Backfiring / Misfiring problem on 4.6V8 P38

Hi all. My RR P38 has recently, and quite suddenly within the space
of a week, developed a backfiring / mis fire problem. I think it's
more a back fire than a misfire.....as you hear the noise of it back
though to the large air scruber filter box. So firstly, am I correct
in thinking it's a back fire, not misfire???

In any event, the firing problem seems to be mildly present even on
tick-over.....but the backfiring becomes more serious at a certain
throttle rev range?...and is enough to make the vehicle judder.
The problem came to light last week on a very damp and wet day
(remember all the rain we'd been having!!) and as I was accelerating
away (doing a rolling start off blocks on the M6 TOLL plaza...as you do
with a V8!!!!!!) it started playing up.

I took it easy and limped home, and I've since checked usual things
like plugs and plug leads for secureness or dampness, and that all
viseable electrical connections look ok, and that all viseable 'air
intake' related hozes and pipework look ok. Which they all visually
look ok and secure. I've had some friends tell me that the symptoms
sound like something really serious and expensive like maybes a burnt
exhaust valve or broken air-meter sensors etc.
Has anyone out there come across similar probs, and / or can offer any
advice.

Cheers, John-o.

SIII 88 County
RR Classic Vogue
RR P38 4.6

<[email protected]> wrote in message
news:[email protected]...
> Hi all. My RR P38 has recently, and quite suddenly within the space
> of a week, developed a backfiring / mis fire problem. I think it's
> more a back fire than a misfire.....as you hear the noise of it back
> though to the large air scruber filter box. So firstly, am I correct
> in thinking it's a back fire, not misfire???
>
> In any event, the firing problem seems to be mildly present even on
> tick-over.....but the backfiring becomes more serious at a certain
> throttle rev range?...and is enough to make the vehicle judder.
> The problem came to light last week on a very damp and wet day
> (remember all the rain we'd been having!!) and as I was accelerating
> away (doing a rolling start off blocks on the M6 TOLL plaza...as you do
> with a V8!!!!!!) it started playing up.
>
> I took it easy and limped home, and I've since checked usual things
> like plugs and plug leads for secureness or dampness, and that all
> viseable electrical connections look ok, and that all viseable 'air
> intake' related hozes and pipework look ok. Which they all visually
> look ok and secure. I've had some friends tell me that the symptoms
> sound like something really serious and expensive like maybes a burnt
> exhaust valve or broken air-meter sensors etc.
> Has anyone out there come across similar probs, and / or can offer any
> advice.
>
> Cheers, John-o.

Without hearing it, I'd say it's ignition related. Is it Gems or Thor
(Motronic)? Best bet is to get it on an analyser if Gems, or
Testbook/Autologic/Rovacom if Motronic and milk the ecu for misfire
detection fault codes. There is a little history of the Thor versions eating
coil packs, my DII did it and I've replaced a good few for others as well,
but never had to replace a coil pack on a Gems as yet.
Badger.

Hi, Thanks for the info. Off hand I think it's the Thor type - but
couldn't be sure of the top of my head. I'd better get it booked in at
my local LR Services garage then for them hook it up to a PC. Cheers
for the info! John.


Badger wrote:
> <[email protected]> wrote in message
> news:[email protected]...
> > Hi all. My RR P38 has recently, and quite suddenly within the space
> > of a week, developed a backfiring / mis fire problem. I think it's
> > more a back fire than a misfire.....as you hear the noise of it back
> > though to the large air scruber filter box. So firstly, am I correct
> > in thinking it's a back fire, not misfire???
> >
> > In any event, the firing problem seems to be mildly present even on
> > tick-over.....but the backfiring becomes more serious at a certain
> > throttle rev range?...and is enough to make the vehicle judder.
> > The problem came to light last week on a very damp and wet day
> > (remember all the rain we'd been having!!) and as I was accelerating
> > away (doing a rolling start off blocks on the M6 TOLL plaza...as you do
> > with a V8!!!!!!) it started playing up.
> >
> > I took it easy and limped home, and I've since checked usual things
> > like plugs and plug leads for secureness or dampness, and that all
> > viseable electrical connections look ok, and that all viseable 'air
> > intake' related hozes and pipework look ok. Which they all visually
> > look ok and secure. I've had some friends tell me that the symptoms
> > sound like something really serious and expensive like maybes a burnt
> > exhaust valve or broken air-meter sensors etc.
> > Has anyone out there come across similar probs, and / or can offer any
> > advice.
> >
> > Cheers, John-o.
>
> Without hearing it, I'd say it's ignition related. Is it Gems or Thor
> (Motronic)? Best bet is to get it on an analyser if Gems, or
> Testbook/Autologic/Rovacom if Motronic and milk the ecu for misfire
> detection fault codes. There is a little history of the Thor versions eating
> coil packs, my DII did it and I've replaced a good few for others as well,
> but never had to replace a coil pack on a Gems as yet.
> Badger.

<[email protected]> wrote in message
news:[email protected]...
> Hi, Thanks for the info. Off hand I think it's the Thor type - but
> couldn't be sure of the top of my head. I'd better get it booked in at
> my local LR Services garage then for them hook it up to a PC. Cheers
> for the info! John.

Another thing to consider if it's the thor type - they use spark plugs with
a different ceramic insulator diameter (smaller) and if you fit the wrong
plugs at any point it stretches the insulator boot on the end of the lead,
rendering them useless at keeping moisture etc out of the end fittings,
especially if you later revert to the correct thin-body platinum plugs. The
thor's motronic management system is extremely complex in the manner in
which it provides misfore detection and it doesn't respond well to the
fitting of plugs other than platinum, or irridium for lpg use.
If testbook etc shows up multiple misfires on any two cylinders that share
opposite ends of a single coil (1&6, 8&5, 4&7, 3&2) then it most likely
points to a fault within that coil pack. The packs are around £40each from
landrover and there are two of them, but the upper half of the inlet
manifold has to be removed to replace them! Best to replace the leads at the
same time, around £90 for the set!! The lead end fittings are not std and as
far as I know there are no decent quality aftermarket ones available... cue
Beamends?

Badger.