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V8 misfiring and down on power


v8discosteve

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Hi all, I am new on here I have been trawling through your posts for information and have now joined as I really need to get some ideas off people in the know.

Sorry to any LR specialists but i can't keep throwing new parts on and getting little improvement . Sorry to waffle!

Anyway the car is 1998 disco 1 V8 Auto with single point LPG system. 75,000 miles no water use issues or overheating and oil use about a pint between 3000mile oil changes.

I have done or had done over the last few months! (been done)

Alarm spider bypass wiring

Complete new distributor with amplifier,coil, cap and rotor arm(powerspark products) Made a big difference initially old distributor was worn.

Magnecor leads when I 1st had car cured lpg inlet bangs!

NGK plugs

LPG service

New mass air flow meter

New catalytic converters due to collapsed one side and blocked pipe! Bit concerned the misfire will destroy these again

I also changed the valley gasket which did improve erratic idle. But it is erratic again just not sure if it is because its misfiring?

Over the last 6 months the car has had a very slight misfire which has now developed into it running on about 7 to 8 cylinders most of the time and strangely it runs smoother and misses LESS on lpg. But the problem is on both fuels so i am pretty sure it is not fuel issue. Also when you try to accelerate hard it feels as if it is holding back getting no fuel in, gradual build up of speed not nearly as bad.

All 8 spark plugs are firing if I try with a spare one.

I did a compression test and all were very similar result can't remember now the figure.

I have added an extra earth cable to the engine and cleaned and checked all the wiring connections etc.

I am thinking it is a worn camshaft the mileage is about right from what has been mentioned on here? Wish I had looked more when I did valley gasket.

I shall remove rocker covers and see if I can see a rocker or rockers not moving as much also there is a slightly noisy tappet but it is a faint noise only but presumably that is enough to be bad!

I have seen on fleebay a full camshaft kit for £122.00 Item number: 390124076701 any thoughts on this or advice where to get a kit would be appreciated.

Thanks and sorry for the long post but hopefully enough info on it to make sense?

regards steve

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This sort of misbehaviour, coming on relatively quickly, is unlikely to be camshaft - most problems with V8's are down to the electrics.

There's a full diagnostics manual for the V8 Hotwire EFI in the technical archive, it should give you some good pointers (and the LPG side of things will probably work along similar lines).

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Hi, thanks for the reply, i can't get my head round why when I remove the plugs they are all normal colour not one oiled or wet etc so can't see why missing and seems to not be on any specific cylinder thought new distributor would have dealt with spark scatter. There was play in the old one, rotor could be moved sideways a bit.

Also it hasn't come on quickly i have just tolerated it for long while, guess its been over last 12k miles just now its really got a lot more noticable. Towing there is a big lack of power when hitting hills etc.

The electrics also seem to have been covered pretty well too. No vacuum leaks either.

Thanks again

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Hi,

The distributor is from simons best british classics, powerspark and the rotor and cap (blue cap) came with it, prior to fitting new distributor I did have a genuine LR new cap and rotor arm. Also the amplifier was fitted on side of dissy too. The plugs ngk as suggested on forum and leads blue magnecor.

thanks

steve

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Get a known good amplifier and try again... they are renowned for being bad out of the box, even genuine!

Change cap and rotor arm for new genuine items.... seriously, don't ever bother putting aftermarket stuff on them.

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Thanks for the replies,

I have changed cap, rotor arm (genuine) and put another amp on with no change. I then started on the wiring again and have got a big improvement in the running after re checking the wires around coil area. I have soldered new connections on as a precaution. Power steering fluid leaks in area so wires were covererd too!

So you were correct as usual a wiring issue I will continue with wiring tests.

But I will do a camshaft soon as I have water leak (very small) between block and housing timing chain is in so will make sense to change the lot then. Is it worth a cam upgrade without doing any other mods or just stick with stock cam? Would be nice to be able to keep up with old mans 4.6hse p38!

Thanks again it is good to have a V8 again not a V? Still find it has no grunt when accelerating hard but this seems to be another issue!

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I'm not expecting the impossible but i've been using our old jeep (I guess that is a swear word on this forum) cherokee 4.0ltd on lpg single point too and boy does it pull and show the shortcommings of my disco! We use the jeep for work and as a back up for when the land rovers are poorly! The jeep is very old (L reg) and it has now covered over 200,000 miles pretty neglected apart from oil changes and it just keeps going. Every year expecting to scrap it MOT time but a few minor jobs and away it goes again. Not wishing to speak out of turn but I wish the land rovers we have owned had beem half as reliable. Needless to say I love the land rovers for comfort, space and for towing especially but wish i could go a few more miles before the next repairs! ABS light now on constant again too but not just sensor gaps! Better read posts for that too!

Bet I have just opened a can of worms saying that a jeep is good!

steve

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It'll probably benefit as much from just having an overhaul with a new std. cam etc. than modifying it. If you're determined to tweak it, looking at MegaSquirt & EDIS would be a good first step to making it go better.

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Check the layout of the HT leads: the Rover V8 is renowned for 'crossfiring' where a spark sent down one HT lead induces a spark in an adjacent lead - if that cylinder's on an intake-stroke when its plug gets fired it can cause all sorts of odd backfires/misfires.

Also, you mention "single point" LPG system - is this a properly-mapped LPG-injector with ECU and lambda-sensor feedback or a crude squirt-and-guess vapouriser/venturi-mixer? If the latter then getting it to run efficiently across all combinations of RPM and throttle-position is really a loser's game.

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