Disco 1 misfire,1995 3.9 V8

V8demon · Oct 20, 2018

discool said:
It will be back when the sun is out, but it’s just a hiss for a few seconds when releasing the fuel cap, although not every time.

Suns been out today, quite a warm day, no sussing. I wasn’t getting a suss though, more like a psshhh, and a big one at that haha

V8demon · Oct 21, 2018

Another warm day here in the south, fuel tank had pressurised again, check engine light flickered and at the same time the engine cut out and fired back up pretty much instantly whilst driving. The whole journey home it didn’t want to idle, I parked in the yard, selected park and sure enough the idle just dropped and it cut out.
I opened the fuel cap to release pressure, started the engine and it idled fine again.

Any suggestions?

Iffy Fuel pump? filler cap? This also seems to happen just before the fuel gauge hits the 1/4 tank mark.

V8demon · Oct 21, 2018

Another warm day here in the south, fuel tank had pressurised again, check engine light flickered and at the same time the engine cut out and fired back up pretty much instantly whilst driving. The whole journey home it didn’t want to idle, I parked in the yard, selected park and sure enough the idle just dropped and it cut out.
I opened the fuel cap to release pressure, started the engine and it idled fine again.

Any suggestions?

Iffy Fuel pump? filler cap?

mr glee · Oct 21, 2018

your tank breather may be blocked creating pressure when standing ( fuel expansion/ evaporation) and a vacuum when driving

wireman · Oct 21, 2018

mr glee said:
your tank breather may be blocked creating pressure when standing ( fuel expansion/ evaporation) and a vacuum when driving

+1 for the above

wireman · Oct 21, 2018

mr glee said:
your tank breather may be blocked creating pressure when standing ( fuel expansion/ evaporation) and a vacuum when driving

+1 for the above

V8demon · Oct 21, 2018

mr glee said:
your tank breather may be blocked creating pressure when standing ( fuel expansion/ evaporation) and a vacuum when driving

Where is the location of the tank breather? Any idea?

V8demon · Oct 21, 2018

Any idea where the tank breather is?

V8demon · Oct 21, 2018

Any idea where the tank breather is?

dieseldog69 · Oct 21, 2018

Check under the bonnet and look for a black plastic canister that is the charcoal filter for the petrol vapour

V8demon · Oct 21, 2018

dieseldog69 said:
Check under the bonnet and look for a black plastic canister that is the charcoal filter for the petrol vapour

Ahh ok, I know about that. I hear the canister itself can cause issues and some reroute the vacuum pipe to the top of the plenum in the stepper motor housing.

V8demon · Oct 21, 2018

dieseldog69 said:
Check under the bonnet and look for a black plastic canister that is the charcoal filter for the petrol vapour

Ahh ok, I know about that. I hear the canister itself can cause issues and some reroute the vacuum pipe to the stepper motor housing.

discool · Oct 21, 2018

V8demon said:
Where is the location of the tank breather? Any idea?

In the fuel filler cap, it has a two way breather.

V8demon · Oct 21, 2018

Apologies for the double postings, I’m assuming there’s an issue somewhere, I’m only posting once.

discool · Oct 21, 2018

V8demon said:
Apologies for the double postings, I’m assuming there’s an issue somewhere, I’m only posting once.

There’s an issue, but just “Post Reply” once and ignore anything else.

V8demon · Oct 21, 2018

So it could

discool said:
In the fuel filler cap, it has a two way breather.

So it could well be a faulty filler cap then.

discool · Oct 21, 2018

No, but the cap can look like it’s clogged up especially it its original, I replaced mine which looked grotty around the two valves but it made no difference at

dieseldog69 · Oct 21, 2018

I did the same but then changed out the charcoal canister thing and it alleviated the problem.

V8demon · Oct 22, 2018

Is there any way to just get rid of the charcoal canister completely? Can I simply disconnect the electrical plug and the tank breather or will that cause a rich fuel mixture?

Irishrover · Oct 22, 2018

Just an aside regarding rough running on the V8's (3.5's & 3.9's).
Years ago when I had my 3.9 Vogue SE, I had a problem with misfiring and backfiring-it was just as if it was running on 3 or 4 cylinders. Did exactly what the poster has done, changed plugs, dizzy cap, rotor arm & leads all to no avail.
Took it to a LR specialist and they could not find the cause.
Eventually, decided to check everything over for the umpteenth time but with one additional check...removed dizzy cap and disconnected main HT coil lead from the cap. Held the end of the HT lead with a well insulated pair of pliers about 10mm away from the brass segment on top of the rotor arm and got the Missus to crank it over...a big fat spark jumped across which meant that the rotor bakelite insulation had broken down. Bought another new Lucas rotor and did the same check-no spark. Fitted everything back and the motor ran perfectly.
Moral of the tale is - never take for granted that a new part can't be faulty !!!

V8demon · Oct 22, 2018

I hear what your saying Irishrover.

I have no doubt that I have faulty parts, when I hit this Disco it was apparent that somebody had been chasing an issue for a while, it had all new ignition components fitted but cheap variants from Eurocarparts and such, hence replacing.

After working through the faults I’ve come to realise that I’m not just chasing one issue, it was infact a combination of three issues.

Misfire was a poor connection on No.8 injector. Poor running was ignition components. Cutting out was temp sensor on inlet manifold.
It now appears that cutting out at 1/4 tank of fuel is a vacuum being created within the tank, which of course will be a breather issue. As a matter of course I will change the filler cap along with the charcoal canister. Google searches show this is quite a common fault and LR even made a new stepper motor housing to move the vacuum pipe higher up on the plenum. I believe a recall was issued but I’m assuming not many got fixed.

It was a tricky issue(s) to approach because I had no idea where to start fault finding at first, but of course it’s a LR product, it’s full of faults haha

discool · Oct 22, 2018

V8demon said:
Another warm day here in the south, fuel tank had pressurised again, check engine light flickered and at the same time the engine cut out and fired back up pretty much instantly whilst driving. The whole journey home it didn’t want to idle, I parked in the yard, selected park and sure enough the idle just dropped and it cut out.
I opened the fuel cap to release pressure, started the engine and it idled fine again.

Any suggestions?

Iffy Fuel pump? filler cap?

Filler cap, the valve in the cap is possible blocked so not letting air into the tank, soak the cap in petrol to remove the muck from the filter element and refit, better still replace the cap.
The hissing is normal, u may hear the tank ticking for 3 or 4 minutes when parked up after a run on a very hot day.

V8demon · Oct 25, 2018

So, went ahead and removed the charcoal canister, fuel tank breather is free to breath ad plenum vacuum is blocked off. Also replaced the fuel filler cap, still has a fair bit of pressure within the tank.

Still cuts out briefly and then refuses to idle after that.

My next thinking is, fuel pump relay or fuel pump? I’ve ordered a relay and already have a pump. Also thinking it could be the ignition amplifier mounted next to the coil?

Any ideas folks? There’s not a lot else to change now, this issue must be coming to an end shortly!!

Disco 1 misfire,1995 3.9 V8

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