Disco 2 V8 Auto misfire

Landrover Bear · Jan 11, 2020

Hi any advice on this would be appreciated !! car has 113,000 miles full history well looked after ive had her for 6 months . Misfire started had good look around and plug leads looked tired , one broke in half when removing it . Fitted new set of plugs , leads and 2 new coil packs ….. I thought the problem had gone , as only tested it on a short run , but its still doing it . Sometimes just a mild vibration , other times power down , very audible misfire . Sometimes it makes a odd noise crossed between a squeak and a chuffing noise, piston speed related . Backing off the power , then on again sometimes settles it down for a short while . Worse under load , have run 2 injector cleaners through her . Hot or cold always starts on the button hardly turns over, idles perfectly . No oil / water contamination etc . From cold start usually fine , once warming up misfire starts . Very frustrating !! Any help would be gratefully received !!

kermit_rr · Jan 11, 2020

Chuffing sounds like compression leakage to me, head gasket, or plug the most likely
Have you done a compression test?
Have you checked all valves are opening and closing fully?

Landrover Bear · Jan 12, 2020

kermit_rr said:
Chuffing sounds like compression leakage to me, head gasket, or plug the most likely
Have you done a compression test?
Have you checked all valves are opening and closing fully?

Landrover Bear · Jan 12, 2020

kermit_rr said:
Chuffing sounds like compression leakage to me, head gasket, or plug the most likely
Have you done a compression test?
Have you checked all valves are opening and closing fully?

Hi Kermit , thanks for the reply ! No I haven't done a compression test yet , was thinking it would be a good idea before taking her to bits! Surly a compression leak would be permanent once it started ? Ironically to day I had a 10 mile blast on a duel carriageway , and she was running sweet as a nut cruising at 60 odd slowed down to come off a the junction , then accelerated on the slip road and she sounded like a machine gun !!!! Will invest in a compression tester ! Presume if its down on 1 or 2 cylinders that would indicate either the head or valves . Do have a ticking lifter occasionally . Presume to check the valves is rocker covers off and spin her over plugs out? Bloody top manifold off again !!!

kermit_rr · Jan 12, 2020

machine gun.. hmm.. depending how loud.. that does sound like a loose plug.. but that would be consistent and wouldn't take long to blow the plug out
Let's see what anyone else comes up with before you start taking bits off

discool · Jan 12, 2020

Machine gun sound will be probably be due to a blown manifold to the Y down pipe gasket, which isn’t unknown with the V8,
I had the machine gun sound issue which stayed like that for six months until my disco had the auto gearbox fluid & filter change, then as the exhaust is separated anyway, new gaskets are used when refitting back the exhaust, so issue was gone.

As for the noise “crossed between a squeak and a chuffing-noise” I haven’t a clue what that would sound like

Landrover Bear · Jan 17, 2020

kermit_rr said:
machine gun.. hmm.. depending how loud.. that does sound like a loose plug.. but that would be consistent and wouldn't take long to blow the plug out
Let's see what anyone else comes up with before you start taking bits off

Yes its odd , I am going to put a compression tester on her tomorrow afternoon . Really scratching my head , yesterday she drove for probably about 10 miles beautifully, then misfires . Cant see it could be head gasket as once its gone its gone !!

DanClarke · Jan 17, 2020

My P38 4.6 had the machine gun sound.
Most noticeable when drivers side was close to a hard sound reflective surface like the central reserve barriers stationary on the M25 (happened a lot).
Was a failed exhaust manifold gasket. When the manifold was removed you could see a thick black carbon line where it had been chuffing.

Landrover Bear · Jan 17, 2020

discool said:
Machine gun sound will be probably be due to a blown manifold to the Y down pipe gasket, which isn’t unknown with the V8,
I had the machine gun sound issue which stayed like that for six months until my disco had the auto gearbox fluid & filter change, then as the exhaust is separated anyway, new gaskets are used when refitting back the exhaust, so issue was gone.

As for the noise “crossed between a squeak and a chuffing-noise” I haven’t a clue what that would sound like

Manifold is fine , the chuffing is only noticeable at slow speed , window down and I would say it relates to piston speed , I half wonder if its a fuel problem and when she's not getting enough fuel to combust in a chamber obviously no combustion so the compressed air is being forced out , but that's a wild guess . Many moons ago had a Saab that was seriously over fuelling in 1 cylinder on start up , not combusting in that cylinder and that was forcing out un burnt fuel via the head gasket , similar squeaky chuff ! changed the injector and it was fine !

Landrover Bear · Jan 17, 2020

DanClarke said:
My P38 4.6 had the machine gun sound.
Most noticeable when drivers side was close to a hard sound reflective surface like the central reserve barriers stationary on the M25 (happened a lot).
Was a failed exhaust manifold gasket. When the manifold was removed you could see a thick black carbon line where it had been chuffing.

Yes , the noise is most noticeable, pulling out of the tole booth on the Tamer bridge , as you accelerate away as its enclosed on the sides , that's not going to give me the misfire and juddering ? Don't you just love a land rover

kermit_rr · Jan 17, 2020

When it's making the noise, is it all the time, or just on hard acceleration? What about at idle or deceleration?

Henry_b · Jan 17, 2020

i'd give it lead with the bonnet up, and listen for the chuffing.

It is either the manifold gaskets, the headgasket or a spark plug.

I've had some in where the plugs were only finger tight because the DIY'er cranked em down like headbolts and wound the threads out!

If you have a misfire with this chuffing it is likely headgasket, have a compression test done,

kermit_rr · Jan 17, 2020

It depends how you define machine gunning.. mine made a really bad noise when one of the exhaust cams rounded off and it dumped into the inlet manifold when the inlet opened

Shippers · Jan 25, 2020

Had a similiar noise when I removed head on one side to extract a snapped plug on my 101 Able engineering supplied fitting kit recomending fitting exhuast gaskets that were not originally fitted spent hours trying to get them to seal and stop the chuffing and machine gun bang when reving on 1 cylinder took them out problem sorted still have the one in shed with black soot marks to prove that was the issue...

Landrover Bear · Jan 26, 2020

Henry_b said:
i'd give it lead with the bonnet up, and listen for the chuffing.

It is either the manifold gaskets, the headgasket or a spark plug.

I've had some in where the plugs were only finger tight because the DIY'er cranked em down like headbolts and wound the threads out!

If you have a misfire with this chuffing it is likely headgasket, have a compression test done,

kermit_rr said:
When it's making the noise, is it all the time, or just on hard acceleration? What about at idle or deceleration?

kermit_rr said:
It depends how you define machine gunning.. mine made a really bad noise when one of the exhaust cams rounded off and it dumped into the inlet manifold when the inlet opened

kermit_rr said:
When it's making the noise, is it all the time, or just on hard acceleration? What about at idle or deceleration?

At idle sounds lovely. Deceleration fine it’s when holding a constant speed or under load . Just done a 50 mile journey twisty roads and 70 mph I would say for about 20 mikes drove absolutely perfectly, then you might got to accelerate and it starts again !!

Landrover Bear · Jan 26, 2020

Henry_b said:
i'd give it lead with the bonnet up, and listen for the chuffing.

It is either the manifold gaskets, the headgasket or a spark plug.

I've had some in where the plugs were only finger tight because the DIY'er cranked em down like headbolts and wound the threads out!

If you have a misfire with this chuffing it is likely headgasket, have a compression test done,

Just replaced the plugs and there fine came in and out perfectly. I wondered about head gasket, and had them go on other cars, from my experience once they go the go not intermittently, same with manifolds / gasket . Did a 50 mile journey today and she ran like a dream for about 20 miles . Random!

Shippers · Jan 26, 2020

Could be heat related something breaking down electrically or expanding to create a gasket leak...

Shippers · Jan 26, 2020

Shippers said:
Could be heat related something breaking down electrically or expanding to create a gasket leak...

Had a plug lead that only showed fault when run for a while then caused backfire due to unburnt fuel igniting ...

diverman62 · Mar 17, 2020

Henry_b said:
i'd give it lead with the bonnet up, and listen for the chuffing.

It is either the manifold gaskets, the headgasket or a spark plug.

I've had some in where the plugs were only finger tight because the DIY'er cranked em down like headbolts and wound the threads out!

If you have a misfire with this chuffing it is likely headgasket, have a compression test done,

What did you do to fix thread problem on spark plug as I have same problem with my V8 4 ltr

Henry_b · Mar 17, 2020

diverman62 said:
What did you do to fix thread problem on spark plug as I have same problem with my V8 4 ltr

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