8 hold the bottom pulley still and move the cam back and forth and see how many degrees it moves, add in the slop for the distributor drive gear and the up/down slop on the dizzy gears add it all together and you will be looking at probably 20 degrees of slop.
best regards
Mike